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The **Drude model** of electrical conduction was proposed in 1900^{ [1] }^{ [2] } by Paul Drude to explain the transport properties of electrons in materials (especially metals). The model, which is an application of kinetic theory, assumes that the microscopic behavior of electrons in a solid may be treated classically and looks much like a pinball machine, with a sea of constantly jittering electrons bouncing and re-bouncing off heavier, relatively immobile positive ions.

The **electron** is a subatomic particle, symbol ^{}e^{−}_{} or ^{}β^{−}_{}, whose electric charge is negative one elementary charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no known components or substructure. The electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the electron include an intrinsic angular momentum (spin) of a half-integer value, expressed in units of the reduced Planck constant, *ħ*. Being fermions, no two electrons can occupy the same quantum state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of both particles and waves: they can collide with other particles and can be diffracted like light. The wave properties of electrons are easier to observe with experiments than those of other particles like neutrons and protons because electrons have a lower mass and hence a longer de Broglie wavelength for a given energy.

The **kinetic theory of gases** describes a gas as a large number of submicroscopic particles, all of which are in constant, rapid, random motion. The randomness arises from the particles' many collisions with each other and with the walls of the container.

**Pinball** is a type of arcade game, in which points are scored by a player manipulating one or more metallic balls on a play field inside a glass-covered cabinet called a **pinball machine**. The primary objective of the game is to score as many points as possible. Many modern pinball machines include a "storyline" where the player must complete certain objectives in a certain fashion to complete the story, usually earning high scores for different methods of completing the game. Different numbers of points are earned when the ball strikes different targets on the play field. A drain is situated at the bottom of the play field, partially protected by player-controlled paddles called flippers. A game ends after all the balls fall into the drain a certain number of times. Secondary objectives are to maximize the time spent playing, and to earn bonus credits by achieving a high enough score or through other means.

- Assumptions
- Explanations
- DC field
- Time-varying analysis
- AC field
- Drude response in real materials
- Accuracy of the model
- See also
- Notes
- References
- General
- External links

The two most significant results of the Drude model are an electronic equation of motion,

and a linear relationship between current density **J** and electric field **E**,

In electromagnetism, **current density** is the amount of charge per unit time that flows through a unit area of a chosen cross section. The **current density vector** is defined as a vector whose magnitude is the electric current per cross-sectional area at a given point in space, its direction being that of the motion of the charges at this point. In SI base units, the electric current density is measured in amperes per square metre.

Here t is the time, ⟨p⟩ is the average momentum per electron and q, n, m, and τ are respectively the electron charge, number density, mass, and mean free time between ionic collisions (that is, the mean time an electron has traveled since the last collision). The latter expression is particularly important because it explains in semi-quantitative terms why Ohm's law, one of the most ubiquitous relationships in all of electromagnetism, should be true.^{ [note 1] }^{ [3] }^{ [4] }

**Ohm's law** states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

The model was extended in 1905 by Hendrik Antoon Lorentz (and hence is also known as the **Drude–Lorentz model**)^{[ citation needed ]} and is a classical model. Later it was supplemented with the results of quantum theory in 1933 by Arnold Sommerfeld and Hans Bethe, leading to the Drude–Sommerfeld model.

**Classical physics** refers to theories of physics that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major paradigm shift, then the previous theories, or new theories based on the older paradigm, will often be referred to as belonging to the realm of "classical physics".

**Arnold Johannes Wilhelm Sommerfeld**, was a German theoretical physicist who pioneered developments in atomic and quantum physics, and also educated and mentored a large number of students for the new era of theoretical physics. He served as doctoral supervisor for many Nobel Prize winners in physics and chemistry.

The Drude model considers the metal to be formed of a mass of positively charged ions from which a number of "free electrons" were detached. These may be thought to have become delocalized when the valence levels of the atom came in contact with the potential of the other atoms.^{ [note 2] }

The Drude model neglects any long-range interaction between the electron and the ions or between the electrons. The only possible interaction of a free electron with its environment is via instantaneous collisions. The average time between subsequent collisions of such an electron is τ, and the nature of the collision partner of the electron does not matter for the calculations and conclusions of the Drude model.^{ [note 2] }

After a collision event, the velocity (and direction) of the electron only depends on the local temperature distribution and is completely independent of the velocity of the electron before the collision event.^{ [note 2] }

