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Radiant Intensity

Radiant intensity

In radiometry, radiant intensity is a measure of the intensity of a light beam. It is defined as power per unit solid angle. The SI unit of radiant intensity is Watts per steradian (W·sr^-1). Note that this is quite distinct from the meaning of intensity in other areas of physics, and the latter definition is sometimes used in optics as well, where irradiance or radiant exitance would be the preferred radiometric term. =See also=
- Luminous intensity
- Intensity Category:Radiometry Category:Physical quantity ja:放射強度

Radiometry

In telecommunications and physics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power. =External links=
- [http://www.optics.arizona.edu/Palmer/rpfaq/rpfaq.htm Radiometry and photometry FAQ] Professor Jim Palmer's Radiometry FAQ page (University of Arizona). Category:Optics

Power (physics)

In physics, power (symbol: P) is the amount of work done per unit of time. This can be modeled as an energy flow, equivalent to the rate of change of the energy in a system, or the time rate of doing work. Power is defined by :

P=\frac=\frac

where :P is power :E is energy :W is work :t is time.

Units

The units of power are units of energy divided by time. The SI unit of power is the watt, which is equal to one joule per second. The power consumption of a human is on average roughly 100 watts, ranging from 85 W during sleep to 800 W or more while playing a strenuous sport. Professional cyclists have been measured at 2000 W output for short periods of time. Non-SI units of power include horsepower (HP), Pferdestärke (PS), cheval vapeur (CV) and foot-pounds per minute. One unit of horsepower is equivalent to 33,000 foot-pounds per minute, or the power required to lift 550 pounds one foot in one second, and is equivalent to about 746 watts. Other units include dBm, a logarithmic measure with 1 milliwatt as reference; and kilocalories per hour (often referred to as Calories per hour).

Mechanical power

In mechanics, the work done on an object is related to the forces acting on it by :

W = \int \mathbf \cdot \mathrm\mathbf

where :F is force :s is the displacement of the object. Therefore :

P = \int \mathbf \cdot \mathrm\mathbf

where :v is velocity

Electrical power

Main article: Electric power

Instantaneous electrical power

The instantaneous electrical power P delivered to a component is given by :

P(t) = I(t) \cdot V(t) \,\!

where :P(t) is the instantaneous power, measured in watts :V(t) is the potential difference (or voltage drop) across the component, measured in volts :I(t) is the current flowing through it, measured in amperes If the component is a resistor, then: :

P=I^2 \cdot R \,\!

or :

P=\frac

where :R is the resistance, measured in ohms

Average electrical power for sinusoidal voltages

The average power consumed by a two-terminal electrical device is a function of the root mean square values of the sinusoidal voltage across the terminals and the sinusoidal current passing through the device. That is, :

P=I \cdot V \cdot \cos\phi \,\!

where :P is the average power, measured in watts :I is the root mean square value of the sinusoidal alternating current (AC), measured in amperes :V is the root mean square value of the sinusoidal alternating voltage, measured in volts :φ is the phase angle between the voltage and the current sine functions. The amplitudes of sinusoidal voltages and currents, such as those used almost universally in mains electrical supplies, are normally specified in terms of root mean square values. This makes the above calculation a simple matter of multiplying the two stated numbers together. This figure can also be called the effective power, as compared to the larger apparent power which is expressed in volt-amperes reactive (VAR) and does not include the cos φ term due to the current and voltage being out of phase. For simple domestic appliances or a purely resistive network, the cos φ term (called the power factor) can often be assumed to be unity, and can therefore be omitted from the equation. In this case, the effective and apparent power are assumed to be equal.

Average electrical power for AC

P =  \int_^ i(t) v(t)\, dt

Where v(t) and i(t) are, respectively, the instantaneous voltage and current as functions of time.

Electrical power transfer

The efficient transfer of electrical power is governed by the maximum power theorem, which states that for the transfer of maximum power from a source with a fixed internal resistance to a load, the resistance of the load must be equal to that of the source.

Peak power and duty cycle

internal resistance In the case of a periodic signal

s(t)

of period

T

, like a train of identical pulses, the instantaneous power

p(t) = |s(t)|^2

is also a periodic funtion of period

T

. The peak power is simply defined by: :

P_0 = \max p(t)

The peak power is not always readily measurable, however, and the measurement of the average power

P_

is more commonly performed by an instrument. If one defines the energy per pulse as: :

\epsilon_ = \int_^p(t) dt

then the average power is: :

P_ = \frac \int_^p(t) dt = \frac

One may define the pulse length

\tau

such that

P_0\tau = \epsilon_

so that the ratios :

\frac = \frac

are equal. These ratios are called the duty cycle of the pulse train.

Power in optics

In optics, the term power sometimes refers to the average rate of energy transport by electromagnetic radiation. The term "power" is also, however, used to express the ability of a lens or other optical device to focus light. It is measured in dioptres (inverse metres), and is equal to one over the focal length of the optical device.

