Section 1 of 5Direct Current
Direct current is produced by cells and batteries, which have fixed positive and negative terminals.
Definition
- A steady current that flows in one direction only, from the positive terminal to the negative terminal of the supply.
Source
- Cells and batteries produce d.c. because their terminals are always fixed as positive and negative.
Circuits
- Circuits powered by cells or batteries, such as a torch, are d.c. circuits in which the current never reverses.
Oscilloscope trace
- A d.c. supply appears as a straight line that stays on one side of the time axis.
Section 2 of 5Alternating Current
Alternating Current
Definition
- A current that continuously changes its direction, so it flows back and forth around the circuit.
Frequency
- The number of times the current changes direction each second, measured in hertz (Hz).
UK mains
- The mains supply is an alternating current with a frequency of 50 Hz and a nominal potential difference of 230 V.
Source
- Mains a.c. is produced by electrical generators, not by cells or batteries.
Section 3 of 5Comparing a.c. and d.c.
The table summarises the main differences, and the steps below show how they appear on an oscilloscope.
1
A d.c. signal appears as a straight line on the oscilloscope screen, because its value stays constant.
2
An a.c. signal appears as a wave that crosses the time axis each time the current reverses direction.
3
The time period of the wave is the time for one complete cycle, which can be read from the time base.
4
Because f = 1 / T, doubling the frequency of an a.c. supply halves its time period.
| Feature | Direct current (d.c.) | Alternating current (a.c.) |
|---|---|---|
| Direction | One direction only | Changes direction repeatedly |
| Source | Cells and batteries | Generators, such as the mains supply |
| Terminals | Fixed positive and negative | Two identical terminals that alternate in polarity |
Section 4 of 5Worked Example
Question
- The UK mains supply has a frequency of 50 Hz.
- Calculate the time period of one complete cycle.
- Write the equation linking frequency and time period: T = 1 / f.
- Substitute the frequencyT = 1 / 50 Hz.
- CalculateT = 0.02 s, which is 20 ms.
Answer The time period of one complete cycle of mains electricity is 0.02 s.
Section 5 of 5Exam Checklist & Tips
2.3.5 – Key Points for the Exam
- Direct current (d.c.) A current that flows steadily in one direction only.
- Alternating current (a.c.) A current that repeatedly reverses its direction.
- Frequency The number of complete cycles per second of an alternating current.
- Hertz (Hz) The unit of frequency, equal to one cycle per second.
- Mains electricity The a.c. supply delivered to homes and buildings, at about 230 V and 50 Hz in the UK.
Common Mistakes
- Saying that a.c. flows from one terminal to the other without reversing; its direction changes repeatedly.
- Giving 50 Hz as the voltage of the mains; 50 Hz is the frequency and 230 V is the potential difference.
- Forgetting to convert frequency into time period with T = 1 / f when a period is asked for.
- Assuming all circuits use a.c.; cell- and battery-powered circuits use d.c.