Physics · Current Electricity · NEET
No. Free electrons already move very fast (about 10^5 m/s) in random directions due to heat. This random motion gives zero net displacement. Drift velocity is only the tiny extra average velocity (about 10^-4 m/s) added by the electric field along the wire. So drift velocity is a net average, not the true instantaneous speed.
The electrons themselves crawl slowly, but the electric field travels through the wire at nearly the speed of light. The field pushes on all free electrons everywhere in the wire almost at once, so every electron starts drifting together immediately. The bulb glows instantly even though a single electron may take hours to travel the full wire.
Free electrons are negative, so they drift opposite to the electric field E (opposite to the direction of conventional current). NCERT states: in an external field the free electrons drift against the direction of the field. Conventional current, however, points along E.
Drift velocity depends on the field E inside the wire through v_d = (eE/m)τ, where E = V/L. For a fixed voltage V, a longer wire means smaller E, so smaller drift velocity. But v_d does NOT depend on the wire's cross-sectional area directly in this relation; area affects current through v_d = I/(nAe).
Thermal velocity is the fast, random zig-zag motion of electrons due to temperature; its average vector is zero, so it carries no net current. Drift velocity is the slow, ordered motion added by the field; it is what actually produces current. Thermal speed is about 10^5 m/s while drift speed is about 10^-4 m/s.
A copper wire of radius 1 mm contains 10^22 free electrons per m^3. The drift velocity for a 10 A current is (e = 1.6 x 10^-19 C):
Match Column-I with Column-II: (A) Drift velocity (B) Electrical resistivity (C) Relaxation period (D) Current density with (P) m/(ne^2 ρ) (Q) ne·v_d (R) (eE/m)τ (S) E/J
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Metre per second (m/s), the same unit as any velocity. Its typical value in a metal is very small, around 10^-4 m/s.
v_d = I/(nAe), where I is current, n is number of free electrons per unit volume, A is cross-sectional area, e is electron charge. It can also be written as v_d = (eE/m)τ, where E is the field and τ is the relaxation time.
There are a huge number of free electrons (n is about 10^28 to 10^29 per m^3). Because so many electrons share the job of carrying current, each one only needs to drift very slowly.
Yes, indirectly. Higher temperature means more collisions, so relaxation time τ decreases. Since v_d = (eE/m)τ, a smaller τ gives a smaller drift velocity for the same field.
Yes. Drift velocity has both magnitude and direction. For electrons it points opposite to the applied electric field.
