How charges drift through conductors and how circuits share current
Electric current is the rate at which charge flows, I = Q/t, measured in amperes. Inside a metal wire, free electrons move randomly at high speed, but an applied electric field gives them a small average drift velocity opposite to the field. The current is linked to this drift by I = nAev, where n is the number of free electrons per cubic metre, A is the cross-sectional area and e is the electron charge.
Resistance depends on the material and shape of a conductor: R = ρL/A, where ρ is resistivity, which rises with temperature for metals. Kirchhoff’s junction rule says the total current entering a junction equals the total leaving it, because charge is conserved. His loop rule says the sum of potential changes around any closed loop is zero, because energy is conserved. These two rules, together with the emf and internal resistance of cells, let you solve networks such as the Wheatstone bridge, which is balanced when P/Q = R/S and no current flows through the galvanometer.
Why it matters
Every battery-powered device, charger and wiring layout depends on how current divides and how much power is lost as heat, P = I²R. Engineers use Kirchhoff’s rules to design circuit boards, and the metre bridge and potentiometer are standard laboratory tools for measuring unknown resistances and emf. This topic is the base for magnetic effects of current, electromagnetic induction and semiconductor electronics.
Key terms
- Drift velocity
- The small average velocity that free electrons gain in the direction opposite to an applied electric field.
- Resistivity
- A property of a material that measures how strongly it opposes current, independent of the conductor’s size.
- Internal resistance
- The resistance inside a cell that causes its terminal voltage to fall below its emf when current flows.
- Kirchhoff’s loop rule
- The algebraic sum of emfs and potential drops around any closed loop of a circuit is zero.