A charge of 90\,C flows through a wire in 30\,s. Calculate the current. [2 marks]

OCR OCR 2018-style practice: A current of 2.0\,A flows through a lamp for 30\,s. Calculate the charge that flows through the lamp.

A P3 Calculate question on the recall equation Q = It. Write the values: current I = 2.0\,A and time t = 30\,s (1 mark for selecting and stating the equation). Substitute: Q = It = 2.0 × 30 = 60\,C (2 marks for the calculation and the unit coulombs). Markers reward the correct equation, substitution, and the answer in coulombs. A common error is to forget the unit or to divide rather than multiply.

EnglandPhysicsSyllabus dot point

What are current, potential difference and resistance, and how are they linked?

Electric current as the rate of flow of charge, the charge equation, potential difference as energy transferred per unit charge, resistance and the equation linking potential difference, current and resistance.

A focused answer to OCR Gateway GCSE Physics A topic P3 on current, potential difference and resistance, covering current as the rate of flow of charge, the charge equation, potential difference as energy per unit charge, resistance, and the equation linking them.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this topic is asking
Electric current and charge
Potential difference
Resistance
What affects resistance
Try this

What this topic is asking

OCR wants you to define electric current as the rate of flow of charge, use the charge equation, define potential difference as energy per unit charge, and link potential difference, current and resistance. This is topic P3.3 of the OCR Gateway Physics A (J249) specification.

Electric current and charge

A larger current means more charge passes each second. One ampere is one coulomb per second. Conventional current is taken to flow from the positive to the negative terminal of the supply (opposite to the electron flow), which is the direction used in circuit diagrams.

Potential difference

So a higher potential difference gives each coulomb of charge more energy to transfer to the components. Across a component such as a lamp, the potential difference tells you how much energy each coulomb gives up there (as heat and light).

Resistance

Rearranging gives $R = \dfrac{V}{I}$ (to calculate resistance) and $I = \dfrac{V}{R}$ (to calculate current). A high resistance lets only a small current flow for a given voltage. Resistance arises because the moving electrons collide with the fixed ions in the conductor, transferring energy and heating it up.

What affects resistance

The resistance of a wire depends on its length (longer means more resistance), its cross-sectional area (thicker means less resistance), the material it is made of, and its temperature. For many metal conductors, heating increases the resistance because the ions vibrate more and obstruct the electrons.

Finding the current through a resistor

A $20\,\Omega$ resistor is connected across a $12\,\text{V}$ supply. Calculate the current through the resistor and the charge that flows in one minute.

step 1: Find the current

Rearrange $V = IR$ for current: $I = \dfrac{V}{R} = \dfrac{12}{20} = 0.60\,\text{A}$ .

step 2: Write down the time in seconds

One minute is $t = 60\,\text{s}$ .

step 3: Find the charge

Use $Q = It = 0.60 \times 60 = 36\,\text{C}$ .

step 4: State the answers

The current is $0.60\,\text{A}$ and the charge that flows in one minute is $36\,\text{C}$ . Both equations ( $V = IR$ and $Q = It$ ) are recall equations, so you must know them without the data sheet.

Try this

Q1. A charge of $90\,\text{C}$ flows through a wire in $30\,\text{s}$ . Calculate the current. [2 marks]

Cue. Rearrange $Q = It$ : $I = \dfrac{Q}{t} = \dfrac{90}{30} = 3.0\,\text{A}$ .

Q2. State what is meant by a potential difference of one volt. [1 mark]

Cue. One joule of energy is transferred per coulomb of charge.

Exam-style practice questions

Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

OCR 20183 marksA current of

2.0\,\text{A}

flows through a lamp for

30\,\text{s}

. Calculate the charge that flows through the lamp.

Show worked answer →

A P3 Calculate question on the recall equation $Q = It$ . Write the values: current $I = 2.0\,\text{A}$ and time $t = 30\,\text{s}$ (1 mark for selecting and stating the equation). Substitute: $Q = It = 2.0 \times 30 = 60\,\text{C}$ (2 marks for the calculation and the unit coulombs). Markers reward the correct equation, substitution, and the answer in coulombs. A common error is to forget the unit or to divide rather than multiply.

OCR 20214 marksA resistor has a potential difference of

6.0\,\text{V}

across it and a current of

0.50\,\text{A}

through it. Calculate its resistance, and explain what happens to the current if the potential difference is doubled while the resistance stays constant.

Show worked answer →

A P3 Calculate and Explain question. Use the recall equation $V = IR$ rearranged: $R = \dfrac{V}{I} = \dfrac{6.0}{0.50} = 12\,\Omega$ (2 marks for the rearrangement and answer with units). If the potential difference is doubled to $12\,\text{V}$ while the resistance stays at $12\,\Omega$ , then $I = \dfrac{V}{R} = \dfrac{12}{12} = 1.0\,\text{A}$ , so the current also doubles (2 marks for the reasoning and the new current). Markers reward the resistance with units and the proportional doubling of current. A common error is to think doubling the voltage has no effect on the current.

Related dot points

Sources & how we know this

OCR GCSE (9-1) Physics A (Gateway Science) J249 specification — OCR (2016)