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Displacement

Displacement

The term displacement can have one of several meanings, depending on context:
- Displacement (distance), a physical quantity in kinematics
- another term for a congruent transformation in geometry
- Electric displacement field, a physical quantity in electrodynamics
- Engine displacement, a property of an internal combustion engine
- Displacement (fencing)
- Displacement (fluid), a different physical quantity, used in fluid mechanics and navigation; used as a measure of a ship's size
- Displacement hull, where the moving hull's weight is supported by buoyancy alone and it must displace water from its path rather than planing on the water's surface
- A ship's displacement, also known as tonnage
- Displacement aka offset used in relative addressing of computer memory
- Particle displacement, acoustics of sound in air
- Displacement of people by persecution or violence
- Displacement (psychology)
- Single or double displacement reaction, a chemical reaction concerning the exchange of ions
- Displacement in Orthopedic surgery refers to change in alignment of the fracture fragments.

Displacement (distance)

In Newtonian mechanics, displacement is one of two subtly different quantities measuring distance and direction. direction (a) Displacement in the sense of position vector is a vector quantity which expresses position by the length and direction of a straight line from one place to another (as opposed to the scalar quantity distance which expresses only the length). The SI unit for either distance or displacement is the metre. (b) Displacement in the sense of movement is a vector quantity which expresses the distance and direction (in a straight line) from the starting to the finishing point. The two meanings are not the same, as the table shows: Category:Length ms:Sesaran

Kinematics

In physics, kinematics is the branch of mechanics concerned with the motions of objects without being concerned with the forces that cause the motion. In this latter respect it differs from dynamics, which is concerned with the forces that affect motion. Because of its relative simplicity, kinematics is usually taught before dynamics or the concept of a force is introduced. The equations of motion are generally taught at secondary school level.

Fundamental equations

Relative motion

This is a simple equation from vector mathematics that restates vector addition: motion of A relative to O is equal to the motion of B relative to O plus the motion of A relative to B:

r_ = r_ + r_ \,\!

Rotating frame

One fundamental equation in kinematics is the equation for the derivative of a vector described in a rotating frame of reference. As a sentence, it is: the time derivative of a vector in a fixed frame is equal to the derivative of the vector relative to the rotating frame plus the cross product of the angular velocity of the frame with the vector. In equation form that is:

\left.\frac\right|_ = \left.\frac\right|_ + \omega \times r(t)

where: r(t) is a vector X,Y,Z is the fixed frame x,y,z is the rotating frame ω is the rate of rotation of the frame.

Coordinate systems

Fixed rectangular coordinates

In this coordinate system, vectors are expressed as an addition of vectors in the x, y, and z direction from a non-rotating origin. Usually

\vec i \, \!

is a unit vector in the x direction,

\vec j \, \!

is a unit vector in the y direction, and

\vec k \, \!

is a unit vector in the z direction. The position vector,

\vec s \, \!

(or

\vec r \, \!

), the velocity vector,

\vec v \, \!

, and the acceleration vector,

\vec a \, \!

are expressed using rectangular coordinates in the following way:

\vec s = x \vec i + y \vec j + z \vec k \, \!

\vec v = \dot  = \dot  \vec  + \dot  \vec  + \dot  \vec  \, \!

\vec a = \ddot  = \ddot  \vec  + \ddot  \vec  + \ddot  \vec  \, \!

Note:

\dot  = \frac

\ddot  = \frac

Two dimensional rotating coordinate frame

This coordinate system only expresses planar motion. This system of coordinates is based on three orthogonal unit vectors: the vector

\vec i

, and the vector

\vec j

which form a basis for the plane in which the objects we are considering reside, and

\vec k

about which rotation occurs. Unlike rectangular coordinates which are measured relative to an origin that is fixed and non rotating, the origin of these coordinates can rotate and translate - often following a particle on a body that is being studied.

Derivatives of unit vectors

The position, velocity, and acceleration vectors of a given point can be expressed using these coordinate systems, but we have to be a bit more careful than we do with fixed frames of reference. Since the frame of reference is rotating, we must take the derivatives of the unit vectors into account when taking the derivative of any of these vectors. If the coordinate frame is rotating at a rate of

\vec \omega \, \!

in the counterclockwise direction (that's

\omega \vec k

using the right hand rule) then the derivatives of the unit vectors are as follows:

\dot \vec i = \omega \vec k \times \vec i = \omega \vec j

\dot \vec j = \omega \vec k \times \vec j = - \omega \vec i

Position, velocity, and acceleration

Given these identities, we can now figure out how to represent the position, velocity, and acceleration vectors of a particle using this coordinate system.

Position

Position is straightforward:

\vec s =  x \vec i + y \vec j

It's just the distance from the origin in the direction of each of the unit vectors.

Velocity

Velocity is the time derivative of position:

\vec v = \frac = \frac + \frac

By the chain rule, this is:

\vec v = \dot x \vec i + x \dot \vec i + \dot y \vec j + y \dot \vec j

Which from the identities above we know to be:

\vec v = \dot x \vec i + x \omega \vec j + \dot y \vec j - y \omega \vec i = (\dot x - y \omega) \vec i + (\dot y + x \omega) \vec j

or equivalently

\vec v = (\dot x \vec i + \dot y \vec j) + (y \dot \vec j + x \dot \vec i) = \vec v_ + \vec \omega \times \vec r

where

\vec v_

is the velocity of the particle relative to the coordinate system.

Acceleration

Acceleration is the time derivative of velocity. We know that:

\vec a = \frac 
= \frac + \frac

Consider the

\frac

part.

\vec v_

has two parts we want to find the derivative of: the relative change in velocity (

\vec a_

), and the change in the coordinate frame (

\omega \times \vec v_

\frac = \vec a_ + \omega \times \vec v_

Next, consider

\frac

. Using the chain rule:

\frac = \dot \vec \omega \times \vec r + \vec \omega \times \dot \vec r

\dot \vec r

we know from above:

\frac = 
\dot \vec \omega \times \vec r + 
\vec \omega \times (\vec \omega \times \vec r) +
\vec \omega \times \vec v_

So all together:

\vec a =  \vec a_ + \omega \times \vec v_ + 
\dot \vec \omega \times \vec r + 
\vec \omega \times (\vec \omega \times \vec r) +
\vec \omega \times \vec v_

And collecting terms:

\vec a =  \vec a_ + 2(\omega \times \vec v_) + 
\dot \vec \omega \times \vec r + 
\vec \omega \times (\vec \omega \times \vec r)

Three dimensional rotating coordinate frame

(to be written)

Kinematic constraints

A kinematic constraint is any condition relating properties of a dynamic system that must hold at all times. Below are some common examples:

Rolling without slipping

An object that rolls against a surface without slipping obeys the condition that the velocity of its center of mass is equal to the cross product of its angular velocity with a vector from the point of contact to the center of mass, or:

v_G(t) = \omega \times r_ \,\!

For the case of an object that does not tip or turn, this reduces to v = R ω .

Gears (no slip)

Similar to the case of rolling without slipping, this involves two bodies with the same motion at their contact point. For any bodies 1 and 2 the constraint is:

r_1 \omega_1 = r_2 \omega_2 \,\!

where r is a radius ω is an angular velocity

Inextensible cord

This is the case where bodies are connected by some cord that remains in tension and cannot change length. The constraint is that the sum of all components of the cord, however they are defined, is the total length, and the derivative of this sum is zero.

