Electricity 1 · Part A — Circuits, symbols and meters
⬅ Back to Electricity 1① Circuits, symbols & meters
Circuit symbols, and how current, voltage and meters behave in series and parallel circuits.
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L1 — Circuit symbols
Every component has an agreed British Standard symbol. You need to draw and recognise these ten:
⭐ Not in your booklet table yet — add it. It is on the test.
Watch the pair: a fuse is a rectangle with a line through it lengthwise. The plain rectangle (which you'll meet properly in Part B) is a resistor. These two are the symbols most often mixed up.
TryQuick recall check:
Check your booklet component functions
- Cell / Battery: supplies energy to the circuit.
- Lamp: converts electrical energy to light energy.
- Switch: opens and closes the circuit.
- Voltmeter: measures the voltage across a component.
- Ammeter: measures the current in the circuit.
- Motor: converts electrical energy to kinetic (movement) energy.
- Microphone: converts sound energy to electrical energy.
- Loudspeaker: converts electrical energy to sound energy.
- Photovoltaic cell: converts light energy to electrical energy.
Can you draw all ten symbols from memory, and say what each one does?
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L2 — Current & voltage
| Term | Definition (SQA) | Symbol | Unit |
|---|---|---|---|
| Voltage (potential difference) |
The energy supplied to each coulomb of charge passing through the supply. Think of it as the electrical push. | V | volt (V) |
| Current | The amount of charge passing a point every second — the flow of charge. | I | ampere (A) |
Measuring with a multimeter
- Black lead → COM port (always).
- Measuring voltage: red lead → V/Ω port, dial to DCV (e.g. the 20 V range to read a 1.5 V cell).
- Measuring resistance: red lead → V/Ω port too, but dial to Ω — never DCV.
- Measuring current: red lead → A port (e.g. 2 A or 10 A), dial to DCA, and the meter goes in series.
- Always pick a range just above the value you expect.
Measuring resistance — the ohmmeter
- An ohmmeter measures resistance directly, in ohms (Ω). Its symbol is the letter Ω inside a circle.
- It goes across the component, like a voltmeter — but the component must be on its own, with the supply disconnected.
- An ohmmeter has its own small battery inside it. It pushes a tiny known current through the component and works the resistance out for you, so it needs no supply of its own.
- On a multimeter: black lead in COM, red lead in V/Ω, dial turned to Ω.
⚠ Common mistake — using an ohmmeter
The component must be disconnected from the supply before you measure its resistance.
Don't leave the circuit switched on — the supply's voltage adds to the ohmmeter's own, so the reading is wrong, and a big supply can damage the meter.
Don't leave the component wired into the rest of the circuit either. Current can sneak round through the other components, so the meter reads the whole network instead of the one component you wanted.
Two ways to find a resistance
| Method | What you connect | What you get |
|---|---|---|
| Ohmmeter | The ohmmeter alone in a loop with the component. No supply. | Reads R directly — one meter, no calculation. |
| Voltmeter + ammeter | A supply, an ammeter in series (in the main line) and a voltmeter across the component. | Reads V and I — then you work out R = V ÷ I. |
So: a meter on its own in a loop with a component, with no supply, is an ohmmeter. In a circuit that has a supply, a meter across a component is a voltmeter, and a meter in the main line is an ammeter.
⚠ Common mistake — where the meters go
An ammeter goes in series (in the line, so all the current flows through it).
A voltmeter goes in parallel — across the component.
Don't put the ammeter across a component, and don't put the voltmeter in the main line — a voltmeter has very high resistance and would block the current.
Series vs parallel rules
| Series | Parallel | |
|---|---|---|
| Current | Same everywhere (one path) | Splits between branches, then rejoins |
| Voltage | Splits across components | Same across every branch |
Everyday uses: Christmas-style series strings share one path; house sockets are in parallel so each appliance gets the full mains voltage and can switch on independently.
Check your booklet definitions & circuit rules
Voltage: energy supplied to each coulomb of charge passing through the supply.
Current: the amount of charge passing a point every second.
| Series | Parallel | |
|---|---|---|
| Current | Same everywhere | Splits then rejoins |
| Voltage | Splits across components | Same across each branch |
Explore · optional Build your own circuits (PhET)
Build circuits, add cells, lamps, resistors and meters, and watch the current flow. Test the series and parallel rules for yourself — nothing here is marked.
Simulation by PhET Interactive Simulations, University of Colorado Boulder — licensed CC BY 4.0.
Could you label a multimeter to read a 0.3 A current, and state the current/voltage rule for both circuit types?
✅ Rate yourself
Sources & credits: The RS Electricity B1 booklet © R Stewart, 2025. Circuit simulation by PhET Interactive Simulations, University of Colorado Boulder (CC BY 4.0).