External links

- [http://www.unitconversion.org/unit_converter/power.html Online Power Converter] - convert between various units of power, such as watt, horsepower, Btu/hour, calorie/hour, volt ampere, joule/hour, and so on
- [http://www.unitconversion.org/unit_converter/power-v.html Interactive Power Conversion Table] - convert selected unit to all other units of power
- [http://www.ibiblio.org/obp/electricCircuits/AC/AC_11.html Power in resistive and reactive AC circuits]
- [http://calc.skyrocket.de/en/ Online Unit Converter - Conversion of many different units] Category:Radiometry Category:Physical quantity Category:Introductory physics ms:Kuasa (fizik) ja:仕事率

Solid angle

The solid angle subtended by a surface at a point is defined to be the surface area of the projection of that surface onto a unit sphere centered at that point. Solid angles so defined are called steradians and are usually denoted by

\Omega

(Omega). Thus the solid angle of a sphere measured at its center is 4π steradians, and the solid angle subtended at the center of a cube by one of its sides is one-sixth of this, that is, 2π/3 steradians. A solid angle is the three dimensional analogue of the ordinary angle, and the steradian is similarly analogous to the radian. The steradian (symbol sr) is the SI unit of solid angle. Solid angles can also be measured in degrees². One way to determine the solid angle that a surface subtends at a point is to imagine a sphere centered at the point. Now, compute the fractional area of the surface relative to the sphere by dividing the surface area of the part of the sphere that is contained within the outline of the projection of the surface onto the sphere by the total area of the sphere. #To obtain the solid angle in steradians or radians squared, multiply the fractional area by 4π. #To obtain the solid angle in degrees squared, multiply the fractional area by 4 x 180²/π which is equal to 129600/π. By analogy with the two dimensional case —
- To get an angle, imagine two lines passing through the center of a unit circle. The length of the arc between the lines on the unit circle is the angle, in radians.
- To get a solid angle, imagine three or more planes passing through the center of a unit sphere. The area of the surface between the planes on the unit sphere is the solid angle, in steradians. The angle between two planes is termed dihedral, between three trihedral, between any number more than three polyhedral. A spherical angle is a particular dihedral angle; it is the angle between two intersecting arcs on a sphere, and is measured by the angle between the planes containing the arcs and the centre of the sphere.

Practical applications

- defining luminosity
- calculating spherical excess E of a spherical triangle
- the calculation of potentials by using the Boundary Element Method (BEM)

Solid angles for common objects

- An efficient algorithm for calculating the solid angle Ω subtended by a triangle with vertices R₁, R₂ and R₃, as seen from the origin has been given by Oosterom and Strackee (IEEE Trans. Biom. Eng., Vol BME-30, No 2, 1983):

\tan \left( \frac \Omega \right) 
= 
\frac

, where: : [R₁R₂R₃] denotes the determinant of the matrix that results when writing the vectors together in a row, e.g. M_ij=R_j(i); : R_i denotes the distance of point i from the origin and R_i is the vector representation of point i; : R_i·R_j denotes the scalar product.
- The solid angle of a hemisphere is 2
- π.
- The solid angle of a cone with apex angle a is 2
- π
- (1 - cos[a/2]).
- The solid angle of a four-sided right regular pyramid with apex angle a (measured to the faces of the pyramid) is 4
- arccos[-(sin[a/2])^2] - 2
- π.
- The sun and moon are both seen from Earth at a fractional area of 0.001% of the celestial hemisphere. Category:Angle Category:Geometry ja:立体角

Watt

:For other uses, see: Watt (disambig) The watt (symbol: W) is the SI derived unit of power.
Definition
One watt is one joule of energy per second. : 1 W = 1 J/s = 1 newton meter per second
Origin
The watt is named after James Watt for his contributions to the development of the steam engine, and was adopted by the Second Congress of the British Association for the Advancement of Science in 1889 and by the 11th Conférence Générale des Poids et Mesures in 1960.
SI multiples

Conversions

- 1 watt ≈ 3.41214163 BTU/h
- 1 horsepower ≈ 745.700 W
- 1 horsepower (electrical British) = 746 W
- 1 horsepower (electrical European) = 736 W
- 1 horsepower ("metric") = 735.498 75 W
Derived and qualified units for power distribution
A watt is a unit of power or the amount of energy per unit time.
Kilowatt-hour, MWd
When paired with a unit of time the term watt is used for expressing energy consumption. For example, a kilowatt hour, is the amount of energy expended by a one kilowatt device over the course of one hour; it equals 3.6 megajoules (1 hour = 3600 seconds). A megawatt day (MWd or MW·d) is equal to 86.4 GJ (1 day = 86400 seconds). These units are often used in the context of power plants and home energy bills. For the use of watts as a measurement of transmitter power in radio, see effective radiated power and nominal power.
MWe, MWt
Watt electrical (abbreviation: We) is a term that refers to power produced as electricity. SI prefixes can be used, for example megawatt electrical (MWe) and gigawatt electrical (GWe). Watt thermal (abbreviation: Wt). This is a term that refers to thermal power produced. SI prefixes can be used, for example megawatt thermal (MWt) and gigawatt thermal (GWt). For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which creates steam to drive a turbine to generate electricity. See nuclear proliferation for discussion of a reactor that generates 200 MWt (50 MWe), and another reactor that generates 800 MWt (200 MWe).
See also