See example

- point mass
- rigid body

Congruent transformation

In mathematics, a congruent transformation (or congruence transformation) is:
- Another term for an isometry; see congruence (geometry).
- A transformation of the form A → P^TAP, where A and P are square matrices, P is invertible, and P^T denotes the transpose of P; see congruence in linear algebra. Category:mathematical disambiguation

Electric displacement field

In physics, the electric displacement field or electric flux density is a vector-valued field :

\vec D

that appears in Maxwell's equations and that generalizes the electric field. "D" stands for "displacement". In most ordinary materials,

\vec D

may be calculated as :

\vec D = \varepsilon \vec E

where

\varepsilon

is the permittivity of the material; in linear isotropic media this will be a constant, and in linear anisotropic media it will be a rank 2 tensor (a matrix)

Interpretation of the displacement field

The electric displacement field is sometimes known as the "macroscopic electric field," in contrast to the electric field E, which is analagously the "microscopic electric field." The difference is that the macroscopic field "averages out" the jumble of electric fields from charged particles that make up otherwise electrically neutral material. Thus D can be considered the field after taking into account the response of a medium to an external field, for instance by means of charge migration, reorientation of electric dipoles, etc. These responses can be summed into a quantity known as the polarisation of a medium, given the symbol P. :

\vec D \ = \varepsilon_ \ \vec E \ + \ \vec P

where ε₀ is the vacuum dielectric constant.

Capacitor interpretation

Imagine a microscopic parallel plate capacitor placed across a point in space (or in a medium) with no charges present except on the capacitor. The charge density on the plates is equal to the value of the D field between the plates. This follows directly from Gauss's law, by integrating over a small rectangular box straddling the edge of one of the capacitors: :

\oint_A \mathbf \cdot d\mathbf = Q

The part of the box inside the capacitor plate has no field, so that part of the integral is zero. On the sides of the box,

d\mathbf

is perpendicular to the field, so that part of the integral is also zero, leaving: :

|\mathbf| = \frac

which is the charge density on the plate.

Units

In the standard SI system of units D is measured in coulombs per square meter (C/m²). This choice of units results in one of the simpliest forms of Maxwell's vorticity equations:

\nabla \times \vec H = \vec j + \frac

If one chooses both B and H to be measured in teslas, and E and D to be measured in newtons per coulomb, then the formula is modified to be:

\nabla \times \vec H = \mu_0 \vec j + \frac \frac

Therefore it is seen as being preferential to express B & H, and D & E in different sets of units. Choice of units has differed in history, for instance in the electromagnetic system of scientific units, in which the unit of charge is defined such that

1 / 4\pi\varepsilon_0 = 1

(dimensionless), D and E are expressed in the same units. Category:Electric and magnetic fields in matter

Electrodynamics

Electrodynamics is the theory of the electromagnetic interaction (see electromagnetism). The theory is divided into
- Classical electrodynamics
- Quantum electrodynamics

Internal combustion engine

The internal combustion engine is a heat engine in which combustion occurs in a confined space called a combustion chamber. Combustion of a fuel creates high temperature/pressure gases, which are permitted to expand. The expanding gases are used to directly move a piston, turbine blades, rotor(s), or the engine itself thus doing useful work. Internal combustion engines can be powered by any fuel that can be combined with an "oxidizer" in the chamber. By way of contrast, an external combustion engine such as a steam engine does work when the combustion process heats a separate working fluid, such as water or steam, which then in turn does work. Jet engines, most rockets and many gas turbines are strictly classed as internal combustion engines, but the term internal combustion engine is also used to refer specifically to reciprocating engines, Wankel engines and similar designs in which combustion is intermittent. Today, in some published discussions, internal combustion engine is abbreviated to the acronym ICE.

History

Wankel engine English inventor Sir Samuel Morland used gunpowder to drive water pumps in the 17th century. For more conventional, reciprocating internal combustion engines the fundamental theory for two-stroke engines was established by Sadi Carnot in France in 1824, whilst the American Samuel Morey received a patent on April 1, 1826 for a "Gas Or Vapor Engine". The Italians Eugenio Barsanti and Felice Matteucci patented the first working, efficient version of an internal combustion engine in 1854 in London (pt. Num. 1072). Despite these and other attempts, it wasn't until 1859 that the Frenchman Étienne Lenoir (1822 - 1900) designed an engine that ran on a mixture of explosive gas and air. In 1860, Jean Joseph Etienne Lenoir produced a gas-fired internal combustion engine not dissimilar in appearance to a steam beam engine. This closelly resembled a horizontal double acting steam engine, with cylinders, pistons, connecting-rods and fly wheel in which the gas essentially took the place of the steam. In 1870 in Vienna Siegfried Marcus put the first mobile gasoline engine on a handcart. Nikolaus Otto working with Gottlieb Daimler and Wilhelm Maybach in the 1870's developed the four-stroke cycle (Otto cycle) engine.

Applications

Internal combustion engines are most commonly used for mobile propulsion systems. In mobile scenarios internal combustion is advantageous, since it can provide high power to weight ratios together with excellent fuel energy-density. These engines have appeared in almost all cars, motorbikes, many boats, and in a wide variety of aircraft and locomotives. Where very high power is required, such as jet aircraft, helicopters and large ships, they appear mostly in the form of gas turbines. They are also used for electric generators and by industry. For low power mobile and many non-mobile applications an electric motor is a competitive alternative. In the future, electric motors may also become competitive for most mobile applications. However, the high cost and weight and poor energy density of batteries and lack of affordable onboard electric generators such as fuel cells has largely restricted their use to specialist applications.

Operation

All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with air, although other oxidisers such as nitrous oxide may be employed. Also see stoichiometry. The most common fuels in use today are made up of hydrocarbons and are derived from petroleum. These include the fuels known as diesel, gasoline and liquified petroleum gas. Most internal combustion engines designed for gasoline can run on natural gas or liquified petroleum gases without modifications except for the fuel delivery components. Liquid and gaseous biofuels of adequate formulation can also be used. Some have theorized that in the future hydrogen might replace such fuels. Furthermore, with the introduction of hydrogen fuel cell technology, the use of internal combustion engines may be phased out. The advantage of hydrogen is that its combustion produces only water. This is unlike the combustion of hydrocarbons, which also produces carbon dioxide, a major cause of global warming, as well as carbon monoxide, resulting from incomplete combustion. The big disadvantage of hydrogen in many situations its storage. Liquid hydrogen has extremely low density- 14 times lower than water and requires extensive insulation, whilst gaseous hydrogen requires very heavy tankage. While hydrogen is light and therefore has a higher specific energy, the volumetric efficiency is still roughly five times lower than petrol. This is why hydrogen must be compressed if there is to be a useful amount of stored energy. All internal combustion engines must have a means of ignition to promote combustion. Most engines use either an electrical or a compression heating ignition system. Electrical ignition systems generally rely on a lead-acid battery and an induction coil to provide a high voltage electrical spark to ignite the air-fuel mix in the engine's cylinders. This battery can be recharged during operation using an alternator driven by the engine. Compression heating ignition systems (Diesel engines and HCCI engines) rely on the heat created in the air by compression in the engine's cylinders to ignite the fuel. Once successfully ignited and burnt, the combustion products (hot gases) have more available energy than the original compressed fuel/air mixture (which had higher chemical energy). The available energy is manifested as high temperature and pressure which can be translated into work by the engine. In a reciprocating engine, the high pressure product gases inside the cylinders drive the engine's pistons. Once the available energy has been removed the remaining hot gases are vented (often by opening a valve or exposing the exhaust outlet) and this allows the piston to return to its previous position (Top Dead Center - TDC). The piston can then proceed to the next phase of its cycle (which varies between engines). Any heat not translated into work is a waste product and is removed from the engine either by an air or liquid cooling system.

