Physics 10.7 - 10.11 - Current and charge

Current in a complete circuit

Pearson supplies an equation sheet for the 2025–2027 GCSE Combined Science Physics exams. Practise selecting equations, understanding their quantities, converting units and rearranging them. The independent calculations here include the equation support they need.

Electric charge can move through conductors. Electric current is the rate at which charge flows past a point in a circuit. Its symbol is $I$ and its unit is the ampere, $\mathrm{A}$, often shortened to amp.

In a metal, some electrons are free to move through the fixed structure of positive ions. When the metal forms part of a complete circuit, these mobile electrons provide the flow of charge. The electrons were already present throughout the metal; they are not created by the cell.

A steady current requires:

• a closed circuit, giving charge a complete conducting path; and
• a source of potential difference, such as a cell, which sets charge in motion around that path.

Opening a switch breaks the conducting path, so there is no sustained current around the circuit. A source does not supply one fixed current in every situation: the current depends on the potential difference and the resistance in the circuit.

An ammeter measures the current in ampere. To measure the current in a component, connect the ammeter in series with that component. This places the meter in the same unbranched path, so the charge passing through the component also passes through the meter. Connecting an ammeter on a separate path would not measure the current in the chosen component.

Charge flow and time

A current of $1\,\mathrm{A}$ means that $1\,\mathrm{C}$ of charge passes a point every second. In unit form:

$1\,\mathrm{A} = 1\,\mathrm{C\,s^{-1}}$

This gives the relationship between charge, current and time. A current of $250\,\mathrm{mA}$ is $0.250\,\mathrm{A}$, since $1000\,\mathrm{mA}=1\,\mathrm{A}$.

The time must be in seconds before values are substituted. Rearranging gives $I = \frac{Q}{t}$ and $t = \frac{Q}{I}$.

Worked example: calculating charge

A current of $0.40\,\mathrm{A}$ flows for $3.0\,\mathrm{min}$. Calculate the charge that passes a point in the circuit.

Solution

1. Convert the time: $3.0\,\mathrm{min} = 3.0 \times 60 = 180\,\mathrm{s}$.
2. Select the equation: $Q = I \times t$.
3. Substitute: $Q = 0.40 \times 180$.
4. Calculate: $Q = 72\,\mathrm{C}$.