Page 38 - Q & A STPM 2022 Physics

Physics Semester 2 STPM Chapter 14 Electric Current
(b) When a potential difference is applied across a conductor, free electron
esperience electric force and move with acceleration, a,
F eE
\ a = =
m m
let t = mean time interval between collisions of free electrons and the
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lattice atoms
mean initial velocity of electrons = 0
eE
Mean final velocity of electron, v = 0 + m t
J = nve
2
2 1
= n 1 eE t e = ne t 2 E
m
m
J = σE
2
\σ = ne t
m
(c) In copper wire, when the temperature increases the lattice ions vibrate
with bigger amplitude. The mean free path of the free electrons decreases
and the rate of collision between free electrons and ions increases.
ne t
2
Hence the mean time interval between the collisions decreases. σ = m ,
conductivity decreases as t decreases.
Silicon is a semiconductor. When the temperature increases, more holes Semester
and free electrons are produced. Holes and electrons are the charge
carriers in semiconductor. Thus the number of charge carriers increases. 2
The mean free time interval between collisions also decreases but the
effect of the number of charge carriers is greater than that of mean free
time interval.
ne τ
2
Hence, σ = , n increases, σ increases.
m
(d) At high temperature, the resistance of metal conductors increases. Hence
the resistivity of metal conductor increases as temperature rises.
At high temperature, the resistance of semiconductor decreases. So, the
resistivity of semiconductor decreases with increasing temperature.
At very low temperature of near-absolute zero, the resistivity of certain
materials becomes zero. Superconductors have zero resistivity at very low
temperature.

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014 STPM Q&A Physics T2.indd 237 17/02/2022 10:12 AM