Parts

heat The parts of an engine vary depending on the engine's type. For a four-stroke engine, key parts of the engine include the crankshaft (purple), one or more camshafts (red and blue) and valves. For a two-stroke engine, there may simply be an exhaust outlet and fuel inlet instead of a valve system. In both types of engines, there are one or more cylinders (grey and green) and for each cylinder there is a spark plug (darker-grey), a piston (yellow) and a crank (purple). A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke and the downward stroke that occurs directly after the air-fuel mix in the cylinder is ignited is known as a power stroke. A Wankel engine has a triangular rotor that orbits in an epitroichoidal (figure 8 shape) chamber around an eccentric shaft. The four phases of operation (intake, compression, power, exhaust) take place in separate locations, instead of one single location as in a reciprocating engine. A Bourke Engine uses a pair of pistons integrated to a scotch yoke that transmits reciprocating force through a specially designed bearing assembly to turn a crank mechanism. Intake, compression, power, and exhaust all occur in each stroke of this yoke.

Classification

There is a wide range of internal combustion engines corresponding to their many varied applications. Likewise there is a wide range of ways to classify internal-combustion engines, some of which are listed below. Although the terms sometimes cause confusion, there is no real difference between an "engine" and a "motor." At one time, the word "engine" (from Latin, via Old French, ingenium, "ability") meant any piece of machinery. A "motor" (from Latin motor, "mover") is any machine that produces mechanical power. Traditionally, electric motors are not referred to as "engines," but combusion engines are often referred to as "motors."

Principles of operation

electric motor Reciprocating:
- Two-stroke engine
- Four-stroke engine
- Bourke Engine Rotary:
- Demonstrated:
- Wankel engine
- Proposed:
- orbital engine
- quasiturbine Continuous combustion:
- gas turbine
- jet engine
- rocket engine

Engine cycle

Engines based on the two-stroke cycle use two strokes (one up, one down) for every power stroke, relying on the action of the bottom of the piston within the crankcase to help move the fuel-air mixture, and are used where small size and weight are important, such as snowmobiles, lawnmowers, mopeds, outboard motors and some motorcycles. Gasoline two-stroke engines are generally louder, less efficient, more polluting, and smaller than their four-stroke counterparts, although large two-stroke diesel engines are not subject to these complaints and are used in many applications, for instance some locomotives built by EMD. Engines based on the four-stroke cycle or Otto cycle have one power stroke for every four strokes (up-down-up-down) and are used in cars, larger boats and many light aircraft. They are generally quieter, more efficient and larger than their two-stroke counterparts. There are a number of variations of these cycles, most notably the Atkinson and Miller cycles. Most truck and automotive Diesel engines use a four-stroke cycle, but with a compression heating ignition system it is possible to talk separately about a diesel cycle. The Wankel engine operates with the same separation of phases as the four-stroke engine (but with no piston strokes, would more properly be called a four-phase engine), since the phases occur in separate locations in the engine; however like a two-stroke piston engine, it provides one power 'stroke' per revolution per rotor, giving it similar space and weight efficiency. The Bourke cycle's combustion phase more closely approximates constant volume combustion than either four stroke or two stroke cycles do. It also uses less moving parts, hence needs to overcome less friction than the other two reciprocating types have to. In addition, its greater expansion ratio also means more of the heat from its combustion phase is utilized than is used by either four stroke or two stroke cycles.

Fuel and oxidiser types

Fuels used include gasoline (aka petrol), Liquified Petroleum Gas, Vapourized Petroleum Gas, Compressed Natural Gas, hydrogen, diesel fuel, JP18 (jet fuel), landfill gas, biodiesel, peanut oil, ethanol, methanol (methyl or wood alcohol). Engines that use gases for fuel are called gas engines and those that use liquid hydrocarbons are called oil engines. However, gasoline engines are often called gas engines for short. The only limitations are that the fuel must be easily transportable through the fuel system to the combustion chamber, and that the fuel release sufficient energy in the form of heat upon combustion to make use of the engine practical. The oxidiser is typically air, but can be pure oxygen, nitrous oxide or hydrogen peroxide. Other chemicals such as chlorine or fluorine have seen experimental use; but mostly are impractical. Diesel engines are generally heavier, noisier and more powerful at lower speeds than gasoline engines. They are also more fuel-efficient in most circumstances and are used in heavy road-vehicles, some automobiles (increasingly more so for their increased fuel-efficiency over gasoline engines), ships and some locomotives and light aircraft. Gasoline engines are used in most other road-vehicles including most cars, motorcycles and mopeds. Note that in Europe, sophisticated diesel-engined cars are far more prevalent, representing around 40% of the market. Both gasoline and diesel engines produce significant emissions. There are also engines that run on hydrogen, methanol, ethanol, liquefied petroleum gas (LPG) and biodiesel. Paraffin and Tractor vaporising oil (TVO) engines are no longer seen. Tractor vaporising oil

Cylinders

Internal combustion engines can contain any number of cylinders with numbers between one and twelve being common, though as many as 28 have been used. Having more cylinders in a engine yields two potential benefits: First. the engine can have a larger displacement with smaller individual reciprocating masses (that is, the mass of each piston can be less) thus making a smoother running engine (since the engine tends to vibrate as a result of the pistons moving up and down). Second, with a greater displacement and more pistons, more fuel can be combusted and there can be more combustion events (that is, more power strokes) in a given period of time, meaning that such an engine can generate more torque than a similar engine with fewer cylinders. The down side to having more pistons is that, over all, the engine will tend to weigh more and tend to generate more internal friction as the greater number of pistons rub against the inside of their cylinders. This tends to decrease fuel efficiency and rob the engine of some of its power. For high performance gasoline engines using current materials and technology (such as the engines found in modern automobiles), there seems to be a break point around 10 or 12 cylinders, after which addition of cylinders becomes an overall detriment to performance and efficiency, although exceptions such as the W-16 engine from Volkswagen exist.
- Most car engines have four to eight cylinders, with some high performance cars having ten, twelve, or even sixteen, and some very small cars and trucks having two or three. In previous years some quite large cars, such as the DKW and Saab 92, had two cylinder, two stroke engines.
- Radial aircraft engines, now obsolete, had from five to 28 cylinders. A row contains an odd number of cylinders, so an even number indicates a two- or four-row engine.
- Motor cycles commonly have from one to four cylinders, with a few high performance models having six.
- Snowmobiles usually have two cylinders. Some larger (not necessarily high-performance, but also touring machines) have four.
- Small appliances such as chainsaws and domestic lawn mowers most commonly have one cylinder, although two-cylinder chainsaws exist.

Ignition system

Internal combustion engines can be classified by their ignition system. Today most engines use an electrical or compression heating system for ignition. However outside flame and hot-tube systems have been used historically. Nikola Tesla gained one of the first patents on the mechanical ignition system with , "Electrical Igniter for Gas Engines", on 16 August 1898.

Fuel systems

Often for simpler reciprocating engines a carburetor is used to supply fuel into the cylinder. However, exact control of the correct amount of fuel supplied to the engine is impossible. Larger gasoline engines such as used in cars have mostly moved to Fuel injection systems. LPG engines use a mix of Fuel injection systems and closed loop carburetors. Diesel engines always use fuel injection. Other internal combustion engines like Jet engines use burners, and rocket engines use various different ideas including impinging jets, gas/liquid shear, preburners and many other ideas.

Engine configuration

Internal combustion engines can be classified by their configuration which affects their physical size and smoothness (with smoother engines producing less vibration). Common configurations include the straight or inline configuration, the more compact V configuration and the wider but smoother flat or boxer configuration. Aircraft engines can also adopt a radial configuration which allows more effective cooling. More unusual configurations, such as "H", "U", "X", or "W" have also been used. Multiple-crankshaft configurations do not necessarily need a cylinder head at all, but can instead have a piston at each end of the cylinder, called an opposed piston design. This design was used in the Junkers Jumo 205 diesel aircraft engine, using two crankshafts, one at either end of a single bank of cylinders, and most remarkably in the Napier Deltic diesel engines, which used three crankshafts to serve three banks of double-ended cylinders arranged in an equilateral triangle with the crankshafts at the corners. It was also used in single-bank locomotive engines, and continues to be used for marine engines, both for propulsion and for auxiliary generators. The Gnome Rotary engine, used in several early aircraft, had a stationary crankshaft and a bank of radially arranged cylinders rotating around it.

