The difference between d.c. and a.c., what the UK mains supply is, the three wires in a cable and why the live wire is dangerous.
What you will learn
- Tell a.c. from d.c. and read the peak p.d., period and frequency off a trace
- State the UK mains p.d. and frequency
- Give the colour and job of the live, neutral and earth wires
- Explain why touching the live wire is dangerous even with the switch off
One way, or back and forth
A cell or a battery pushes charge round a circuit in one direction only.
Its potential difference is direct: one terminal is always positive. The current is direct current (d.c.).
The mains supply is different. Its potential difference keeps swapping direction, so the current flows one way, then the other, many times a second. That is alternating current (a.c.).
The flat line is a 6 V d.c. supply. The wave is an a.c. supply with a peak of 10 V. The peak potential difference is measured from 0 V up to the top of a peak, not from the bottom of a trough. One complete cycle (peak to the next peak) is the period, here 20 ms. The frequency is the number of cycles each second:
frequency = 1 ÷ period
f = 1 ÷ T
Frequency in hertz (Hz), period in seconds (s).
- T = 20 ms = 0.020 s
- f = 1 ÷ 0.020
- f = 50 Hz
Change milliseconds to seconds before you divide: 1 ÷ 20 gives 0.05, not 50.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- One way, or back and forth
- The UK mains supply
- Why the live wire is dangerous
- Try it: Read an a.c. trace
10 practice questions, marked as you go; about 20 minutes in all.
Key terms
Peak, Period, Frequency, Fuse, Circuit breaker
In this unit: Mains electricity, the National Grid and static
Direct and alternating current, the three-core cable and plug, electrical power and energy, the cost of electricity, the National Grid and transformers, and static charge and electric fields.
- Direct and alternating current, and the mains supply
- Electrical power, P = VI and P = I²R
- Energy transferred, kilowatt-hours and cost
- The National Grid and transformers
- Static electricity
- Electric fields