if i had a cube of pure steel, 1cm x 1cm x 1cm, at 20 degrees celcius and a potential difference of 20 volts between two opposite faces, how would i work out its resistance to the flow of this charge? without knowing the current beforehand i mean, surely if you know al this you can work it out?

Originally posted by god-of-course
if i had a cube of pure steel, 1cm x 1cm x 1cm, at 20 degrees celcius and a potential difference of 20 volts between two opposite faces, how would i work out its resistance to the flow of this charge? without knowing the current beforehand i mean, surely if you know al this you can work it out?

use R=ρ/1 cm. you need to know the resistivity as well, of course. you can look this up in a table of physical constants and such.

where p is?

Originally posted by god-of-course
where p is?

resistivity.

so its just resitivity X length / cross-sectional area? what about heat? surely it has a measurable effect? perhaps not in conditions such as i described but at a high enough temperature im gonna need a formula that includes it?

Originally posted by god-of-course
so its just resitivity X length / cross-sectional area? what about heat? surely it has a measurable effect? perhaps not in conditions such as i described but at a high enough temperature im gonna need a formula that includes it?

well the resistivity might have temperature dependence. if you want to account for that, then:

ρ = ρ₀(1+α(T-T₀))

where ρ₀ is the resistivity at T₀, and ρ is the resistivity at the temperature that you want to find, T. α is the coefficient of termal resistances expansion or something like that.

Excellent. Cheers Lethe.

The effect of temperature on resistivity in metals is largely non-existant due to the fermi energy being in the middle of the conduction band, and core electrons being extremely tightly bound. However the resistivity of semi-conductors is extremely sensitive to temperature, given the nature of the band gap, and the abilty of valence electrons to overcome this band-gap to be promoted to the conduction band. These figure can be calculated using the fermi-dirac distribution, with the only required known properties being the fermi energy of the material, and the temperature. The rest is implicit in the distribution.

Look up a book called "Modern Physics" by Serway for a good introduction to this stuff.

Originally posted by ryans
The effect of temperature on resistivity in metals is largely non-existant due to the fermi energy being in the middle of the conduction band, and core electrons being extremely tightly bound.
the formula i gave is really a first order approximation to an exponential. do you suppose that the first order equation is usually used because the effect is so weak? after all, it wouldn t be much harder to use the exponential version.

However the resistivity of semi-conductors is extremely sensitive to temperature, given the nature of the band gap, and the abilty of valence electrons to overcome this band-gap to be promoted to the conduction band. These figure can be calculated using the fermi-dirac distribution, with the only required known properties being the fermi energy of the material, and the temperature. The rest is implicit in the distribution.

hmm... i don t know much about it, but i ll bet that the linear relationship between the rate of change of temperature to the temperature (leading to an exponential equation) would fail for the semiconductor, given the more complicated structure.

what do you think?

Originally posted by ryans

Look up a book called "Modern Physics" by Serway for a good introduction to this stuff.

don t have that book. and i know precious little solid state. so i ll leave the rest to you. in fact, the only reason i know what stuff i posted up there is because i have to teach it in some general physics course. i never learned this stuff.