Engine capacity

An engine's capacity is the displacement or swept volume by the pistons of the engine. It is generally measured in litres or cubic inches for larger engines and cubic centimetres (abbreviated to cc's) for smaller engines. Engines with greater capacities are usually more powerful and provide greater torque at lower rpms but also consume more fuel. Apart from designing an engine with more cylinders, there are two ways to increase an engine's capacity. The first is to lengthen the stroke and the second is to increase the piston's diameter. In either case, it may be necessary to make further adjustments to the fuel intake of the engine to ensure optimal performance. An engine's quoted capacity can be more a matter of marketing than of engineering. The Morris Minor 1000, the Morris 1100, and the Austin-Healey Sprite Mark II all had engines of the same stroke and bore according to their specifications, and were from the same maker. However the engine capacities were quoted as 1000cc, 1100cc and 1098cc respectively in the sales literature and on the vehicle badges.

Engine pollution

Generally internal combustion engines, particularly reciprocating internal combustion engines, produce moderately high pollution levels, due to incomplete combustion of carbonaceous fuel, leading to carbon monoxide and some soot along with oxides of nitrogen & sulphur and some unburnt hydrocarbons depending on the operating conditions and the fuel/air ratio. Diesel engines produce a wide range of pollutants including aerosols of many small particles that are believed to penetrate deeply into human lungs.
- Many fuels contain sulfur leading to sulfur oxides (SOx) in the exhaust, promoting acid rain.
- The high temperature of combustion creates greater proportions of nitrogen oxides (NOx), demonstrated to be hazardous to both plant and animal health.
- Net carbon dioxide production is not a necessary feature of engines, but since most engines are run from fossil fuels this usually occurs. If engines are run from biomass, then no net carbon dioxide is produced as the growing plants absorb as much, or more carbon dioxide while growing.
- Hydrogen engines only produce water, in theory.

Bibliography

- Singer, Charles Joseph; Raper, Richard, A history of technology : The Internal Combustion Engine, edited by Charles Singer ... [et al.], Clarendon Press, 1954-1978. pp.157-176[http://proxy.bib.uottawa.ca:2398/cgi/t/text/pageviewer-idx?c=acls&cc=acls&idno=heb02191.0005.001&q1=bicycle&frm=frameset&seq=5]

External links

- [http://www.keveney.com/Engines.html Animated Engines] - explains a variety of types
- [http://auto.howstuffworks.com/engine3.htm How Internal Combustion Works] - with animation Category:Energy conversion Category:Engines ja:内燃機関

Displacement (fencing)

In fencing, a displacement is a movement that avoids or dodges an attack. Category:Fencing

Navigation

: This article concerns navigation in the sense of determination of position and direction on the surface of the Earth. See navigation (disambiguation) for other meanings. There are several traditions of navigation. In the pre-modern history of human migration and discovery of new lands by navigating the oceans, a few peoples have excelled as sea-faring explorers. Prominent examples are the Phoenicians, the Ancient Greeks, the Malays, the Persians, Arabians, the Norse and, perhaps more than any others, the peoples of the Pacific Ocean, particularly Polynesians and Micronesians.

Polynesian navigation

The Polynesian navigators routinely crossed thousands of miles of open ocean, to tiny inhabited islands, using only their own senses and knowledge, passed by oral tradition, from navigator to apprentice. In Eastern Polynesia, navigators, in order to locate directions at various times of day and year, memorized extensive facts concerning:
- the motion of specific stars, and where they would rise and set on the horizon of the ocean
- weather
- times of travel
- wildlife species (which congregate at particular positions)
- directions of swells on the ocean, and how the crew would feel their motion
- colors of the sea and sky, especially how clouds would cluster at the locations of some islands
- angles for approaching harbors These, and outrigger canoe construction methods, were kept as guild secrets. Generally each island maintained a guild of navigators who had very high status, since in times of famine or difficulty, only they could trade for aid or evacuate people. The guild secrets might have been lost, had not one of the last living navigators trained a professional small boat captain so that he could write a book. The first settlers of the Hawaiian Islands were said to have used these navigation methods to sail to the Hawaiian Islands from the Marquesas Islands. In 1973, the Polynesian Voyaging Society was established in Hawaii to research Polynesian navigation methods. They built a replica of an ancient double-hulled canoe called the Hokule'a, whose crew, in 1976, successfully navigated the Pacific Ocean from Hawaii to Tahiti using no instruments.
- [http://pvs.kcc.hawaii.edu/navigate/navigate.html Wayfinding Summary]
- [http://pvs.kcc.hawaii.edu/L2wayfind.html Wayfinding Main Page]