The simplest analysis of the Drude model assumes that electric field **E** is both uniform and constant, and that the thermal velocity of electrons is sufficiently high such that they accumulate only an infinitesimal amount of momentum *d***p** between collisions, which occur on average every τ seconds.^{ [note 1] }

Then an electron isolated at time t will on average have been travelling for time τ since its last collision, and consequently will have accumulated momentum

During its last collision, this electron will have been just as likely to have bounced forward as backward, so all prior contributions to the electron's momentum may be ignored, resulting in the expression

Substituting the relations

results in the formulation of Ohm's law mentioned above:

The dynamics may also be described by introducing an effective drag force. At time *t* = *t*_{0} + *dt* the average electron's momentum will be

because, on average, a fraction of 1 − *dt*/*τ* of the electrons will not have experienced another collision, and the ones that have will contribute to the total momentum to only a negligible order.^{ [note 3] }

With a bit of algebra and dropping terms of order *dt*^{2}, this results in the differential equation

where ⟨**p**⟩ denotes average momentum and q the charge of the electrons. This, which is an inhomogeneous differential equation, may be solved to obtain the general solution of

for *p*(*t*). The steady state solution, *d* ⟨**p**⟩/*dt* = 0, is then

As above, average momentum may be related to average velocity and this in turn may be related to current density,

and the material can be shown to satisfy Ohm's law with a DC-conductivity *σ*_{0}:

The Drude model can also predict the current as a response to a time-dependent electric field with an angular frequency ω. The complex conductivity is

Here it is assumed that:

In engineering, i is generally replaced by −i (or −j ) in all equations, which reflects the phase difference with respect to origin, rather than delay at the observation point traveling in time. The imaginary part indicates that the current lags behind the electrical field. This happens because the electrons need roughly a time τ to accelerate in response to a change in the electrical field. Here the Drude model is applied to electrons; it can be applied both to electrons and holes; i.e., positive charge carriers in semiconductors. The curves for *σ*(*ω*) are shown in the graph.

If a sinusoidally varying electric field with frequency is applied to the solid, the negatively charged electrons behave as a plasma that tends to move a distance *x* apart from the positively charged background. As a result, the sample is polarized and there will be an excess charge at the opposite surfaces of the sample.

The dielectric constant of the sample is expressed as

where is the electric displacement and is the polarization density.

The polarization density is written as

and the polarization density with *n* electron density is

After a little algebra the relation between polarization density and electric field can be expressed as

The frequency dependent dielectric function of the solid is

At a resonance frequency , called the **plasma frequency**, the dielectric function changes sign from negative to positive and real part of the dielectric function drops to zero.

The plasma frequency represents a plasma oscillation resonance or plasmon. The plasma frequency can be employed as a direct measure of the square root of the density of valence electrons in a solid. Observed values are in reasonable agreement with this theoretical prediction for a large number of materials.^{ [5] } Below the plasma frequency, the dielectric function is negative and the field cannot penetrate the sample. Light with angular frequency below the plasma frequency will be totally reflected. Above the plasma frequency the light waves can penetrate the sample.

The characteristic behavior of a Drude metal in the time or frequency domain, i.e. exponential relaxation with time constant τ or the frequency dependence for *σ*(*ω*) stated above, is called Drude response. In a conventional, simple, real metal (e.g. sodium, silver, or gold at room temperature) such behavior is not found experimentally, because the characteristic frequency *τ*^{−1} is in the infrared frequency range, where other features that are not considered in the Drude model (such as band structure) play an important role.^{ [6] } But for certain other materials with metallic properties, frequency-dependent conductivity was found that closely follows the simple Drude prediction for *σ*(*ω*). These are materials where the relaxation rate *τ*^{−1} is at much lower frequencies.^{ [6] } This is the case for certain doped semiconductor single crystals,^{ [7] } high-mobility two-dimensional electron gases,^{ [8] } and heavy-fermion metals.^{ [9] }

Historically, the Drude formula was first derived in a limited way, namely by assuming that the charge carriers form a classical ideal gas. Arnold Sommerfeld considered quantum theory and extended the theory to the free electron model, where the carriers follow Fermi–Dirac distribution. Amazingly, the conductivity predicted turns out to be the same as in the Drude model, as it does not depend on the form of the electronic speed distribution.

