🔽 m means milli
One thousandth. To change into the plain unit, divide by 1000.
250 mA = 250 / 1000 = 0.25 A
Units and Circuit Symbols » Electrical Units and Circuit Symbols
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.1, Electrical circuit symbols
- The seven electrical quantities, their symbols and their units
- How to change mA, kΩ and kW into amperes, ohms and watts
- How to recognise every circuit symbol used in the exam papers
- How to read a simple circuit diagram, including where ammeters and voltmeters go
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Unlock This CourseThe language of electricity
Electricity has its own short language: a small set of quantities to measure, and a set of little drawings that stand for the parts of a circuit. Learn both now and the rest of this topic becomes much easier. The equations that link the quantities come in later lessons.
Every quantity has a symbol (the letter used in equations) and a unit (what it is measured in). Do not mix the two up.
| Quantity | Symbol | Unit | What it tells you |
|---|---|---|---|
| Current | I | ampere (A) | How quickly charge is flowing round the circuit |
| Charge | Q | coulomb (C) | How much electric charge has passed |
| Energy | E | joule (J) | How much energy has been transferred |
| Resistance | R | ohm (Ω) | How hard it is for current to get through a component |
| Time | t | second (s) | How long the current flows for |
| Voltage | V | volt (V) | How big the 'push' is that drives the current |
| Power | P | watt (W) | How quickly energy is being transferred |
Three things to notice:
- The symbol for current is I, not C. The letter C is the unit of charge, the coulomb.
- Voltage is the only one where the symbol and the unit use the same letter. In 'V = 6 V' the first V means voltage and the second means volts.
- 'Amp' is a short form of ampere. Both are fine, and both are written A.
Small and large values: m and k
Some values are awkward to write in the plain unit, so a letter called a prefix goes in front.
🔼 k means kilo
One thousand. To change into the plain unit, multiply by 1000.
4.7 kΩ = 4.7 × 1000 = 4700 Ω
2 kW = 2 × 1000 = 2000 W
Small currents are often given in milliamperes (mA), large resistances in kilohms (kΩ) and the power of a kettle or heater in kilowatts (kW). Always change to the plain unit before you put a value into an equation.
Worked example
An ammeter reads 35 mA. State this current in amperes.
milli means one thousandth, so divide by 1000
35 / 1000 = 0.035 A
Circuit symbols
A circuit diagram is a tidy drawing that uses one agreed symbol for each component (part) of a circuit. These are the symbols used in the exam papers. Draw each one as you read its description.
| Component | How the symbol is drawn | What it is for |
|---|---|---|
| Conductors crossing with no connection | Two straight lines that cross, with no dot | Wires not joined |
| Junction of conductors | Lines that meet, with a solid dot where they join | Wires joined |
| Open switch | A gap in the wire with one side lifted up at a slope, like an open gate | Breaks the circuit |
| Cell | Two upright lines side by side across the wire, one long and thin, one short and thick | Provides the voltage. Long line = positive side |
| Battery of cells | Two cell symbols joined by a dashed line | Two or more cells joined together |
| Power supply (d.c.) | Two small circles with a gap between them, marked + and − | Supplies direct current |
| Power supply (a.c.) | Two small circles with a short wavy line (~) between them | Supplies alternating current |
| Transformer | Two coils of loops facing each other, with two straight lines between them | Changes an alternating voltage |
| Ammeter | A circle with the letter A inside | Measures current |
| Voltmeter | A circle with the letter V inside | Measures voltage |
| Fixed resistor | A plain empty rectangle | Resistance is set |
| Variable resistor | A rectangle with a sloping arrow drawn right through it | Resistance can be changed |
| Heater | A rectangle split into four small boxes by three upright lines | Gets hot |
| Thermistor | A rectangle with a sloping line through it that ends in a short flat foot (no arrowhead) | Resistance changes with temperature |
| Light-dependent resistor (LDR) | A rectangle with two small arrows pointing towards it | Resistance changes with brightness of light |
| Diode | A triangle on the wire with its point touching a short bar | Current in one direction only |
| Light-emitting diode (LED) | A diode symbol with two small arrows pointing away from it | A diode that gives out light |
| Lamp | A circle with a diagonal cross inside | Gives out light |
| Loudspeaker | A small rectangle with a wider cone shape joined to it | Produces sound |
| Microphone | A circle with a short straight bar touching its edge | Detects sound |
| Electric bell | A dome (half a circle) sitting on a short stem | Rings |
| Earth or ground | A line going down to three flat lines, each one shorter than the one above | A connection to the ground |
| Motor | A circle with the letter M inside | Spins |
| Generator | A circle with the letter G inside | Produces a voltage when turned |
| Fuse / circuit breaker | A rectangle with the wire drawn straight through the middle of it | Breaks the circuit if the current is too large |
You only need to recognise and draw these for now. How the components behave comes in later lessons.