Western navigation

Modern methods

There are several different branches of navigation, including but not limited to:
- celestial navigation - navigation by observation of the sun, moon and stars
- pilotage - using visible natural and man made features such as sea marks and beacons
- dead reckoning - using compass and log to monitor expected progress on a journey
- waypoint navigation - using electronic equipment such as radio navigation and satellite navigation system to follow a course to a waypoint
- position fixing - determining current position by visual and electronic means
- collision avoidance using radar Knowing the ship's current position is the main problem for all navigators. Early navigators used pilotage, relying on local knowledge of land marks and coastal features, forcing all ships to stay close to shore. The magnetic compass allowing a course to be maintained and estimates of the ship's location to be calculated. Nautical charts were developed to record new navigational and pilotage information for use by other navigators. The development of accurate systems for taking lines of position based on the measurement of stars and planets with the sextant allowed ships to navigate the open ocean without needing to see land marks. Later developments included the placing of lighthouses and buoys close to shore to act a marine signposts identifying ambiguous features, highlighting hazards and pointing to safe channels for ships approaching some part of a coast after a long sea voyage. The invention of the radio lead to radio beacons and radio direction finders providing accurate land-based fixes even hundreds of miles from shore. These were made obsolete by satellite navigation systems. Traditional maritime navigation with a compass uses multiple redundant sources of position information to locate the ship's position. A navigator uses the ship's last known position and dead reckoning, based on the ship's logged compass course and speed, to calculate the current position. If the set and drift, due to tide and wind, can be determined, an estimated position can also be calculated. Periodically, the navigator needs to confirm the accuracy of the dead reckoning or estimated position calculations using position fixing techniques. This is done by correctly identifying reference points and measuring their bearings from the ship. These lines of position can be plotted on a nautical chart, with the intersection being the ship's current location. Addition lines of position can be measured in order to validate the results taken against other reference points. This is known as a fix. Celestial navigation systems are based on observation of the positions of the Sun, Moon and stars relative to the observer and a known location. Anciently the home port was used as the known location, currently the Greenwich Meridian or Prime Meridian is used as the known location for celestial charts. Navigators could determine their latitude by measuring the angular altitude of Polaris any time that it was visible (excepting, of course, in those southern latitudes from where it cannot be observed). Determining latitude by the sun was a little more difficult since the sun's altitude at noon during the year changes for a given location. Calculating the anticipated altitude of the sun for a given day and known position is done easily using Calculus. However, prior to the development and formulation of its key principles in the latter part of the 17th century by Isaac Newton and Gottfried Leibniz, tables of the sun's altitude during the year for a known port were used. The sun's angle over the horizon at noon was measured, and compared to the known angle at the same date as the known port. Local noon is easily determined by recording periodic readings of the altitude of the sun. Since periodic readings of the altitude will plot a sine wave, the maximum reading is the one used for local noon. Longitude is calculated as a time difference between the same celestial event at different locations. Noon was an easy event to observe. Local noon is determined while shooting the azimuth as described above. The time of the maximum altitude is easily determined by interpolating between periodic readings. The time of noon at the known location is carried by the navigator on an accurate clock. Then the local time of local noon is observed by the navigator. The difference of longitude is determined knowing that the sun moves to the west at 15 degrees per hour. The need for accurate navigation led to the development of progressively more accurate clocks. Once accurate clocks were available, detailed tables for celestial bodies were created so that navigational activities could take place anytime during the day or night, rather than at noon. In modern celestial navigation, a nautical almanac and trigonometric sight-reduction tables permit navigators to measure the Sun, Moon, visible planets or any of 57 navigational stars at any time of day or night. From a single sight, a time within a second and an estimated position, a position can be determined within a third of a mile (500 m). Conceptually, the angle to the celestial object establishes a ring of possible positions on the surface of the Earth. A second sighting on a different object establishes an intersecting ring. Usually the navigator knows his position well enough to pick which of the two intersections is the current position. The math required for sight reduction is simple addition and subtraction, if sight-reduction tables are available. The numerous celestial objects permit navigators to shoot through holes in clouds. Most navigation is performed with the sun and moon. Accurately knowing the time of an observation is important. Time is measured with a chronometer, a quartz watch or a shortwave radio broadcast from an atomic clock. A quartz wristwatch normally keeps time within a half-second per day. If it is worn constantly, keeping it near body heat, its rate of drift can be measured with the radio, and by compensating for this drift, a navigator can keep time to better than a second per month. Traditionally, three chronometers are kept in gimbals in a dry room near the center of the ship, and used to set a watch for the actual sight, so that the chronometers themselves do not risk exposure to the elements. Winding the chronometers was a crucial duty of the navigator. The angle is measured with a special optical instrument called a "sextant." Sextants use two mirrors to cancel the relative motion of the sextant. During a sight, the user's view of the star and horizon remains steady as the boat rocks. An arm moves a split image of the star relative to the split image of the horizon. When the image of the star touches the horizon, the angle can be read from the sextant's scale. Some sextants create an artificial horizon by reflecting a bubble. Inexpensive plastic sextants are available, though they have less accuracy than the more expensive metal models. The LORAN system is based on measuring the phase shift of radio waves sent simultaneously from a master and slave station. Signals from these two point establish a hyperbolic curve for possible positions. A third source along with dead-reckoning will generally resolve to a single position. GPS uses 3D trilateration based on measuring the time-of-flight of radio waves using the well-known speed of light to measure distance from at least three satelites. This can be accomplished using low-cost quartz clocks because the satellites send time correction signals to the GPS receivers.

History

In the West, navigation was at first performed exclusively by dead-reckoning, the process of estimating one's present position based on the navigators' experience with wind, tide and currents. Most sailors have always been able find absolute north from the stars, which currently rotate around Polaris, or by using a dual sundial called a diptych. When combined with a plumb bob, some diptychs could also determine latitude. Basically, when the diptych's two sundials indicated the same time, the diptych was aligned to the current latitude and true north. diptych Another early invention was the compass rose, a cross or painted panel of wood oriented with the pole star or diptych. This was placed in front of the helmsman. Latitude was determined with a "cross staff" an instrument vaguely similar to a carpenter's angle with graduated marks on it. Most sailors could use this instrument to take sun sights, but master navigators knew that sightings of Polaris were far more accurate, because they were not subject to time-keeping errors involved in finding noon. Time-keeping was by precision hourglasses, filled and tested to 1/4 of an hour, turned by the helmsman, or a young boy brought for that purpose. The most important instrument was a navigators' diary, later called a rutter. These were often crucial trade secrets, because they enabled travel to lucrative ports. The above instruments were a powerful technology, and appear to have been the technique used by ancient Cretan bronze-age trading empire. Using these techniques, masters successfully sailed from the eastern Mediterranean to the south coast of the British Isles. Some time later, around 300, the magnetic compass was invented in China. This let masters continue sailing a course when the weather limited visibility of the sky. China Around 400, metallurgy allowed construction of astrolabes graduated in degrees, which replaced the wooden latitude instruments for night use. Diptychs remained in use during the day, until shadowing astrolabes were constructed. After Isaac Newton published the Principia, navigation was transformed. Starting in 1670, the entire world was measured using essentially modern latitude instruments and the best available clocks. In 1730 the sextant was invented and navigators rapidly replaced their astrolabes. A sextant uses mirrors to measure the altitude of celestial objects with regard to the horizon. Thus, its "pointer" is as long as the horizon is far away. This eliminates the "cosine" error of an astrolabe's short pointer. Modern sextants measure to 0.2 minutes of arc, an error that translates to a distance of about 0.2 nautical miles (400 m). At first, the best available "clocks" were the moons of Jupiter, and the calculated transits of selected stars by the moon. These methods were too complex to be used by any but skilled astronomers, but they sufficed to map most of the world. A number of scientific journals during this period were started especially to chronicle geography. Later, mechanical chronometers enabled navigation at sea and in the air using relatively unskilled procedures. In the late 19th century Nikola Tesla invented radio and direction-finding was quickly adapted to navigation. Up until 1960 it was commonplace for ships and aircraft to use radio direction-finding on commercial stations in order to locate islands and cities within the last several miles of error. Around 1960, LORAN was developed. This used time-of-flight of radio waves from antennas at known locations. It revolutionized navigation by permitting semiautomated equipment to locate geographic positions to less than a half mile (800 m). An analogous system for aircraft, VOR and DME, was developed around the same time. At about the same, TRANSIT, the first satellite-based navigation system was developed. It was the first electronic navigation system to provide global coverage. Other radionavigation systems include:
- Decca
- Omega, a longwave system developed by the United States Navy
- Alpha, a longwave system developed by the Soviet Union In 1974, the first GPS satellite was launched. The GPS system now permits accurate geographic location with an error of only a few metres, and precision timing to less than a microsecond. GLONASS is a positioning system launched by the Soviet Union. It relies on a slightly different geodesic model of the Earth. Galileo is a competing system, that will be placed into service by the European Union.

"Point" measure of direction

A "point" is defined as one eighth of a right angle, and therefore equals exactly 11.25 degrees. For example, a bearing of northwest by north differs by one point from a northwest bearing, and by a point from a north-northwest one.

External links

- [http://www.wildernessmanuals.com/manual_6/chpt_2/index.html Navigation] - U.S Army Manual.
- [http://www.irbs.com/bowditch Bowditch Online] - complete online edition of Nathaniel Bowditch's American Practical Navigator Category:Navigation zh-min-nan:Tō-hâng ja:航海

Weight

:See also weight function. For the 1994 album by the group Rollins Band, see Weight (album). In the physical sciences, weight is the interaction of matter with a gravitational field. It is equal to the mass of the object multiplied by the magnitude of the gravitational field. The word weight entered Old English sometime around the 9th century, and meant the quantity measured with a balance -- the same as mass in both common and scientific usage. In common usage, weight still means the same as mass.

