Direct current

1. In a direct current, written d.c., the charge flows in one direction only and never reverses.
2. A cell or a battery has a fixed positive terminal and a fixed negative terminal, so the potential difference it supplies always acts the same way round and the conventional current in the external circuit always passes from the positive terminal to the negative terminal.
3. