DC and AC
DC = one direction (cells). AC = oscillating (mains).
Direct current (DC) flows always in the same direction. Examples: torch batteries, mobile-phone batteries, USB outputs.
Alternating current (AC) flows first one way then the other, reversing many times per second. The UK national grid supplies AC at 50 Hz — meaning 50 complete cycles per second.
Why AC for mains? AC can be transformed up or down in voltage using a transformer (covered in 4.2.4.3). This lets electricity be transmitted across the National Grid at very high voltage (low current = low energy loss) and stepped down for safe domestic use.
UK mains values to memorise:
- Voltage: 230 V (this is the rms or 'effective' voltage; the peak is about 325 V).
- Frequency: 50 Hz.
- Period: = 20 ms.
DC: constant direction (battery).
AC: oscillating (mains).
UK mains: 230 V, 50 Hz.
AC enables transformer use → efficient transmission.