Factors affecting electrical resistivity, Physics

TEMPERATURE:

When the conduction electrons are only scattered by thermal vibrations of thermal ions, then τ in the mobility expression refers to the mean time between scattering events by this process. The resulting conductivity and resistivity are denoted by σT and ρt, where the subscript T represents "thermal" vibration scattering". To find the temperature dependence of the mean free time τ, since this determines the drift mobility. An electron moving with a means speed u is the scattered when its path crosses the cross-sectional area S of a scattering centre. The scattering centre may be vibrating atom, impurity, vacancy, or some other crystal defect. Since τ is the mean time taken for one scattering process, the mean free path l of the electron between scattering process is uτ. If Ns is the concentration of scattering centres, then in the volume SI, there is one scattering centre, that is, Ns=1 thus the mean free path is given . The mass speed u of conduction electrons in a metal can be shown to only slightly temperature dependent. Because the atomic vibrations are random the atoms cover a cross-sectional area A= ∏r2 where a is the amplitude of the vibrations. If the electron's path crosses A= ∏r2 it gets scattered. Therefore the mean time between scattering events is inversely proportional to the area that scatters the electron, that is. As the temperature raises, the amplitudes of the atomic vibrations increases thus,

                                                        Τ=C/T

Where C is a temperature independent constant. Substituting the values we obtain

                                    ΡT= AT

Where A is the temperature independent constant. This shows that the resistivity of a pure metal wire increases linearly with the temperature, and that the resistivity is due simply to the scattering of conduction electrons by the thermal vibrations of the atoms. We term this conductivity lattice-scattering-limited conductivity. The change in resistance of a material per ohm per degree change in temperature is called the temperature coefficient of resistance of that material. The resistance of a conductor changes with temperature according to the law:

                                 Rt = R0 (1+αt)

Where Rt, and R0 are respectively the resistance of the conductor at t degrees and zero degree centigrade's and α, the temperature coefficient of resistance. Based on temperature effects, electrical materials can be classified into two groups:

Positive temperature coefficient of materials: It means that the resistance of some of the metals and alloys increases when their temperature is raised.

Negative temperature coefficient of materials: It means that the resistance of some of the materials, carbon and insulators decreases when their temperature is raised.

Posted Date: 7/21/2012 7:40:51 AM | Location : United States







Related Discussions:- Factors affecting electrical resistivity, Assignment Help, Ask Question on Factors affecting electrical resistivity, Get Answer, Expert's Help, Factors affecting electrical resistivity Discussions

Write discussion on Factors affecting electrical resistivity
Your posts are moderated
Related Questions
Q. Match the units to the physical amount that is measured in those units. Answer: The major purpose of this question is to familiarize you with the jargon "physical quant

In X-ray tube, when high speed electrons strike the target, they move into the target. They lose their kinetic energy and come to rest inside the solid. The electron because of fin

Briefly explain the equations describing under-damped, over damped and critically damped one-dimensional harmonic oscillator?

Define, with the help of diagram, the principle and working of an a.c. Generator. Write the expression for the e.m.f. generated in the coil in terms of its speed of rotation

I don''t understand how pressure energy arises

When reverse bias is enhanced the electric field at the junction also enhances. At some stage the electric field becomes so high that it breaks the covalent bonds building electron

A water skier lets go of the tow rope upon leaving the end oaf jump ramp at a speed of 14.0 m/s. The skier has a speed of 13.0m/s at the highest point of the jump. Ignoring air res

A monochromatic light of wavelength 6000Å is incident on a plane diffraction grating with grating element 6 X 10 -5 cm. Evaluate the maximum order of spectrum that will be observe

how would the results change if the angle of the third load in second experiment increased ? state the differences

Verify Gauss' theorem in electrostatics. Apply it to search the electric field strength at a point near a long straight conductor carrying electric charge.