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Catégorie:Comics

Catégorie:Comics

Comics l'article principal de cette Catégorie: Comic

Catégorie:Genre de bande dessinée

Catégorie:Bande dessinée Catégorie:Genre et forme littéraire

Comic

Comics est le terme généralement utilisé aux États-Unis pour désigner les bandes dessinées. On peut classer les comics en deux catégories:
- les comic strips, composés de seulement quelques cases racontant le plus souvent une courte histoire humoristique, parfois une aventure, sous forme de feuilleton ; les comic strips sont publiés dans la presse (quotidiens, hebdomadaires…).
- les comic books, composés de plusieurs dizaines de pages racontant une histoire développée. Même si tous les genres sont représentés (western, science-fiction, polar, biographie…), les plus connus sont ceux mettant en scène des super-héros. Ils sont généralement publiés en fascicules généralements mensuels, mais font parfois l'objet de romans graphiques. Les notions d'univers partagé et de continuité sont importantes pour les comics des grands éditeurs (DC Comics et Marvel Comics notamment, les deux éditeurs qui dominent le marché) dont les personnages partagent un même univers et ce depuis plusieurs dizaines d'années, ce qui pose des problèmes de cohérence et d'évolution des personnages

Exemples de Comic Strips

- Yellow Kid (Outcault)
- Little Nemo in Slumberland (Winsor McCay)
- Flash Gordon (Alex Raymond)
- Terry & the Pirates (Milton Caniff)
- Male Call (Milton Caniff)
- Calvin et Hobbes (Bill Watterson)
- Dilbert (Scott Adams)
- Donald Duck (Al Taliaferro) dans les années 1930 et 1940
- Garfield (Jim Davis)
- Peanuts (avec le chien Snoopy) (Charles M. Schulz)

Exemples de Comic Books

Il existe plusieurs sortes de comics : Les Funny Animals :
- La Jeunesse de Picsou (Don Rosa)
- Les aventures de Picsou (Carl Barks)
- Les aventures de Donald (Carl Barks) Les Super-héros :
- Superman (créé par Jerry Siegel / Joe Shuster)
- Batman (créé par Bob Kane / Bill Finger)
- Wonder Woman (créé par William Moulton Marston)
- Green Lantern (trois personnages principaux…)
- Flash (trois personnages principaux…)
- Les Quatre Fantastiques (Fantastic Four) (créés par Stan Lee / Jack Kirby)
- Hulk (créé par Stan Lee / Jack Kirby)
- Spider-Man (créé par Stan Lee / Steve Ditko)
- Les X-Men (créés par Stan Lee / Jack Kirby)
- Wolverine (créé par Len Wein / Herb Trimpe ; développé par Chris Claremont / Dave Cockrum)
- Les Vengeurs (Avengers) (créés par Stan Lee / Jack Kirby), association de super-héros, comme :
- Iron Man (créé par Stan Lee / Don Heck)
- Thor (créé par Stan Lee / Jack Kirby),
- Hulk (créé par Stan Lee / Jack Kirby),
- Ant-Man (créé par Stan Lee / Jack Kirby), puis
- Captain America (créé par Joe Simon / Jack Kirby)
- et d'autres personnages tels Hawkeye, Scarlet Witch, Quicksilver, Hercules, Vision… )
- Daredevil (créé par Stan Lee / Bill Everett)
- Silver Surfer (créé par Stan Lee / Jack Kirby)
- The Punisher (créé par Gerry Conway / Ross Andru) Le Fantastique :
- Fables (Bill Willingham / Ian Medina et Mark Buckingham)
- Grimjack (John Ostrander / Tim Truman)
- Hellblazer (Jamie Delano / John Ridgway ; personnage créé par Alan Moore)
- Hellboy (Mike Mignola)
- Sandman (Dessinateurs : Divers / Scénariste : Neil Gaiman) En ce qui concerne la dernière décennie de l'histoire du comic book, ce titre est incontournable. La Science-Fiction :
- American Flagg! (Howard Chaykin)
- Nexus (Mike Baron / Steve Rude)
- Ronin (Frank Miller) Le Policier :
- Sin City (Frank Miller)
- 100 Bullets (Brian Azzarello / Eduardo Risso)
- Powers (Brian Michael Bendis / Michael Avon Œming)

Liens externes:

- [http://www.comicsvf.com/ Comics VF]
- [http://www.france-comics.com/ France-Comics]
- [http://members.shaw.ca/tom.t/unh/dir.html Liste d'onomatopées utilisées dans les Comics américains]
- [http://comics.my-underworld.net/ L'encyclopédie française des Comics]
- [http://www.belfry.com/comics/ Referenceur de comic gratuit] (anglais) Catégorie:Comics

Debye approximation

In thermodynamics and solid state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat (heat capacity) in a solid. The Debye model treats the vibrations of the atomic lattice (heat) as phonons in a box, in contrast to Einstein model, which treats the solid as many individual, non-interacting quantum harmonic oscillators. This model correctly predicts the low temperature dependence of the heat capacity, which is proportional to

T^3

. Just like the Einstein model, it also recovers the Dulong-Petit law at high temperatures. But due to simplifying assumptions, its accuracy suffers at intermediate temperatures.

