9.1 Electric current

Current is I=∆Q/∆t: the rate at which charge passes a point, measured in amperes, with 1 A=1 C s⁻¹.

Choose a direction convention and distinguish conventional current from the motion direction of negative electrons.

If 12 C passes a wire section in 3.0 s, the current is 4.0 A.

Current is not consumed by a component in a series circuit; charge flow rate is continuous around the circuit.

Electric charge occurs in discrete amounts q=ne, where n is an integer and e≈1.60×10⁻¹⁹ C.

Use the sign to identify positive or negative carriers and interpret n as a count, not a continuously adjustable fraction.

A charge of −3.2×10⁻¹⁹ C corresponds to two excess electrons.

The quantisation statement does not mean every macroscopic measurement visibly jumps by e; huge carrier counts make charge appear continuous.

For constant current, charge transferred in time t is Q=It; for changing current, use the area under an I–t graph.

Use seconds and coulombs, and state whether Q is magnitude or signed charge according to the chosen direction.

A 0.50 A current flowing for 4.0 minutes transfers Q=120 C.

Do not use minutes directly with amperes, and do not assume Q=It for a changing current without integrating or finding graph area.

For charge carriers of number density n, cross-sectional area A, drift speed v and charge magnitude q, I=Anvq.

The formula counts carriers crossing the section each second; use the conductor’s actual area and the carrier type appropriate to the material.

A thinner wire with the same carrier density and drift speed carries less current because A is smaller.

Drift speed is not the same as the electromagnetic signal speed, and n is a volume number density, not total carrier count.