Section 1 of 8Direct current (d.c.)
In a direct current the charge flows in one direction only. The current may be steady or it may vary in size, but it never reverses. Cells and batteries are the usual d.c. sources: one terminal is always positive and the other always negative, so electrons are always pushed round the circuit the same way.
Section 2 of 8Alternating current (a.c.)
In an alternating current the charge flow repeatedly reverses direction. The potential difference of the supply swaps its polarity again and again, so the current rises to a maximum in one direction, falls to zero, rises to a maximum in the opposite direction, and falls to zero again. This is one complete cycle.
An alternator (a.c. generator) produces this naturally: a coil turning in a magnetic field has a voltage induced in it which changes direction every half turn. The faster the coil turns, the higher the frequency and the larger the peak voltage.
The electrons do not travel round the circuit and reach the appliance. They shuffle back and forth about a fixed position, yet energy is still transferred, because the lamp or heater warms up whichever way the current passes through it.
Section 3 of 8Period, frequency and peak voltage
Three quantities describe an a.c. supply.
Section 4 of 8Reading an oscilloscope trace
A cathode-ray oscilloscope (CRO) plots potential difference on the vertical axis against time on the horizontal axis. Two dials set the scale.
Reading the trace:
A d.c. source gives a flat horizontal line, above or below the centre line depending on its polarity. An a.c. source gives a wave. A larger peak voltage makes the wave taller; a higher frequency squeezes more waves across the screen when the time base is unchanged.
Section 5 of 8UK mains electricity
Mains electricity in the UK is a.c. at about 230 V and 50 Hz.
A.C. is used for the national grid because its voltage can be changed easily with transformers. Raising the voltage for long-distance transmission lowers the current for the same power, which reduces energy lost as heat in the cables.
Linksthe power rule P = IV from the previous sub-topics applies to a.c. appliances, and the fuse and earth wire in a plug protect against faults in the a.c. supply.
Section 6 of 8Worked examples
A generator produces an alternating supply with a frequency of 400 Hz. Calculate its period in milliseconds.
T = 1 / f = 1 / 400 = 0.0025 s.
What the symbols mean: T = period, the time for one complete wave (s) · f = frequency (Hz)0.0025 s × 1000 = 2.5 ms.
On an oscilloscope the Y-gain is set to 2 V/div and the time base to 5 ms/div. One complete wave occupies 4 horizontal divisions, and each crest is 3 divisions above the centre line. Find (a) the peak voltage, (b) the period and (c) the frequency.
(a) V0 = 3 × 2 = 6 V.
(b) T = 4 × 5 = 20 ms = 0.020 s.
(c) f = 1 / T = 1 / 0.020 = 50 Hz.
How many complete cycles does 50 Hz mains complete in 2 minutes, and how many times does the current change direction in 1 second?
2 minutes = 120 s. Cycles = 50 × 120 = 6000 cycles.
Each cycle contains two reversals, so in 1 s there are 50 × 2 = 100 changes of direction.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Direct current (d.c.) | Current that flows in one direction only. |
| Alternating current (a.c.) | Current that repeatedly reverses its direction. |
| Frequency | Number of complete cycles per second, measured in hertz (Hz). |
| Period | Time taken for one complete cycle, in seconds. |
| Peak voltage | The maximum potential difference of an a.c. wave, measured from zero. |
| Oscilloscope | Instrument that displays a graph of potential difference against time. |
| Time base | Oscilloscope setting that gives the time represented by each horizontal division. |
Section 8 of 8Exam technique
- frequency is cycles per second (Hz), period is seconds per cycle;
- one is the reciprocal of the other.