How Current Flows in Conductors: Free Electrons Explained
Physics · Current Electricity · NEET
In a metal conductor, current is carried by free electrons — loose outer electrons that leave their atoms and move around inside the metal. Without a battery they move fast but in random directions, so the net current is zero. When a battery is connected, an electric field E pushes all of them slowly in one direction (this small extra speed is called drift velocity v_d), and this ordered movement of charge is the electric current. Memory hook: "Free electrons are always busy, but a battery makes them all walk the same way."
Left: with no battery, free electrons move fast but randomly, so arrows cancel and net current is zero. Right: the battery's field E gives every electron the same small extra drift (opposite to E, since electrons are negative), and this ordered drift is the current I = neAv_d.
Your doubts, answered
Do electrons or protons carry the current in a metal wire?
Only free electrons carry current in a metal. The positive ions are heavy and locked in the fixed lattice — they only vibrate, they do not travel. So the moving charge is the negatively charged electrons in the background of fixed positive ions. This is why conventional current (direction of positive flow) points opposite to the actual electron motion.
If electrons are always moving, why is the current zero without a battery?
Without a field, electrons move due to thermal motion and keep colliding with the fixed ions. After each collision the direction becomes completely random. So at any moment, as many electrons go one way as go the opposite way, and their average velocity is zero. No net charge crosses any section, so the net current is zero.
How exactly does a battery make current flow?
The battery sets up an electric field E inside the wire. This field applies a force F = -eE on every free electron, giving each a small extra acceleration between collisions. On top of their random motion, all electrons now gain a tiny common velocity in one direction — the drift velocity v_d. This ordered drift of charge is the electric current.
Are the 'free electrons' and the 'current' the same thing?
No. Free electrons are the charge carriers (the material). Current is the rate at which their charge crosses a section, I = neAv_d (n = free electrons per m3, A = area, v_d = drift speed). The electrons are always present; current only exists when they drift together because a field is applied.
Electrons drift very slowly (mm/s), so why does the bulb light up at once?
The electric signal (the field) travels through the wire at nearly the speed of light, so every free electron along the whole circuit starts drifting almost instantly. The bulb lights because electrons already inside its filament start moving right away — you do not wait for an electron from the switch to travel all the way there.
⚠️ The NEET trap ✗ Higher current means the free electrons must be moving very fast, so drift velocity is large (metres per second). ✓ Drift velocity is extremely small (about 10^-4 to 10^-3 m/s) even for a 10 A current, because the free-electron density n is huge (~10^28–10^29 per m3). In I = neAv_d, a large n makes v_d tiny. 🧠 Big current, tiny drift — the crowd is enormous, so each electron only needs to shuffle.
Real NEET questions
NEET 2023
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 = 1.6x10^-19 C):
A · 6.25/pi m/s
B · (6.25x10^3)/pi m/s ✓
C · (6.25x10^4)/pi m/s
D · (6.25/pi)x10^2 m/s
Solution: Use I = n e A v_d, so v_d = I/(n e A). Step 1 area: A = pi r^2 = pi (10^-3)^2 = pi x 10^-6 m^2. Step 2 substitute: v_d = 10 / (10^22 x 1.6x10^-19 x pi x 10^-6). Step 3 multiply the denominator: 10^22 x 10^-19 x 10^-6 = 10^-3, and 10^-3 x 1.6 = 1.6x10^-3, so denominator = 1.6x10^-3 x pi. Step 4 divide: v_d = 10/(1.6x10^-3 pi) = (6.25x10^3)/pi m/s. This matches option B (a known key discrepancy exists, but the computation gives B).
NEET 2021
Match Column-I with Column-II: (A) Drift velocity (B) Electrical resistivity (C) Relaxation period (D) Current density | (P) m/(n e^2 rho) (Q) n e v_d (R) (eE/m) tau (S) E/J
A · A-R, B-P, C-S, D-Q
B · A-R, B-Q, C-S, D-P
C · A-R, B-S, C-P, D-Q ✓
D · A-R, B-S, C-Q, D-P
Solution: Work each microscopic result. (A) The field accelerates electrons for time tau between collisions, so drift velocity v_d = (eE/m) tau -> R. (B) Resistivity is defined by rho = E/J -> S. (C) Rearranging rho = m/(n e^2 tau) gives relaxation time tau = m/(n e^2 rho) -> P. (D) Current per unit area J = n e v_d -> Q. So A-R, B-S, C-P, D-Q, which is option C.
Solved Current Electricity NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a loosely bound outer (valence) electron that has broken away from its atom and can move freely through the whole metal. Metals have huge numbers of them (about 10^28–10^29 per cubic metre), which is why metals conduct so well.
What is the difference between random motion and drift?
Random (thermal) motion is fast but has no preferred direction, so it gives zero net current. Drift is a slow, extra velocity in one direction added by the applied field; it is what actually produces the current.
Why do positive ions not carry current in a metal?
The positive ions form the fixed lattice. They are heavy and bound in place, so they only vibrate about fixed positions. Only the light free electrons are able to travel and carry charge.
What is the formula linking current and free electrons?
I = n e A v_d, where n is the number of free electrons per m3, e is the electron charge, A is the cross-section area, and v_d is the drift velocity. It links the microscopic electrons to the measured current.
Why is drift velocity so small yet current can be large?
Because n is enormous. In I = neAv_d, the large number of carriers means each electron only needs a very small drift speed to give a large current.
