The big idea: Flick a light switch and the bulb glows at once — charged particles are already sitting in the wire, and flicking the switch just sets them flowing. That charge is carried by electrons; its unit is the coulomb (C).
Current is how fast that charge flows — the charge passing a point each second (unit: the ampere, A).
Potential difference (voltage) is the energy given to each coulomb as it passes through a component (unit: the volt, V).
A cell pushes charge round the loop. The current is how much charge passes each second; an ammeter (A), placed in the line, counts it.
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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.
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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)
Δq = charge, I = current, Δt = time. Cover the one you want: two side by side → multiply; one above the other → divide.
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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)
W = energy, V = voltage, q = charge. Cover the one you want: two side by side → multiply; one above the other → divide.
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A charge of 12 C flows through a wire in 4.0 s. Find the current in the wire.
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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.
The 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.
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.
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