Electrical power and mains electricity
Mains electricity in UK homes is an alternating supply. This page covers how plugs keep you safe, the power equations, the energy an appliance transfers, and why the National Grid uses very high voltages.
Part 1 of 3: Learn it
In short
- UK mains: alternating current at 230 V and 50 Hz.
- P = VI and P = I²R; energy transferred E = Pt or E = QV.
- The National Grid steps the voltage up to reduce the current, so less energy is lost heating the cables.
Where this is in your specification
Spec points: AQA 4.2.3 and 4.2.4 (Combined Trilogy 6.2.3 and 6.2.4), Edexcel Topic 10, OCR P3.3
| Board | Topic: Mains electricity, the National Grid and static |
|---|---|
| AQA 8463 | 4.2.3, 4.2.4 and 4.2.5 (8463); 6.2.3 and 6.2.4 (8464) |
| Edexcel 1PH0 | Topic 10 (static: 1PH0 Topic 11) |
| OCR J249 | P3.3 (static: J249 P3.1) |
| Eduqas C420 | 7.3, 7.4 |
| Cambridge IGCSE 0625 | 4.2, 4.4 |
| National 5 C857 75 | Electricity: electrical power |
ac and dc
Direct current (dc) flows in one direction only, as from a cell or battery. Alternating current (ac) keeps changing direction. The UK mains supply is ac at a frequency of 50 Hz and about 230 V.
The three-core cable
| Wire | Colour | Job |
|---|---|---|
| Live | brown | carries the alternating potential difference from the supply |
| Neutral | blue | completes the circuit, at or close to 0 V |
| Earth | green and yellow | a safety wire that only carries current if there is a fault |
The live wire is dangerous even when a switch is off, because it is still at a high potential compared with you. Touching it would let current flow through your body to earth.
Power and energy
P is power (W), V is potential difference (V), I is current (A), R is resistance (Ω), E is energy (J), t is time (s) and Q is charge (C). The power rating on an appliance tells you how much energy it transfers each second.
The National Grid
Power stations are connected to homes by a network of cables and transformers. Step-up transformers raise the potential difference to very high values for transmission. For the same power, a higher potential difference means a smaller current, and a smaller current means much less energy wasted heating the cables. Step-down transformers lower it again to a safe level for homes.
What current does a 2300 W heater draw from the 230 V mains?
Show the answer
I = P ÷ V = 2300 ÷ 230 = 10 A.
Part 2 of 3: See it worked
Worked examples
Example 1
A hairdryer draws a current of 5.0 A from the 230 V mains. What is its power?
- P = V × I
- P = 230 × 5.0
Answer: 1150 W.
Example 2
A current of 0.30 A flows through a 40 Ω resistor. What is the power transferred?
- P = I² × R
- I² = 0.30² = 0.09
- P = 0.09 × 40
Answer: 3.6 W.
Example 3
A 1.5 kW heater is switched on for 20 minutes. How much energy does it transfer?
- P = 1500 W, t = 20 × 60 = 1200 s
- E = P × t = 1500 × 1200
Answer: 1 800 000 J (1.8 MJ).
Common mistakes
- Forgetting to square the current in P = I²R.
- Using minutes in E = Pt instead of seconds.
- Mixing up the colours of the live and neutral wires.
- Saying the National Grid uses high voltage to make electricity travel faster. It is to make the current smaller and cut heating losses.
What colour is the earth wire, and when does it carry current?
Show the answer
Green and yellow; only when there is a fault.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What current does a 2300 W heater draw from the 230 V mains?
I = P ÷ V = 2300 ÷ 230 = 10 A.
What colour is the earth wire, and when does it carry current?
Green and yellow; only when there is a fault.
30 C of charge moves through a potential difference of 12 V. How much energy is transferred?
E = QV = 30 × 12 = 360 J.
Jobs that use this
- Electrician (s'ouvre dans un nouvel onglet)
- Electrical engineer (s'ouvre dans un nouvel onglet)
- Renewable energy engineer (s'ouvre dans un nouvel onglet)
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Ces fiches sont en anglais car elles suivent les programmes d'examen britanniques.
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses