Topic 1 of 5
Units and prefixes
Every electrical calculation starts with the right units. Get the prefix wrong and your answer is out by a thousand.
The main quantities
| Quantity | Symbol in formulas | Unit | Unit symbol |
|---|---|---|---|
| Current | I | ampere (amp) | A |
| Voltage (potential difference) | V or U | volt | V |
| Resistance | R | ohm | Ω |
| Power | P | watt | W |
| Energy | E or W | joule (or kilowatt-hour) | J (kWh) |
| Charge | Q | coulomb | C |
| Frequency | f | hertz | Hz |
Watch the trap: the symbol for current in a formula is I, but its unit is the amp, A. UK mains is 230 V at 50 Hz.
Prefixes
| Prefix | Symbol | Multiply by | Where you'll see it |
|---|---|---|---|
| mega | M | 1,000,000 | Insulation resistance, e.g. 200 MΩ |
| kilo | k | 1,000 | Power and loads, e.g. 9.5 kW shower |
| milli | m | ÷ 1,000 | RCD trip current, e.g. 30 mA |
| micro | µ | ÷ 1,000,000 | Capacitors, e.g. 10 µF |
Worked examples
- 30 mA = 30 ÷ 1,000 = 0.03 A
- 7.2 kW = 7.2 × 1,000 = 7,200 W
- 2 MΩ = 2 × 1,000,000 = 2,000,000 Ω
- 0.5 kΩ = 0.5 × 1,000 = 500 Ω
Exam tip: convert everything to base units (A, V, Ω, W) before you put it into a formula, then convert the answer back at the end.
Mini test
6 questions. Pick an answer to see if you're right and why.