- SI
- Kilowatt hour (kW·h)
- Watt balance
- Conversion of units
- Orders of magnitude (power)
- James Watt
- RMS
- Back to the Future
External links

- Nelson, Robert A., "[http://www.aticourses.com/international_system_units.htm The International System of Units] Its History and Use in Science and Industry". Via Satellite, February 2000. Category:SI derived units Category:Units of power ko:와트 ja:ワット simple:Watt

Steradian

The steradian (symbol: sr) is the SI unit of solid angle. It is the 3-dimensional equivalent of the 2-dimensional radian. The name is partly derived from the Greek stereos for solid. The steradian is dimensionless, since 1 sr = m²·m^–2 = 1. It is useful, however, to distinguish between dimensionless quantities of different nature, so in practice the symbol "sr" is used where appropriate, rather than the derived unit "1" or no unit at all. As an example, radiant intensity can be measured in watts per steradian (W·sr^-1). The steradian is defined as "the solid angle subtended at the center of a sphere of radius r by a portion of the surface of the sphere having an area r²." Since the surface area of this sphere is 4πr², then the definition implies that a sphere measures 4π steradians. By the same argument, the maximum solid angle that can be subtended at any point is 4π sr. A steradian can also be called a squared radian. A steradian is also equal to the spherical area of a polygon having an angle excess of 1 radian, to 1/4π of a complete sphere, or to (180/π)² or 3282.80635 square degrees. The steradian was formerly an SI supplementary unit, but this category was abolished from the SI in 1995.

SI multiples

Category:Natural units Category:SI derived units Category:Units of measure ko:스테라디안 ja:ステラジアン

Irradiance

Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface. The other two terms are used interchangably for radiation emerging from a surface. The SI units for all of these quantities are watts per square metre (W/m²). These quantities are sometimes called intensity, but this usage leads to confusion with radiant intensity, which has different units. All of these quantities characterize the total amount of radiation present, at all frequencies. It is also common to consider each frequency in the spectrum separately. When this is done for radiation incident on a surface, it is called spectral irradiance, and has SI units W/m³, or commonly W·m^-2·nm^-1. If a point source radiates light uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance from the object squared, since the total power is constant and it is spread over an area that increases with the square of the distance from the source. =See also=
- Illuminance Category:Radiometry Category:Physical quantity ja:放射照度

Radiant exitance

Intensity

:For other senses of this word, see intensity (disambiguation). In physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area (i.e. watt/m²). It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently with waves (i.e. sound or light). In physics, the word "intensity" is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless as the parameters of those variables do not match. If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity drops off in proportion to distance from the object squared. This is due to physics and geometry. Physically, conservation of energy applies. The consequence of this is that the net power coming from the source must be constant, thus: :

P = \int I\, dA

where P is the net power radiated, I is the intensity as a function of position, and dA is a differential element of a closed surface that contains the source. That P is a constant. If the source is radiating uniformly, i.e. the same in all directions, and we take A to be a sphere centered on the source (so that I will be constant on its surface), the equation becomes: :

P = |I| \cdot 4 \pi r^2 \,

where I is the intensity at the surface of the sphere, and r is the radius of the sphere. (

4 \pi r^2

is the expression for the surface area of a sphere). Solving for I, we get: :

|I| = \frac

Anything that can carry energy can have an intensity associated with it. If the medium is damped (i.e. both sound and light in air slowly lose energy), then the intensity drops off more quickly than the above equation suggests.

Photometry and radiometry

In photometry and radiometry, intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion in optics, where "intensity" can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term. Radiance is also sometimes called intensity, especially by astronomers and astrophysicists.

Category:Radiometry

In telecommunications and physics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power. Category:measurement Category:optics Category:telecommunications

AP

Aliança Popular (Alianza Popular), és uns dels partits d'ideologia conservadora que va sorgir durant el periode de la transició democràtica a Espanya, dirigit per Manuel Fraga Iribarne, i que incorporava molts antics partidaris del règim franquista (com ell mateix, ex-ministre de Interior). Durant els anys 80, es va incorporar el PDP, amb el nom de coalició popular, i finalment es va refundar en l'actual Partit Popular.

Vegeu

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