Lesson focus: potential differenceThe energy transferred per unit of charge between two points in a circuit, also called voltage, measured in volts (V). measures energy transferred per unit charge between two points and is measured across a component.
Measure and calculate potential difference, connect voltmeterA meter that measures potential difference (voltage), connected across a component. correctly and interpret source and component voltages.
Build on earlier learning
Circuits and currentCurrent described charge flow. Potential difference now describes the energy transferred for each coulomb of that charge.
Practical
Measuring potential difference
- Connect a voltmeter in parallel across the cell, then across each bulb.
- Compare the readings.
- Use the model on this page to see how the potential difference is shared.
Record: Voltmeter readings across each component.
Circuit builder
Choose how the bulbs are connected, add bulbs and change the cell. The meters read like real ones.
Build the circuit
Each bulb has a resistance of 6 Ω (illustrative). Brighter bulbs transfer more energy each second.
You will be successful when you can…
- Define potential difference.
- Use V = E ÷ Q.
- Connect a voltmeter in parallel.
- Interpret source and component readings.
Words that carry the science
- potential difference
- Energy transferred per unit charge between two points.
- volt
- One joule transferred per coulomb of charge.
- voltmeter
- A high-resistance meter connected across two points.
- cell
- A source that maintains a potential difference using a chemical energy store.
- energy transfer
- Change in the energy stored by a system, measured in joules.
- parallel connection
- A connection across the same two circuit points.
Voltage is an energy-per-charge quantity
01Voltage is an energy-per-charge quantity
potential differenceThe energy transferred per unit of charge between two points in a circuit, also called voltage, measured in volts (V). is given by V = E ÷ Q. One voltThe unit of potential difference (V); 1 volt means 1 joule of energy transferred per coulomb of charge. means one joule of energy is transferred per coulomb of charge.
A 6 V source transfers 6 J to the circuit for each coulomb passing through it. A 6 V component transfers 6 J from the electrical pathway for each coulomb passing through that component.
02A voltmeter compares two points
A voltmeterA meter that measures potential difference (voltage), connected across a component. is connected parallelDescribes a circuit with more than one path, so components are on separate branches. across a component. It has very high resistance so little current passes through the meter and the circuit is disturbed as little as possible.
Connecting a voltmeter in series can greatly reduce circuit current and does not measure the intended component potential difference.
03Cells supply; components transfer
cellA component that provides a potential difference to push charge around a circuit, using energy stored in chemicals. in series with the same orientation provide potential differences that add. Reversing one cell makes its effect oppose the others.
Around a complete loop, the potential rises supplied by sources equal the potential drops across components, within measurement uncertainty. This is an energy account per coulomb.
Find energy transferred
A motor has 9.0 V across it while 15 C passes.
- Use V = E ÷ Q.
- Rearrange to E = V × Q.
- Substitute E = 9.0 × 15.
- Calculate E = 135 J.
How to make the reasoning visible: The motor transfers 135 joules from the electrical pathway for that charge.
Circuit-measurement
All readings are from idealised steady circuits.
| Test | Source / V | Component reading / V | Interpretation |
|---|---|---|---|
| single lamp | 3.0 | 3.0 | 3 J transferred per C |
| two equal series lamps | 6.0 | 3.0 each | drops share equally |
| motor + lamp series | 9.0 | 5.5 and 3.5 | drops sum to source |
| two cells opposed | 3.0 and 1.5 | 1.5 total | source effects oppose |
| voltmeter in series | 6.0 | circuit current nearly zero | incorrect connection |
Why do the motor and lamp readings in row 3 add to 9.0 V?
Why should a voltmeter have high resistance?
Check your understanding
Answer from memory, then check against the suggested answer.
What does one volt mean?
How is a voltmeter connected?
What does a cell supply?
A 12 V lamp transfers energy as 20 C passes. Calculate the energy.
Why can two components have the same current but different potential differences in series?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A 9 V battery is connected to two different lamps in series. Voltmeters across the lamps read 6 V and 3 V.
State what a potential difference of 1 V means.
(1)Explain the voltmeter readings.
(4)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., translate every voltage into joules per coulomb and balance the loop.