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AQA GCSE Physics · Topic 4.2 Electricity
Circuit Symbols, Current, Potential Difference and Resistance
In this lesson you'll learn to: draw and recognise the standard circuit symbols, connect an
ammeter in series and a voltmeter in parallel, describe current as the rate of flow of
charge and use Q = I t, explain what a potential difference (voltage) does to each charge, and
use V = I R to find voltage, current or resistance.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. Each glowing ball is 1 coulomb of charge. On tab 1, press Count for 1 second and count the balls crossing the gold line, then compare your count with the ammeter. On tab 2, follow one ball round and watch what happens to its red joules in the battery and in the bulb, then add a second bulb. On tab 3, change the battery, then change the resistor, and see which one changes the resistance.
1 coulomb of chargedashed gold line = counting point
Variable resistor
high resistance
low resistance
2 A
ammeter
0.00 s
stopwatch
0
coulombs counted
What the ammeter is telling you
Try this: press Count for 1 second and count the balls crossing the gold line yourself. Then turn the variable resistor up and count again.
1 coulomb of charge1 joule of energy
Battery pack
2 cells · 3 V
Follow one charge round
1 volt = 1 joule per coulomb. The voltmeter does not count charges. It measures how much energy each coulomb hands over between its two wires.
1 coulomb of charge1 joule of energy
Battery pack
2 cells · 3 V
Variable resistor
low
high
V = I R
V potential difference, volts (V)
I current, amps (A)
R resistance, ohms (Ω)
I current, amps (A)
R resistance, ohms (Ω)
Read the meters, then work out R
| V (volts) | I (amps) | R = V ÷ I (ohms) |
|---|---|---|
| Record a few readings: change the battery, then change the resistor. | ||
Charges are shown flowing in the direction of conventional current, from + to −. Cells are 1.5 V.
▲ The balls move at exactly 1 coulomb per second for every amp, so what you count is what the ammeter reads.
Seen it move? Now learn to write it down.
The exam wants the symbols drawn correctly, the meters in the right place, and the two equations Q = I t and V = I R used with the right units. The notes below cover all three.
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Revision notes
4.2.1.1 Standard circuit symbols
You need to recognise and draw every one of these. Draw wires with a ruler, as straight lines that meet at right angles, and never leave a gap in a circuit unless it is an open switch.
Switch (open)
Switch (closed)
Cell
Battery
Diode
Resistor
Variable resistor
LED
Lamp (bulb)
Fuse
Voltmeter
Ammeter
Thermistor
LDR
💡 Exam tip — the three that get mixed up
Cell vs battery: a battery is two or more cells joined together. The long line is the positive (+) terminal.
Thermistor vs variable resistor: the thermistor's line has a flat foot at the bottom; the variable resistor's line ends in an arrowhead.
Resistor vs fuse: a fuse has a line running all the way through the box.
Ammeters and voltmeters
| Meter | Measures | How it is connected |
|---|---|---|
| Ammeter | current, in amps (A) | in series — in the same loop, so the charge flows through it |
| Voltmeter | potential difference, in volts (V) | in parallel — across the component, one wire on each side of it |
4.2.1.2 Electrical charge and current
For a current to flow you need two things: a closed circuit (no gaps) and a source of potential difference, such as a cell or battery.
⭐ Definition
Electric current is the rate of flow of electrical charge.
In other words, current tells us how much charge passes a point every second. A current of 1 amp means 1 coulomb of charge passes each second; 3 A means 3 coulombs every second.
Q
Q = I t
charge flow = current × time
Q charge, coulombs (C) · I current, amps (A) · t time, seconds (s)⟳
Same everywhere in a loop
In a single closed loop the current is the same at every point. Charge is not used up by the bulb; it all comes back round to the battery.
💡 Exam tip — units first
Q = I t only works in amps and seconds. Convert before you substitute: mA ÷ 1000 → A (45 mA = 0.045 A) and minutes × 60 → s (2 min = 120 s).
Potential difference (voltage)
Potential difference, also called voltage, tells us how much energy is transferred to or from each coulomb of charge. The battery gives energy to the charge; the bulb or resistor takes it and transfers it away, as light and heat.
- A 3 V battery gives each coulomb 3 J of energy.
- A voltmeter reading 3 V across a bulb means each coulomb transfers 3 J to the bulb.
- Two identical bulbs in series share it: 1.5 J each, so a voltmeter across one reads 1.5 V.
4.2.1.3 Current, resistance and potential difference
The current through a component depends on both the potential difference across it and its resistance. More voltage pushes more current; more resistance lets less current through.
Ω
V = I R
potential difference = current × resistance
V potential difference, volts (V) · I current, amps (A) · R resistance, ohms (Ω)÷
Rearranged
I = V ÷ R to find the current.
R = V ÷ I to find the resistance: voltmeter reading ÷
ammeter reading.
⭐ Key idea — resistance is a ratio
For a fixed resistor, doubling the voltage doubles the current, so V ÷ I stays the same. Changing the battery does not change the resistor. Tab 3 of the animation lets you prove it.
Watch for kΩ: 1 kΩ = 1000 Ω. Convert before you calculate.
On the workbook page: resistors in series add up
(Rtotal = R1 + R2). Resistors in parallel give a total that is
less than the smallest one, because the charge has more than one path. You'll meet the full rules in
4.2.2 Series and parallel circuits.
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How to write it in the exam
Every calculation on this page is worth marks for the same four steps. Write all four, even when you can do it in your head.
The four-line calculation
1. Equation — write it in symbols: Q = I t or V = I R
2. Convert — mA → A, minutes → s, kΩ → Ω
3. Substitute — put the numbers in, then rearrange if you need to
4. Answer + unit — C, A, V or Ω
2. Convert — mA → A, minutes → s, kΩ → Ω
3. Substitute — put the numbers in, then rearrange if you need to
4. Answer + unit — C, A, V or Ω
equation & answerconvertsubstitute
Worked example · charge · 3 marks
A phone charger supplies a current of 500 mA for 2 minutes. Calculate the charge that flows.
- Equation: Q = I t
- Convert: 500 mA = 0.5 A · 2 minutes = 120 s
- Substitute: Q = 0.5 × 120
- Answer: Q = 60 C
Where marks go missing: 500 × 2 = 1000 C. Neither number is in the right unit, so the answer is wrong by a long way.
Worked example · V = I R · 2 marks
A current of 0.4 A flows through a 15 Ω resistor. Calculate the potential difference across it.
- Equation: V = I R
- Substitute: V = 0.4 × 15
- Answer: V = 6 V
Exam-style question · rearranging · 3 marks
The potential difference across a 2.2 kΩ resistor is 11 V. Calculate the current through it. Give your answer in milliamps.
- Equation: V = I R, so I = V ÷ R (1)
- Convert: 2.2 kΩ = 2200 Ω
- Substitute: I = 11 ÷ 2200 = 0.005 A (1)
- Answer: I = 5 mA (1)
Check your answer has all 4 parts: equation, conversion, substitution, and an answer with its unit. A number with no unit can lose the final mark.
📘 Now do it in your workbook
Name all 15 circuit symbols, add the meters to two circuits, work out the charge through a torch, and find the total resistance of two resistors in series and in parallel, then check it with the meters.
AQA GCSE Physics Workbook · ELECTRICITY · pages 3, 4 & 6The full workbook covers the whole of Topic 4.2 Electricity, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 3, 4 and 6 of the workbook have the circuit symbols to name, the meters to add, the torch charge question and the series-and-parallel resistance question, each with full answers at the back.
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