What is potential difference? GCSE Physics explained
Potential difference, often called voltage, is the energy transferred per unit charge between two points in an electric circuit. It is measured in volts (V), and one volt means one joule of energy is transferred for every coulomb of charge that flows. In plain language: A potential difference of 1.5 V across a bulb means each coulomb of charge passing through that bulb hands over 1.5 joules of energy.
This guide explains the definition, the two formulas you need for GCSE, how potential difference behaves in series and parallel circuits, and the mistakes that cost students marks every year.
Measured in volts (V)
1 volt = 1 joule of energy transferred per coulomb of charge. The symbol for potential difference is V (sometimes U in older textbooks).
Two formulas to know
V = W/Q (energy per charge) and V = IR (Ohm's law). You will use both across Paper 1 and Paper 2.
Measured with a voltmeter
A voltmeter is always connected in parallel across the component you are measuring. Never in series.
Defining potential difference properly
The full mark-scheme definition is: Potential difference is the work done per unit charge moved between two points in a circuit. Another way of saying the same thing is that it is the energy transferred per coulomb of charge.
This definition matters because GCSE mark schemes are strict on the wording. Students who write "the amount of electricity" or "the power of the battery" lose easy marks. Stick to "energy transferred per unit charge" or "work done per unit charge" and you will pick up the definition mark every time.
Voltage and potential difference In GCSE Physics, voltage and potential difference mean the same thing. Examiners use both terms interchangeably. EMF (electromotive force) is slightly different and is only formally introduced at A-Level, but you might see the term on a battery datasheet.
The two formulas you must know
There are two formulas for potential difference at GCSE. Both appear in the AQA and Edexcel equation sheets, but you should know them well enough to use them without looking.
| Formula | What each letter means | When to use it |
|---|---|---|
| V = W / Q | V = potential difference (V), W = work done or energy transferred (J), Q = charge (C) | Questions that give you energy and charge and ask for voltage, or vice versa |
| V = I × R | V = potential difference (V), I = current (A), R = resistance (Ω) | Questions about resistors, bulbs, or any component with a known resistance and current |
Which formula to reach for first If the question mentions joules or coulombs, use V = W/Q. If the question mentions current, resistance, or Ohm's law, use V = IR. Most circuit questions use V = IR. Energy questions almost always use V = W/Q.
Potential difference in series circuits
In a series circuit, the potential difference of the source (usually a battery or cell) is shared between the components. If you add up the voltage across every component in the circuit, you get the voltage of the supply.
This is one of Kirchhoff's voltage rules, and at GCSE it is usually written as: The sum of the potential differences across components in a series loop equals the potential difference of the supply.
| Circuit setup | Total supply voltage | How it splits |
|---|---|---|
| 6 V cell with two identical bulbs in series | 6 V | 3 V across each bulb |
| 12 V cell with a 4 Ω and an 8 Ω resistor in series | 12 V | 4 V across the 4 Ω resistor, 8 V across the 8 Ω resistor |
| 9 V cell with three identical resistors in series | 9 V | 3 V across each resistor |
Potential difference in parallel circuits
In a parallel circuit, the potential difference across each branch is the same as the potential difference of the source. This is because each branch is connected directly across the supply.
So if you have a 6 V battery with three bulbs in parallel, every bulb has 6 V across it, regardless of its resistance. What changes between branches is the current, not the voltage.
A quick check for your circuit diagrams If you are unsure whether two components are in series or parallel, trace the wire with your finger. If you can travel through both components in a single loop without splitting off, they are in series. If the wire splits and rejoins around them, they are in parallel.
Where students lose marks on potential difference questions
Examiner reports from AQA and Edexcel flag the same handful of mistakes every year. Most of them are not about physics knowledge, they are about being careful with definitions and units.
Common mistakes that cost easy marks Using the wrong formula because the question wording was misread. Forgetting to add component voltages in a series circuit. Saying the voltage across a voltmeter is zero (it is the voltage across the component the voltmeter is measuring). Mixing up V = IR (Ohm's law) with V = W/Q (energy per charge). Writing the unit as v instead of V (capital V is the symbol for volts, named after Volta).
Worked example: Finding the voltage across a resistor
A 6 V cell is connected in series with a 2 Ω resistor and a 4 Ω resistor. Calculate the potential difference across the 2 Ω resistor.
Step 1: Find the total resistance. R(total) = 2 + 4 = 6 Ω.
Step 2: Use V = IR to find the current in the circuit. I = V/R = 6/6 = 1 A.
Step 3: Use V = IR for the 2 Ω resistor. V = 1 × 2 = 2 V.
The potential difference across the 2 Ω resistor is 2 V. As a check, the voltage across the 4 Ω resistor is 1 × 4 = 4 V, and 2 + 4 = 6 V, which matches the supply.
How a voltmeter measures potential difference
A voltmeter measures the potential difference across two points by connecting in parallel with the component. It has a very high internal resistance, so it draws almost no current. This matters because if the voltmeter drew current, it would change the circuit it is trying to measure.
A common Paper 1 question shows a circuit diagram and asks where the voltmeter should be placed. The answer is always: In parallel across the component whose voltage you want to measure. Putting it in series instead is a reliable way to lose the mark.
Key facts to memorise for the exam
- Definition: Potential difference is the energy transferred per unit charge between two points in a circuit
- Unit: Volts (V), where 1 V = 1 J per coulomb
- Formula 1: V = W / Q (use when given energy and charge)
- Formula 2: V = IR (use when given current and resistance)
- Series: The supply voltage is shared across components in proportion to their resistance
- Parallel: The voltage across each branch equals the supply voltage
- Voltmeter: Always connected in parallel, never in series, and has very high internal resistance
- Voltage and potential difference mean the same thing at GCSE