Weight and mass

"Weight" is often used as a synonym for mass. For instance, when we buy or sell goods "by weight", we are interested in the amount of goods exchanged, not how hard it presses down on the table. Similarly, in measurements of body weight we are primarily interested in the amount of tissue (fat, muscle, etc.) present. Correspondingly, weight is often given in kilograms and other units of mass. In the physical sciences, people usually distinguish between weight and mass. Under most circumstances, this ambiguity is not a problem, because the weight of an object is directly proportional to its mass, and the constant of proportionality -- the strength of the gravitational field -- is approximately constant everywhere on the surface of the Earth (around 9.8 m/s²). For instance, a body will exert less force if it is located on the Moon than if it is on the Earth, since the gravitational field of the Moon is weaker; its mass, on the other hand, does not depend on position. Although terms such as "atomic weight", "molecular weight", and "formula weight" may still be encountered, such usage is often discouraged; terms like atomic mass are used instead. Mass is measured using a balance which compares an item in question to matter of known mass; this method is independent of gravity. Alternately, a spring scale or Hydraulic or pneumatic scale is used to measure force (which physicists call weight). Most scales measure weight using a spring. Related to the historical identification of mass and weight, the pound has been used both as a unit of mass and as a unit of force. In the United States, United Kingdom, and elsewhere, the pound is and always has been officially defined as a unit of mass. The corresponding force is called a pound-force, and similarly the weight of a kilogram of material on Earth is called a kilogram-force. However, the use of pounds to measure forces is still common in engineering, and it occurs in derived units like p.s.i. (pounds per square inch). In most countries, scientists have adopted SI units, which use kilogram for mass and newton for force non-interchangeably.

Weight as a force

The SI unit for weight is the newton (N), or kilogram metres per second squared (kg m s⁻²). The weight force that we sense is actually the normal force exerted by the surface we stand on, which prevents us from being pulled to the center of the Earth, and not the weight itself. This normal force, that we can call the apparent weight is the one that is measured by a weighing scale, not the weight itself. A good evidence of this is given by the fact that a person moving up and down on his toes does see the indicator moving, telling that the measured force is changing while his weight, that depends only on his mass, the Earth mass and the distance between his center of mass and the center of Earth obviously do not change. In contrast, in free-fall, there is no apparent weight because we are not in contact with any surface to provide such a normal force. The experience of having no apparent weight is known as weightlessness or microgravity.

Comparative weights on bodies of the solar system

The following is a list of the weights of a mass on some of the bodies in the solar system, relative to its weight on Earth: For weight variations on Earth, see gee, physical geodesy and gravity anomaly.

Human weight in the medical sciences and ordinary language

Although many people prefer the less-ambiguous term body mass to body weight, the term weight is overwhelmingly used in daily English speech and in biological and medical science contexts. Body weight is measured in kilograms throughout the world. Most hospitals in the United States use kilograms for calculations, but use kilograms and pounds simultaneously for other purposes (a pound is 0.45 kg). Many people in the United Kingdom still measure their weight using the stone equal to 14 lb (6.35 kg).

Sports usage

Participants in sports such as boxing, wrestling, judo, and weight-lifting are classified according to their body weight, measured in units of mass such as pounds or kilograms. See, e.g., wrestling weight classes, boxing weight classes, judo at the 2004 Summer Olympics, boxing at the 2004 Summer Olympics. In horse racing, weight is used to handicap horses. A weight also refers to the physical objects used in weight-lifting and other sports such as the hammer throw.

Planing (sailing)

A planing boat's hull skims across the surface of the water rather than pushing through the water in the way a traditional displacement hull works. In the U.S. the term Hydroplaning is sometimes used instead of planing, but primarily to describe racing motorboats which plane. Hydroplaning is also an important concern in car safety (see Hydroplaning (car)). Planing allows the boat to go faster by using its speed and hull shape to lift the front part of the hull out of the water. The boat travels on top of the water, greatly reducing the hydrodynamic drag on the vessel. The increase in aerodynamic drag is small by comparison, and can be compensated for by the increased power from the sails due to the faster speed of the craft, and by the crew trimming the sails. Planing is usually a sailing term, but other watercraft are designed to plane to some extent including most small and some medium size powered boats. A rigid-hulled inflatable boat is an example of a powered hydroplaning craft. Hydroplane is sometimes used as a noun synonym for a hydrofoil, a motorized boat designed to hydroplane on special wings attached to the keel. The principle of a keel wing to lift heavier boats out of the water more efficiently has recently been translated into a surf board model whose wing-shaped skeg allows planing at very low speeds (about 4 knots according to one talk-show enthusiast).

History

The earliest documented planing sailboat was a proa built in 1898 by Commodore Ralph Munroe; it was capable of speeds of more than twice the hull speed. Planing a sailing dinghy was first popularised by Uffa Fox in Britain. In 1928 Uffa Fox introduced planing to the racing world in his International 14 dinghy, the Avenger. It had been designed with a hull shape which permitted planing. He gained 52 first places, two seconds and three third places out of 57 race starts that year. Obviously this performance had an impact: other designers took on his ideas and developed them. Over the years, most dinghies have acquired some ability to plane, and there are now many high-performance dinghies (usually called skiffs, [http://www.18footer.org/ see these examples], or these in [http://www.skiff.org.nz/]), which will plane even in light winds, at all points of sail.

How planing works

Normally a non-planing, displacement, hull is restricted in its maximum speed by a formula related to its overall length

HSPD = \sqrt - 1.34

, where HSPD (in knots) is maximum hull speed, and LWL is the hull length at waterline. This speed is maximised when the boat sits between the bow and stern waves, with no intervening self-caused waves along its length. At low speeds, a hydroplaning hull acts as a displacement hull. But, when the speed increases the hull begins acting as a planing hull. However, when the boat begins to plane the formula becomes irrelevant since the boat is climbing its own bow-wave. The bow rises slightly as it starts by mounting its own bow wave. When it reaches the speed where it overtakes the bow wave, the bow resumes its normal attitude. The boat can often be seen to leave its stern-wave some distance behind it. The hull is now planing. Beginning to plane is the aquatic, and less dramatic, equivalent of an aeroplane breaking the sound barrier. The aeroplane at Mach 1 begins to pierce and go beyond its own 'bow wave', i.e. the compressed layers of air on its front surfaces and ahead of it. A hydroplaning hull travels faster and more efficiently than a displacement hull of comparable size due to two factors: # less area of the hull is in contact with the water. This reduces the friction on the hull caused by water. # the hull is displacing less water from its path. Water is relatively heavy and a displacing hull must displace its own weight of water. The characteristics of a planing hull are that it is narrow at the prow, with a broader beam towards the rear. The shape of the underneath of the rear of a larger, planing, powerboat is often V shaped. To plane, the power to weight ratio must be high; sailing boats need a good sail area and powerboats need a highly powered engine. Note that under some high wind conditions, very light craft (such as windsurfers and kitesurfers) can actually be pulled up onto the surface of the water, or into the air, by the upward lift of the sail alone. Although this certainly reduces water resistance, it is probably better described as flying, rather than hydroplaning, which requires forward velocity to exploit the shape of the hull.

How to plane in a sailing boat

Planing can happen in a suitably designed boat in moderate to strong winds if the crew do some or all of the following:
- Sail on a reach or broad reach to begin
- Slacken the jib
- Raise the centreboard
- Increase the speed
- Keep the hull level, trapeze if necessary
- Observe the wake until it is smooth and fast
- Move the crew weight increasingly towards the rear to begin and to sustain planing
- Sheet in as speed increases, and apparent wind correspondingly moves forward
- Keep the boat flat and level
- Bear away to maintain speed as necessary
- Flick or pump the sails (although there are restrictions on doing this in a race) While planing, keep control of the waves and steer through them, avoiding to increase speed to collide with the wave in front. Also, in dinghies, keep good control of the sail power. A small change in wind direction can easily cause a capsize, watch also out for gybes. Boat control becomes easier as planing begins, but fast reactions are often needed to get there, to keep the speed up and to keep the boat level. Crew balance and trim are vital, as are sail trimming and minimal centreboard.

