The big idea: Charge is a property of tiny particles (electrons carry it). Its unit is the coulomb (C).
Current is how fast that charge flows — the charge passing a point each second. Its unit is the ampere (A).
Potential difference (also called voltage) is the energy given to each coulomb of charge as it passes through a component. Its unit is the volt (V).
[Diagram: phys-circuit] - Available in full study mode
Spot it: More charge per second → bigger current.
More energy handed to each coulomb → bigger voltage.
Current flows through a component; voltage is measured across it.
Current is the rate of flow of charge — the charge Δq that flows divided by the time Δt it takes:
- electric current (A, amperes)
- charge that flows past a point (C, coulombs)
- time taken for that charge to flow (s)
[Diagram: phys-formula-triangle] - Available in full study mode
Potential difference is the energy per unit charge — the energy W given to the charge divided by the amount of charge q:
- potential difference / voltage (V, volts)
- energy given to the charge (J, joules)
- amount of charge moved (C, coulombs)
[Diagram: phys-formula-triangle] - Available in full study mode
Worked example — current from charge
A charge of 12 C flows through a wire in 4.0 s. Find the current in the wire.
Solution
- Start with the given formula:
- Put in the numbers (Δq = 12 C, Δt = 4.0 s):
- Work it out — keep the unit:
Final answer
current I = 3.0 A (3.0 coulombs of charge pass each second).
See how examiners mark answers
Access past paper questions with model answers. Learn exactly what earns marks and what doesn't.
How this is tested: These definitions sit under every circuit question.
- Paper 1A: a one-mark calculation — most often find a current from a charge delivered over a time (charge ÷ time). - Paper 2: longer problems that start by defining current or voltage before the circuit work.
Classic trap: mixing up the two formulas. Current uses charge ÷ time; voltage uses energy ÷ charge — don't divide by time for voltage.
Read the question for the rate: If a question gives a charge delivered every second (or 'per second'), that rate is the current directly. Otherwise divide the total charge by the total time.
IB-style question — current from a charged belt
A moving rubber belt carries charge up to a metal dome. The belt delivers 0.80 C of charge into a wire every 5.0 s. Find the current in the wire.
Solution
- Current is the rate of flow of charge, so use the given formula:
- Put in the numbers (Δq = 0.80 C, Δt = 5.0 s):
- Work it out — keep the unit:
Final answer
current I = 0.16 A (about 160 mA).