Charge on the move
Switch on a torch. Something starts moving inside the wire.
- A closed loop: a cell, a bulb, a switch.
- Check the wire before the bulb. It reads 2 A.
- Check the wire after the bulb too. Also 2 A.
Slide the seconds. Then open the switch.
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The something that moves in the wire is charge. Tiny particles in the wire carry it.
Current — the charge passing one point each second. Its unit is the ampere (A).
The rule
I = Q ÷ t
Current is charge passing one point each second. The charge that leaves a cell comes back. A lamp spends energy, not charge. In a one-loop circuit, a break stops the current everywhere.
In the formula, I is the current. Q is the charge. t is the time, in seconds.
Charge is measured in coulombs (C). A cell drives 12 C past a point in 4 seconds (s). So I = Q ÷ t = 12 ÷ 4 = 3 A.
Arrows on a circuit show conventional current. It points the way positive charge moves. In a metal wire, the particles are electrons. They drift the other way.