External links

- [http://www.boatbuilding.com/content/ratios.html Information on boat design]
- [http://www.ukwindsurfing.com/pics_n_vids/ Some videos of planing sailboards]
- [http://naca.larc.nasa.gov/reports/1958/naca-report-1355/ Seminal 1958 NACA technical report on hydroplaning] Category:Sailing manoeuvres

Offset

Offset is used in :
- Offsets for material on the word offset as relating to the propagation of plants
- Offset printing for material on the offset printing technique.
- Offset account for a description of an account used against a mortgage to reduce interest payments.
- Offset (computer)
- Offset antenna.

Computer memory

The term computer memory refers to the parts of a digital computer which retain physical state (data) for some interval of time. In its most common usage, "memory" refers to very fast storage which does not retain its stored data when the power is turned off. Compare this to "storage", such as hard drive space, which is slow but keeps its data even without power. An analogy is to think of the storage as human memory, with the hard disk as long-term memory, and the memory as short-term memory. In a home computer, memory will often take the form of:
- Random access memory, or RAM, which is used to temporarily store things such as programs and data while the computer is using them. Since RAM can be accessed at very high speeds, it is well suited for this task.
- Cache memory is a small amount of very high speed dedicated memory. Cache memory is used to allow quicker access to data which ordinarily is slow to retrieve. Because of cache memory's high speed nature, storing data into cache memory before it is actually accessed can allow quicker response times. Cache memory is often found in microprocessors and hard drives among many others.

Different types and different purposes

Memory can be categorized in different ways by technology or properties:
- primary access by the CPU or secondary (indirect) access by the CPU. This distinction is primarily based on the speed of access to the memory.
- volatile or non-volatile is a distinction based on technology (magnetic vs. electrical, etc.). Volatile memory requires power to maintain its stored information.
- Read-only memory, or read-write is a distinction based on properties of the memory. Read only memory, or "ROM", is not modifiable.
- Random-Access or Sequential-Access, is a distinction based on the mechanism of reading the memory.
- Mutable (Read-write) vs. Immutable (Read only) storage. Historically, "memory" referred to "magnetic core memory" in the 1950s, and then to semiconductor-based storage in the 1970s. The evolution of memory is closely tied to the costs of various technologies, as can be seen in the history of computing hardware. Each type of storage is suited for different purposes, and most computers contain several types: primary, secondary, and volatile.

Primary vs. secondary storage

In traditional parlance, primary storage contains data that are actively being used (for example, the programs currently being run and the data they are operating on). It is typically high-speed, relatively small, is often (but not always) volatile. It is sometimes referred to as "Main Memory." It can be accessed immediately and randomly. Secondary storage, also known as peripheral storage, is where the computer stores information that is not necessarily in current use. It is typically slower and higher-capacity than primary storage. It is almost always non-volatile. It is slow due to serial access (thus it is also termed Serial Access Memory). Confusingly, these terms are sometimes used differently. Primary storage can be used to refer to local random-access disk storage, which should properly be called secondary storage. If this type of storage is called primary storage, then the term secondary storage would refer to offline, sequential-access storage like tape media. This usage usually occurs in contexts where only the slower, larger forms of storage are being discussed.

Volatile storage

Volatile storage loses its contents when it loses power; non-volatile storage does not.

Mutable vs. immutable storage

Data stored in mutable storage can be overwritten at any time. Data stored in immutable storage cannot be overwritten. Systems can be made more secure by storing programs and static data in immutable storage, where they cannot be changed by an attacker. Dynamic data is stored in mutable storage because it must be changed from time to time. Most operating systems store all programs and data on hard disk drives, which are inherently mutable storage devices. File system permissions can be used to make certain areas of the hard disk logically immutable. However, the superuser is normally not affected by these permissions thus allowing some attacks to succeed. Some operating systems, such as Linux, extend this logical immutability so data remains immutable even if an attacker gains superuser access. Attackers may be able to destroy the data but they can't change it.

A list of storage devices

- Bubble memory
- Cache memory
- Core memory also known as ferrite core memory
- Core rope memory
- Delay line memory
- Holographic memory
- Memory stick
- Selectron tube
- Semiconductor memory:
- DRAM
- EPROM
- EEPROM
- Flash memory
- NVRAM
- MRAM
- RAM
- ROM
- SRAM
- VRAM
- WRAM
- Thin film memory
- Williams tube Internal storage areas in the computer. The term memory identifies data storage that comes in the form of chips, and the word storage is used for memory that exists on tapes or disks. Moreover, the term memory is usually used as a shorthand for physical memory, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk. Every computer comes with a certain amount of physical memory, usually referred to as main memory or RAM. You can think of main memory as an array of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (or characters) of information. There are several different types of memory:
- RAM (random-access memory): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
- SRAM (static RAM): Stores data in flip flops. Only requires steady flow of electricity to maintain its contents.
- DRAM (dynamic RAM): Stores data in capacitors, which gradually lose charge and "forget". Requires periodic Refresh cycles to restore the data.
- ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.
- PROM (programmable ROM): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.
- EPROM (erasable programmable ROM): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light. These must be removed from the board, erased, then reprogrammed.
- EEPROM (electrically erasable programmable ROM): An EEPROM is a special type of PROM that can be erased by electrical signals, often the same ones that program it with new data. These can stay on the board and be reprogrammed in system.

A list of memory-related software

- Aard
- QEMM

Acoustics

Acoustics is a branch of physics and is the study of sound, mechanical waves in gases, liquids, and solids. A scientist who works in the field of acoustics is an acoustician. The application of acoustics in technology is called acoustical engineering. There is often much overlap and interaction between the interests of acousticians and acoustical engineers. "... acoustics is characterized by its reliance on combinations of physical principles drawn from other sources; and that the primary task of modern physical acoustics is to effect a fusion of the principles normally adhering to other sciences into a coherent basis for understanding, measuring, controlling, and using the whole gamut of vibrational phenomena in any material medium." Origins in Acoustics. F.V. Hunt. Yale University Press, 1978 The main sub-disciplines of acoustics are
- Aeroacoustics is the study of aerodynamic sound, generated when a fluid flow interacts with a solid surface or with another flow. It has particular application to aeronautics, examples being the study of sound made by jets and the physics of shock waves (sonic booms).
- Architectural acoustics is the study of how sound and buildings interact including the behavior of sound in concert halls and auditoriums but also in office buildings, factories and homes.
- Bioacoustics is the study of the use of sound by animals such as whales, dolphins and bats.
- Biomedical acoustics is the study of the use of sound in medicine, for example the use of ultrasound for diagnostic and therapeutic purposes.
- Loudspeaker acoustics is an engineering discipline behind the design of the loudspeaker
- Psychoacoustics is the study of how people react to sound, hearing, perception, and localization.
- Psychological Acoustics is the study of the mechanical, electrical and biochemical function of hearing in living organisms.
- Physical acoustics is the study of the detailed interaction of sound with materials and fluids and includes, for example, sonoluminescence (the emission of light by bubbles in a liquid excited by sound) and thermoacoustics (the interaction of sound and heat).
- Speech communication is the study of how speech is produced, the analysis of speech signals and the properties of speech transmission, storage, recognition and enhancement.
- Vibration acoustics Structural Acoustics and Vibration is the study of how sound and mechanical structures interact; for example, the transmission of sound through walls and the radiation of sound from vehicle panels.
- Ultrasonics is the study of high frequency sound, beyond the range of human hearing.
- Wolffian Accousitics is the study of salient features of pediatric ultrasound insofar as it reviews technologic factors, technique, and the normal anatomy used to evaluate the pediatric tract for abnormality.
- Musical acoustics is the study of the physics of musical instruments
- Underwater acoustics is the study of the propagation of sound in the oceans. Closely associated with sonar research and development.
- Acoustic engineering is the study of how sound is generated and measured by loudspeakers, microphones, sonar projectors, hydrophones, ultrasonic transducers, sensors, Electro Acoustics, and all other topics on this list. (see external links) A sound wave is characterized by its speed, its wavelength and its amplitude. The speed of sound depends on the medium through which the sound travels and also depends on temperature and not on the air pressure. The speed of sound is about 340 m/s in air and 1500 m/s in water. The wavelength is the distance from one wave peak to the next. The wavelength,

