Beginner · 10 min
Electric charge and current
Calculate charge passing a point and explain what a current reading means.
Current is a rate of charge flow
Electric charge is measured in coulombs (C). Current tells us how much charge passes a point each second: I = Q ÷ t. One ampere means one coulomb per second. In a metal wire, mobile electrons carry charge, but conventional current direction is opposite to their drift direction. The numerical calculations here use the magnitude of charge passing a point.
Worked example
If 12 C passes a point in 4 s, the average current is 12 ÷ 4 = 3 A.
Charge is not used up
A circuit transfers energy, but it does not use up charge as that charge passes through a component. In a steady series circuit, the same amount of charge per second passes each point. If current is constant, Q = I × t. With changing current, this equation gives the total charge when I is the average current over the stated interval.
Worked example
A constant current of 0.5 A flows for 6 s. Charge passing a point = 0.5 × 6 = 3 C
Interpret a current reading
- Calculate the charge passing a point in 10 s when a steady current is 2 A.
- For two resistors in series, explain whether the charge passing the second resistor each second is smaller than that passing the first.