The Drude model provides a very good explanation of DC and AC conductivity in metals, the Hall effect, and the magnetoresistance ^{ [note 3] } in metals near room temperature. The model also explains partly the Wiedemann–Franz law of 1853. However, it greatly overestimates the electronic heat capacities of metals. In reality, metals and insulators have roughly the same heat capacity at room temperature. The model can also be applied to positive (hole) charge carriers, as demonstrated by the Hall effect.

One note of trivia surrounding the theory is that in his original paper, Drude made an error, estimating the Lorenz number of Wiedemann–Franz law to be in fact twice of what it classically should have been, thus making it seem in agreement with the experimental measurement. Another surprise is that the experimental value for the specific heat is about 100 times smaller than the classical prediction but this factor cancels out with the mean electronic speed that is actually about 100 times bigger than Drude's calculation.^{ [note 4] }

- 1 2 Ashcroft & Mermin 1976 , pp. 6–7
- 1 2 3 Ashcroft & Mermin 1976 , pp. 2-6
- 1 2 Ashcroft & Mermin 1976 , p. 11
- ↑ Ashcroft & Mermin 1976 , p. 23

**Acoustic theory** is a scientific field that relates to the description of sound waves. It derives from fluid dynamics. See acoustics for the engineering approach.

In mechanics, the **virial theorem** provides a general equation that relates the average over time of the total kinetic energy, ⟨*T*⟩, of a stable system consisting of N particles, bound by potential forces, with that of the total potential energy, ⟨*V*_{TOT}⟩, where angle brackets represent the average over time of the enclosed quantity. Mathematically, the theorem states

In electromagnetism, **absolute permittivity**, often simply called **permittivity**, usually denoted by the Greek letter *ε* (epsilon), is the measure of capacitance that is encountered when forming an electric field in a particular medium. More specifically, permittivity describes the amount of charge needed to generate one unit of electric flux in a given medium. A charge will yield more electric flux in a medium with low permittivity than in a medium with high permittivity. Permittivity is the measure of a material's ability to store an electric field in the polarization of the medium.

In physics, **Langevin equation** is a stochastic differential equation describing the time evolution of a subset of the degrees of freedom. These degrees of freedom typically are collective (macroscopic) variables changing only slowly in comparison to the other (microscopic) variables of the system. The fast (microscopic) variables are responsible for the stochastic nature of the Langevin equation.

In classical statistical mechanics, the **equipartition theorem** relates the temperature of a system to its average energies. The equipartition theorem is also known as the **law of equipartition**, **equipartition of energy**, or simply **equipartition**. The original idea of equipartition was that, in thermal equilibrium, energy is shared equally among all of its various forms; for example, the average kinetic energy per degree of freedom in translational motion of a molecule should equal that in rotational motion.

The **adiabatic theorem** is a concept in quantum mechanics. Its original form, due to Max Born and Vladimir Fock (1928), was stated as follows:

In mathematics, the **Hamilton–Jacobi equation** (**HJE**) is a necessary condition describing extremal geometry in generalizations of problems from the calculus of variations, and is a special case of the Hamilton–Jacobi–Bellman equation. It is named for William Rowan Hamilton and Carl Gustav Jacob Jacobi.

The **Lawson criterion** is a figure of merit used in nuclear fusion research. It compares the rate of energy being generated by fusion reactions within the fusion fuel to the rate of energy losses to the environment. When the rate of production is higher than the rate of loss, and enough of that energy is captured by the system, the system is said to be **ignited**.

In solid-state physics, the **free electron model** is a simple model for the behaviour of charge carriers in a metallic solid. It was developed in 1927, principally by Arnold Sommerfeld, who combined the classical Drude model with quantum mechanical Fermi–Dirac statistics and hence it is also known as the **Drude–Sommerfeld model**.

In quantum optics, correlation functions are used to characterize the statistical and coherence properties of an electromagnetic field. The **degree of coherence** is the normalized correlation of electric fields. In its simplest form, termed , it is useful for quantifying the coherence between two electric fields, as measured in a Michelson or other linear optical interferometer. The correlation between pairs of fields, , typically is used to find the statistical character of intensity fluctuations. First order correlation is actually the amplitude-amplitude correlation and the second order correlation is the intensity-intensity correlation. It is also used to differentiate between states of light that require a quantum mechanical description and those for which classical fields are sufficient. Analogous considerations apply to any Bose field in subatomic physics, in particular to mesons.

In theory of vibrations, **Duhamel's integral** is a way of calculating the response of linear systems and structures to arbitrary time-varying external perturbation.