\lambda

of a sound wave is related to the speed of sound

c

and its frequency

f

by :

\lambda = \frac

Sound pressure level (SPL)

The amplitude of a sound wave is most commonly characterized by its sound pressure. In a normal working environment, a very wide range of pressures can occur and it is therefore a convention that sound pressure is measured on a logarithmic scale using the decibel. If

p

is the rms sound pressure amplitude then the sound pressure level (SPL) is defined as 20 times the logarithm of the ratio of the pressure to some reference pressure. Sound pressure level SPL is calculated in decibels as :

L_p =20\, \log_\left(\frac\right)=10\, \log_\left(\frac\right)\mbox SPL

The reference sound pressure in air is by convention the threshold of hearing: :

p_0 = 2 \cdot 10^ \mbox

:= 20 µPa in air and 1 µPa in water. (Pa = pascal = N / m²; N = newton) When speaking of sound levels, one must be sure to differentiate between sound pressure levels and sound power levels. Sound pressure levels are recorded by microphones and other devices. This is a measurement of the amount of pressure in the air being sensed at a given location. It follows that its value can be determined through direct experimentation. In comparison, sound power levels are a measurement of the actual energy being put into use by a given device to create noise. Because of environmental factors, and other influences, the amount of energy a device devotes to creating sound may not be equal to the actual level of the sound as it's perceived. It can be useful to express sound pressure in this way when dealing with hearing, as the perceived loudness of a sound correlates roughly logarithmically to its sound pressure. Both microphones and eardrums respond to the sound pressure level. They cannot convert the sound intensity. Sound power measurements cannot be directly measured, and must be inferred through other data.

Measurement methods

There are two popular ways for scientists to perform acoustical measurements. They include a "direct method", and a "comparison method". The direct method computes sound power levels by computing an equation of environmental factors (such as room temperature, humidity, reverberation time, etc.) and sound pressure levels. A more precise implementation of this method can be found in the ISO3745 acoustics standard. The comparison method however, is conducted by measuring sound pressure levels from a reference sound source which emits a known, constant, sound power level, and then comparing that level with the sound pressure level of the object being recorded. Each way is equally valid and accurate.

Reverberation and anechoic rooms

Experiments such as the two methods mentioned above are sometimes performed in reverberation rooms, or in some cases, anechoic rooms. The design of a reverberation room is to create long lasting echoes of sound waves. This helps create a highly averaged and omnidirectional sound level throughout the entire chamber. A typical example of rooms with characteristics similar to reverberation rooms are concrete tunnels, caves, etc. Anechoic rooms, such as hemi-anechoic rooms, or fully anechoic rooms are created to simulate what is called a free field. A free field is the representation of a theoretical infinite space, in which no sound wave reflections, or echoes, take place. In rooms such as these, the only sounds which exist are being emitted directly from the source, and are not reflected from another part of the chamber. Anechoic rooms have the characteristic of being muted and muffled.

Helmholtz resonator

A Helmholtz resonator is a container with an open hole or neck. It is sometimes used as a passive noise control device. It behaves essentially as a mass-spring-damper system, and its resonant frequency can be calculated as follows:
- f = resonant frequency
- s = speed of sound in air
- r = radius of neck
- a = area of neck
- l = length of neck
- L′ = effective length of neck :L′ = l + 1.7r (outer end flanged) :L′ = l + 1.4r (outer end unflanged)
- v = volume :

f = (s/2 \pi)(\sqrt)

(A container with a hole, rather than a neck, behaves as being flanged, with a neck length of 0.) The Helmholtz resonator is an example of the lumped component model of acoustic systems which is useful when the wavelength of interest is significantly larger than the physical dimensions of the system. Familiar examples of Helmholtz resonators include blowing across the top of a bottle, whistling, and the ocarina.

Rectangular boxes

- f = frequency of standing wave of a rectangular box
- s = speed of sound in air
- x, y, z = dimensions of box
- N_x, N_y, N_z = any integers :

f = (s/2)(\sqrt)

External links

- [http://physics.kenyon.edu/EarlyApparatus/Rudolf_Koenig_Apparatus/Helmholtz_Resonator/Helmholtz_Resonator.html Helmholtz Resonator]
- [http://www.phys.unsw.edu.au/~jw/Helmholtz.html Helmholtz Resonance]
- [http://www.physics.umd.edu/lecdem/services/demos/demosh3/h3-41.htm Helmholtz Resonator with oscilloscope]
- [http://www.fci.uach.cl/escuela/ingacustica/index.htm Acoustic Engineering at Universidad Austral de Chile]
- [http://www.acoustics.salford.ac.uk/schools/index.htm Sounds Amazing: a learning resource for sound and waves]
- [http://www.fe.up.pt/~carvalho/igrejase.htm Church Acoustics]
- Category:Applied and interdisciplinary physics Category:Sound ko:음향학 ja:音響学

Air

Air is a name for the mixture of gases present in the Earth's atmosphere. Compressed air is often used in scuba diving as a shallow water breathing gas and to inflate buoyancy devices. Compressed air is also used as the means of transmission of energy to pneumatic tools.

Composition of air

By volume, air is about:
- 78.084% Nitrogen (N₂)
- 20.947% Oxygen (O₂)
- 0.934% Argon (Ar)
- 0.033% Carbon Dioxide (CO₂) With trace amounts of:
- Neon (Ne)
- Helium (He)
- Krypton (Kr)
- Sulfur dioxide (SO₂)
- Methane (CH₄)
- Hydrogen (H₂)
- Nitrous Oxide (N₂O)
- Xenon (Xe)
- Ozone (O₃)
- Nitrogen dioxide (NO₂)
- Iodine (I₂)
- Carbon monoxide (CO)
- Ammonia (NH₃) The amount of water vapor in the air varies considerably depending on weather, climate, and altitude. See Humidity. The molecular mass of air is approximately 28.96443 g/mole (molecular weight of standard air - CRC, 1983).

External link

- [http://mistupid.com/chemistry/aircomp.htm Composition of Air] Category:Atmosphere Category:Psychrometrics Category:HVAC ko:대기 ms:Udara ja:空気 simple:Air

Forced migration

Forced migration refers to the coerced movement of a person or persons away from their home or home region. It often connotes violent coercion, and is used interchangeably with the terms "displacement" or forced displacement. A specific form of forced migration is population transfer, which is a coherent policy to move unwanted persons, perhaps as an attempt at ethnic cleansing. Someone who has experienced forced migration is a "forced migrant" or "displaced person". Forced migration has accompanied religious and political persecution, as well as war, throughout human history but has only become a topic of serious study and discussion relatively recently. This increased attention is the result of greater ease of travel, allowing displaced persons to flee to nations far removed from their homes, the creation of an international legal structure of human rights, and the realizations that the destabilizing effects of forced migration, especially in parts of Africa, the Middle East, south and central Asia, ripple out well beyond the immediate region.

External links

- [http://www.eurasylum.org Eurasylum] Many relevant documents on asylum and refugee policy, immigration and human trafficking/smuggling internationally
- [http://www.forcedmigration.org/discussion/ Forced Migration Discussion List] (Archives of FORCED-MIGRATION@JISCMAIL.AC.UK) focuses on issues concerning refugees and internal displacement.
- [http://www.forcedmigration.org/ Forced Migration Online] provides access to a diverse range of relevant information resources on forced migration, including a searchable digital library consisting of full-text docum