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.
| Quantity | What it is | Unit |
|---|---|---|
| Charge | carried by electrons | the coulomb (C) |
| Current | how fast that charge flows — the charge passing a point each second | the ampere (A) |
| Potential difference (voltage) | the energy given to each coulomb as it passes through a component | 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.
Drawing the circuit
Charge carriers: A current is a flow of charge carriers: free electrons in a metal, positive and negative ions in a salt solution. The direction of a current is always given as conventional current, the way positive charge would flow: from + to −.
Conductor
- Has many charge carriers that are free to move (high mobility).
- Example: copper, where each atom gives up an electron that drifts through the metal.
Insulator
- Its electrons are held tightly in their atoms: almost no mobile charge carriers.
- Example: plastic, rubber or glass round a wire.
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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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Potential difference, exactly: The pd between two points is the work done per unit charge moving a positive charge from one point to the other: V = W/q.
Through a resistor, positive charge moves from the higher-potential end (nearer the + terminal) to the lower one, and the field does positive work on it. A negative charge, such as −1 C, moves the opposite way; 2.0 J of work across 2.0 V means 1 C of charge, far more than one electron.
The same current all the way along: Charge cannot pile up or vanish in a wire, so in a single path the current is the same at every point, even where the wire is thicker or thinner.
A charge of 12 C flows through a wire in 4.0 s. Find the current in the wire.
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A belt 0.40 m wide carries charge 2.0 mC m⁻² on its surface and moves at 3.0 m s⁻¹. All the charge is removed at one point into a wire. Calculate 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.
The words to know
| Word | What it means |
|---|---|
| Charge carrier | A charged particle free to move: an electron in a metal, an ion in a solution |
| Conventional current | The direction positive charge would flow: from + to − outside the cell |
| Potential difference | Work done per unit charge moving a positive charge between two points |
| Conductor | A material with many mobile charge carriers |
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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