Derivation

The Debye model is a solid-state equivalent of Planck's law of black body radiation, where one treats electromagnetic radiation as a gas of photons in a box. The Debye model treats atomic vibrations as phonons in a box (the box being the solid). Most of the calculation steps are identical. Consider a cube of side

L

. From the particle in a box article, the resonating modes of the sonic disturbances inside the box (considering for now only those aligned with one axis) have wavelengths given by :

\lambda_n =

where

n

is an integer. The energy of a phonon is :

E_n\ =h\nu_n

where

h

is Planck's constant and

\nu_n

is the frequency of the phonon. We make the approximation that the frequency is inversely proportional to the wavelength, giving: :

E_n=h\nu_n==

in which

c_s

is the speed of sound inside the solid. In three dimensions we will use: :

E_n^2=E_^2+E_^2+E_^2=\left(\right)^2\left(n_x^2+n_y^2+n_z^2\right)

The approximation that the frequency is inversely proportional to the wavelength (giving a constant speed of sound) is good for low-energy phonons but not for high-energy phonons. (See the article on phonons.) This is one of the limitations of the Debye model. Let's now compute the total energy in the box :

U = \sum_n E_n\,\bar(E_n)

where

\bar(E_n)

is the number of phonons in the box with energy

E_n

. In other words, the total energy is equal to the sum of energy multiplied by the number of phonons with that energy (in one dimension). In 3 dimensions we have: :

U = \sum_\sum_\sum_E_n\,\bar(E_n)

Now, this is where Debye model and Planck's law of black body radiation differ. Unlike electromagnetic radiation in a box, there is a finite number of phonon energy states because a phonon cannot have infinite frequency. Its frequency is bound by the medium of its propagation -- the atomic lattice of the solid. Consider an illustration of a transverse phonon below. :::::400px It is reasonable to assume that the minimum wavelength of a phonon is twice the atom separation, as shown in the lower figure. There are

N

atoms in a solid. Our solid is a cube, which means there are

\sqrt[3]

atoms per side. Atom separation is then given by

L/\sqrt[3]

, and the minimum wavelength is :

\lambda_\min =

making the maximum mode number

n

(infinite for photons) :

n_\max = \sqrt[3]

This is the upper limit of the triple energy sum :

U = \sum_^\sum_^\sum_^E_n\,\bar(E_n)

For slowly-varying, well-behaved functions, a sum can be replaced with an integral (a.k.a Thomas-Fermi approximation) :

U \approx\int_0^\int_0^\int_0^ E(n)\,\bar\left(E(n)\right)\,dn_x\, dn_y\, dn_z

So far, there has been no mention of

\bar(E)

, the number of phonons with energy

E

. Phonons obey Bose-Einstein statistics. Their distribution is given by the famous Bose-Einstein formula :

\langle N\rangle_ =

Because a phonon has three possible polarization states (one longitudinal and two transverse) which do not affect its energy, the formula above must be multiplied by 3 :

\bar(E) =

Substituting this into the energy integral yields :

U  = \int_0^\int_0^\int_0^ E(n)\,\,dn_x\, dn_y\, dn_z

The ease with which these integrals are evaluated for photons is due to the fact that light's frequency, at least semi-classically, is unbound. As the figure above illustrates, this is not true for phonons. In order to approximate this triple integral, Debye used spherical coordinates :

\ (n_x,n_y,n_z)=(n\cos \theta \cos \phi,n\cos \theta \sin \phi,n\sin \theta )

and boldly approximated the cube by an eighth of a sphere :

U \approx\int_0^\int_0^\int_0^R E(n)\,n^2 \sin\theta\, dn\, d\theta\, d\phi

where

R

is the radius of this sphere, which is found by conserving the number of particles in the cube and in the eighth of a sphere. The volume of the cube is

N

unit-cell volumes, :

N = \pi R^3

so we get: :

R = \sqrt[3]

The substitution of integration over a sphere for the correct integral introduces another source of inaccuracy into the model. The energy integral becomes :

U = \int_0^R \, \,dn

Changing the integration variable to

x =

, :

U =  kT \left(\right)^3\int_0^ \, dx

To simplify the look of this expression, define the Debye temperature

T_D

-- a shorthand for some constants and material-dependent variables.

: $T_D\equiv = \sqrt[3] = \sqrt[3]$

We then have the specific internal energy:

: $\frac = 9T \left(\right)^3\int_0^ \, dx = 3T D_3 \left(\right)$

where

D_3(x)

is the (third) Debye function. Differentiating with respect to

T

we get the dimensionless heat capacity:

: $\frac = 9 \left(\right)^3\int_0^ \, dx$

These formulae give the Debye model at all temperatures. The more elementary formulae given further down give the asymptotic behavior in the limit of low and high temperatures.

