Physics · Current Electricity · NEET
Free electrons do not move through empty space. They keep bumping into the metal ions, which are vibrating in the lattice. Each collision randomises the electron's motion and cancels the speed it gained from the field. This repeated stopping is what opposes current flow, and that opposition is resistivity. So resistivity comes from electron-ion collisions, not from the field.
Almost, but it is an average. Different electrons travel different times before hitting an ion. Relaxation time tau is the average of all these free-flight times over the huge number of electrons. That is why we call it the mean (average) time between successive collisions.
No. tau depends on how tightly packed and how fast the thermal motion of ions is, which depends on temperature, not on the applied field. A stronger field makes electrons drift faster, but the average time before a collision stays the same. This is exactly why current is proportional to voltage (Ohm's law).
When temperature rises, the metal ions vibrate more strongly and take up more room in the electron's path. Collisions happen more often, so the average free time tau gets smaller. Since rho = m/(n e^2 tau), a smaller tau gives a larger rho. That is why metal wires resist more when hot.
Relaxation time tau is a time (seconds) -- the average gap between collisions. Drift velocity v_d is a speed (m/s) -- the slow net speed electrons gain in the field. They are linked: v_d = (eE/m) tau. Bigger tau means electrons drift faster before being stopped.
Match Column-I with Column-II. Column-I: (A) Drift velocity, (B) Electrical resistivity, (C) Relaxation period, (D) Current density. Column-II: (P) m/(ne^2 rho), (Q) ne v_d, (R) (eE/m) tau, (S) E/J. Choose the correct match.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the average time a free electron moves freely between two successive collisions with the metal ions.
rho = m/(n e^2 tau), where m is electron mass, n is free-electron density, e is electron charge, and tau is relaxation time.
The second (s), because it is a time interval. Its typical value in metals is about 10^-14 s.
Smaller tau means electrons are stopped more often, so they drift slower and the material's resistivity rho becomes larger, making it a poorer conductor.
No. In the free-electron (Drude) model used for NEET, resistivity comes mainly from collisions of electrons with the vibrating metal ions, not with each other.