Symbols that look alike
Several symbols differ by one small detail, and that detail is what examiners check.
📦 The rectangle family
Empty = fixed resistor. Arrow through it = variable resistor. Line with a flat foot = thermistor. Arrows pointing in = LDR. Wire through the middle = fuse. Four small boxes = heater.
⭕ The circle family
A = ammeter. V = voltmeter. M = motor. G = generator. A cross = lamp. The letter or cross must be inside the circle.
💡 Arrows in or out
Light goes in to an LDR, so its arrows point towards it. Light comes out of an LED, so its arrows point away.
Two more pairs to watch. A crossing with no dot means the wires are not connected, but a dot means they are. A single pair of lines is a cell, but cells joined together make a battery.
Where ammeters and voltmeters go
- An ammeter is connected in series with a component. In series means in the same loop, one after the other, so the current goes through the ammeter and then through the component.
- A voltmeter is connected in parallel with a component. In parallel means on its own side branch, with one lead joined to each side of the component.
To add an ammeter you break the circuit and put the meter in the gap. To add a voltmeter you break nothing. You just clip it across the component.
Reading a circuit diagram
In a circuit diagram the straight lines are the wires. Sketch this one as you read.
- Draw a large rectangle with a ruler. Its four sides are the wires.
- In the middle of the top side, draw a battery of two cells.
- On the right-hand side, draw an open switch.
- In the middle of the bottom side, draw a lamp.
- On the left-hand side, draw an ammeter.
- Put a dot on the wire just to the left of the lamp and another just to the right. Join the two dots with a wire that loops below the lamp, and draw a voltmeter in the middle of that loop.
Now read it. Start at the battery and follow the wire round. You pass through the switch, the lamp and the ammeter, one after the other, and arrive back at the battery. They are in one loop, so they are in series. The voltmeter is on a side branch across the lamp, so it is in parallel with the lamp. The switch is open, so there is a gap in the loop and the lamp is off. Close the switch and the lamp lights.
Common mistakes
- Forgetting the prefix. 20 mA is 0.02 A, not 20 A. Change mA, kΩ and kW before you calculate.
- Leaving gaps in a diagram. Any gap in a wire is a break in the circuit. Draw the wires with a ruler so they touch every symbol.
- Drawing the voltmeter in the main loop. A voltmeter goes across a component, in parallel. Only the ammeter goes in the loop.
Exam-style question
A student builds a circuit to test a heater. The circuit diagram shows one loop containing a d.c. power supply, a fuse, a heater and an ammeter. A voltmeter is connected across the heater.
(a) Describe the circuit symbol for a heater. [1 mark]
(b) Describe how the symbol for a fuse is different from the symbol for a fixed resistor. [1 mark]
(c) State how the ammeter and the voltmeter are each connected to the heater. [2 marks]
(d) The ammeter reads 450 mA. State this current in amperes. [1 mark]
(e) The heater is labelled 0.02 kW. State the quantity that is measured in watts, and give this value in watts. [2 marks]
Model answer
(a) A rectangle divided into four small boxes by three lines (1).
(b) The fuse has a line (the wire) drawn straight through the rectangle, but the fixed resistor is an empty rectangle (1).
(c) The ammeter is in series with the heater (1). The voltmeter is in parallel with the heater (1).
(d) 450 / 1000 = 0.45 A (1)
(e) Power (1). 0.02 × 1000 = 20 W (1)
Exam tip
The words 'in one loop' and 'across' in the question tell you the answer to part (c) before you start. In one loop means in series, and across means in parallel. For the unit changes in (d) and (e), check the size of your answer: milli makes the number smaller, kilo makes it bigger.