Debye's derivation

Actually, Debye derived his equation somewhat differently and more simply. Using the solid mechanics of a continuous medium, he found that the number of vibrational states with a frequency less than a particular value was asymptotic to :

n \sim  \nu^3 V F

in which

V

is the volume and

F

is a factor which he calculated from elasticity coefficients and density. Combining this with the expected energy of a harmonic oscillator at temperature T (already used by Einstein in his model) would give an energy of :

U = \int_0^\infty \,\, d\nu

if the vibrational frequencies continued to infinity. This form gives the

T^4

behavior which is correct at low temperatures. But Debye realized that there could not be more than

3N

vibrational states for N atoms. He made the assumption that in an atomic solid, the spectrum of frequencies of the vibrational states would continue to follow the above rule, up to a maximum frequency

\nu_m

chosen so that the total number of states is

3N

: :

3N =  \nu_m^3 V F

Debye knew that this assumption was not really correct (the higher frequencies are more closely spaced than assumed), but it guarantees the proper behavior at high temperature (the Dulong-Petit law). The energy is then given by: :

U = \int_0^ \,\, d\nu

= V F kT (kT/h)^3 \int_0^ \,\, dx

::where

T_D

h\nu_m/k

. ::

= 9 N k T (T/T_D)^3 \int_0^ \,\, dx

= 3 N k T D_3(T_D/T)

where

D_3()

is the function later given the name of third-order Debye function.

Low temperature limit

The temperature of a Debye solid is said to be low if

T \ll T_D

, leading to :

\frac \sim 9 \left(\right)^3\int_0^ \, dx

This definite integral can be evaluated exactly:

: $\frac \sim \left(\right)^3$

In the low temperature limit, the limitations of the Debye model mentioned above do not apply, and it gives a correct relationship between (phononic) heat capacity, temperature, the elastic coefficients, and the volume per atom (the latter quantities being contained in the Debye temperature).

High temperature limit

The temperature of a Debye solid is said to be high if

T >> T_D

e^x - 1\approx  x

|x|<<1

, leads to :

\frac \sim 9 \left(\right)^3\int_0^ \, dx

: $\frac \sim 3$

This is the Dulong-Petit law, and is fairly accurate although it does not take into account anharmonicity, which causes the heat capacity to rise further. The total heat capacity of the solid, if it is a conductor or semiconductor, may also contain a non-negligible contribution from the electrons.

Debye versus Einstein

semiconductor So how closely do the Debye and Einstein models correspond to experiment? -- Surprisingly close, but Debye is correct at low temperatures whereas Einstein is not. How different are the models? To answer that question one would naturally plot the two on the same set of axes... except one can't. Both the Einstein model and the Debye model provide a functional form for the heat capacity. They are models, and no model is without a scale. A scale relates the model to its real-world counterpart. One can see that the scale of the Einstein model, which is given by :

C_V = 3Nk\left(\right)^2

\epsilon/k

. And the scale of the Debye model is

T_D

, the Debye temperature. Both are usually found by fitting the models to the experimental data. (The Debye temperature can theoretically be calculated from the speed of sound and crystal dimensions.) Because the two methods approach the problem from different directions and different geometries, Einstein and Debye scales are not the same, that is to say :

\ne T_D

which means that plotting them on the same set of axes makes no sense. They are two models of the same thing, but of different scales. If one defines Einstein temperature as :

T_E \equiv

then one can say :

T_E \ne T_D

and, to relate the two, we must seek the ratio :

= ?

The Einstein solid is composed of single-frequency quantum harmonic oscillators,

\epsilon = \hbar\omega = h\nu

. That frequency, if it indeed existed, would be related to the speed of sound in the solid... even though there is no sound in Einstein solid. If one imagines the propagation of sound as a sequence of atoms hitting one another, then it becomes obvious that the frequency of oscillation must correspond to the minimum wavelength sustainable by the atomic lattice,

\lambda_

. :

\nu =  =  = \sqrt[3]

which makes the Einstein temperature :

T_E =  =  = \sqrt[3]

and the sought ratio is therefore :

= \sqrt[3]

Now both models can be plotted on the same graph. Note that this ratio is the cube root of the ratio of the volume of one octant of a 3-dimensional sphere to the volume of the cube that contains it, which is just the correction factor used by Debye when approximating the energy integral above.

Debye temperature table

Even though the Debye model is not completely correct, it is quite accurate at low temperatures for any kind of solid, as long as one uses the correct Debye temperature (and as long as other contributions such as electronic heat capacity are negligible). The following table lists Debye temperatures for several substances:

References

- 'Zur Theorie der spezifischen Warmen', Annalen der Physik 39(4), p. 789 (1912)
- CRC Handbook of Chemistry and Physics, 56th Edition (1975-1976) Category:Condensed matter physics Category:Thermodynamics

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Catégorie:Comics

Catégorie:Genre de bande dessinée

Comic

Exemples de Comic Strips

Exemples de Comic Books

Principaux éditeurs de comics aux États-Unis (en activité)

Principaux éditeurs de comics aux États-Unis (ne sont plus en activité)

Principaux éditeurs de comics en France (en activité)

Principaux éditeurs de comics en France (ne sont plus en activité)

Autres éditeurs de comics (en activité)

Voir aussi :

Liens externes:

Debye approximation

Derivation

Debye's derivation

Low temperature limit

High temperature limit

Debye versus Einstein

Debye temperature table

See also

References

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