**Kinetic inductance** is the manifestation of the inertial mass of mobile charge carriers in alternating electric fields as an equivalent series inductance. Kinetic inductance is observed in high carrier mobility conductors and at very high frequencies.

The **Jaynes–Cummings model** is a theoretical model in quantum optics. It describes the system of a two-level atom interacting with a quantized mode of an optical cavity, with or without the presence of light. It was originally developed to study the interaction of atoms with the quantized electromagnetic field in order to investigate the phenomena of spontaneous emission and absorption of photons in a cavity.

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In spectroscopy, the **Autler–Townes effect**, named after American physicists Stanley Autler and Charles Townes, is a type of the dynamical Stark effect, corresponding to the case when an oscillating electric field is tuned in resonance to the transition frequency of a given spectral line, and resulting in a change of the shape of the absorption/emission spectra of that spectral line.

In many-body theory, the term **Green's function** is sometimes used interchangeably with correlation function, but refers specifically to correlators of field operators or creation and annihilation operators.

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**Magnetic resonance** is a phenomenon in quantum mechanics that affects a magnetic dipole when placed in a uniform static magnetic field. Its energy is split into a finite number of energy levels, depending on the value of quantum number of angular momentum. This is similar to energy quantization for atoms, say ^{}e^{−}_{} in H atom; in this case the atom, in interaction to an external electric field, transitions between different energy levels by absorbing or emitting photons. Similarly if a magnetic dipole is perturbed with electromagnetic field of proper frequency( ), it can transit between its energy eigenstates, but as the separation between energy eigenvalues is small, the frequency of the photon will be the microwave or radio frequency range. If the dipole is tickled with a field of another frequency, it is unlikely to transition. This phenomenon is similar to what occurs when a system is acted on by a periodic force of frequency equal to its natural frequency.

- ↑ Drude, Paul (1900). "Zur Elektronentheorie der Metalle".
*Annalen der Physik*.**306**(3): 566–613. Bibcode:1900AnP...306..566D. doi:10.1002/andp.19003060312.^{[ dead link ]} - ↑ Drude, Paul (1900). "Zur Elektronentheorie der Metalle; II. Teil. Galvanomagnetische und thermomagnetische Effecte".
*Annalen der Physik*.**308**(11): 369–402. Bibcode:1900AnP...308..369D. doi:10.1002/andp.19003081102.^{[ dead link ]} - ↑ Edward M. Purcell (1965).
*Electricity and Magnetism*. McGraw-Hill. pp. 117–122. ISBN 978-0-07-004908-6. - ↑ David J. Griffiths (1999).
*Introduction to Electrodynamics*. Prentice-Hall. p. 289. ISBN 978-0-13-805326-0. - ↑ C. Kittel (1953–1976).
*Introduction to Solid State Physics*. Wiley & Sons. ISBN 978-0-471-49024-1. - 1 2 M. Dressel; M. Scheffler (2006). "Verifying the Drude response".
*Ann. Phys.***15**(7–8): 535–544. Bibcode:2006AnP...518..535D. doi:10.1002/andp.200510198. - ↑ M. van Exter; D. Grischkowsky (1990). "Carrier dynamics of electrons and holes in moderately doped silicon".
*Phys. Rev. B*.**41**(17): 12140–12149. Bibcode:1990PhRvB..4112140V. doi:10.1103/PhysRevB.41.12140. hdl:11244/19898. - ↑ P. J. Burke; I. B. Spielman; J. P. Eisenstein; L. N. Pfeiffer; K. W. West (2000). "High frequency conductivity of the high-mobility two-dimensional electron gas".
*Appl. Phys. Lett.***76**(6): 745–747. Bibcode:2000ApPhL..76..745B. doi:10.1063/1.125881. - ↑ M. Scheffler; M. Dressel; M. Jourdan; H. Adrian (2005). "Extremely slow Drude relaxation of correlated electrons".
*Nature*.**438**(7071): 1135–1137. Bibcode:2005Natur.438.1135S. doi:10.1038/nature04232. PMID 16372004.

- Ashcroft, Neil; Mermin, N. David (1976).
*Solid State Physics*. New York: Holt, Rinehart and Winston. ISBN 978-0-03-083993-1.

- Heaney, Michael B (2003). "Electrical Conductivity and Resistivity". In Webster, John G. (ed.).
*Electrical Measurement, Signal Processing, and Displays*. CRC Press. ISBN 9780203009406.

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