An electric current is nothing more mysterious than charge flowing past a point. Count the charge per second and you have the current; look inside the wire and the same current becomes a sea of drifting electrons.
Electric current is the rate of flow of charge, I = Q / t, so the charge passing in a time t is Q = It. Charge is quantised in units of e = 1.6 × 10⁻¹⁹ C. Inside a conductor the current is carried by moving charge carriers, and I = Anvq, where A is the cross-sectional area, n the number of carriers per unit volume, v their mean drift speed and q the charge on each.
Watch the charge carriers drift along the conductor. Change the cross-section, the carrier density and the drift speed, and see the current respond exactly as I = Anvq predicts.
One relation for the circuit, one for the microscopic picture.
| Relation | Meaning | Unit |
|---|---|---|
| Q = It | charge from current and time | coulomb (C) |
| I = Anvq | current from the drifting carriers | ampere (A) |
| e = 1.6 × 10⁻¹⁹ C | the quantum of charge | coulomb (C) |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
Electric current is defined as the:
A charge of 30 C passes a point in 6.0 s. The current is:
In the equation I = Anvq, the symbol n represents the:
A current of 2.0 A flows for 5.0 minutes. The charge transferred is:
The I = Anvq equation hides a surprise about how fast electrons actually move.
Keep the symbols in I = Anvq straight: n is the number of carriers per unit volume, not the total number in the wire. The drift speed is genuinely slow; the current appears instantly because the field is established almost at the speed of light, not because the electrons race along. And remember to convert time to seconds in Q = It.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A copper wire of cross-sectional area 1.0 × 10⁻⁶ m² carries a current of 1.6 A. With n = 1.0 × 10²⁹ m⁻³ and q = 1.6 × 10⁻¹⁹ C, the drift speed is:
How many electrons pass a point each second when the current is 1.0 A? (e = 1.6 × 10⁻¹⁹ C)
For a fixed current, a wire that narrows to half its cross-sectional area has a drift speed that is:
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