Mains electricity in the UK is an alternating current (a.c.) at 230 V and 50 Hz; in the USA it is about 120 V and 60 Hz. Batteries supply direct current (d.c.), which flows one way. A three-core cable has a live wire (brown) at 230 V, a neutral wire (blue) near 0 V, and an earth wire (green and yellow) that carries current to the ground if there is a fault. Touching the live wire is dangerous because a large potential difference is set up across your body.
Power is P = V × I, or P = I² × R. Energy transferred is E = P × t, or E = Q × V. The National Grid uses step-up transformers to raise the voltage for transmission, which lowers the current and so reduces heating losses in the cables; step-down transformers lower the voltage for homes.
Worked example
A 230 V hairdryer draws 5 A. What is its power?
- P = V × I.
- P = 230 × 5.
Answer:P = 1150 W.
Key words
- alternating current
- current that keeps reversing direction
- live wire
- brown wire at 230 V
- earth wire
- safety wire that carries fault current to ground
- National Grid
- network of cables and transformers
New questions every time.
NGSS-referenced: HS-PS3-3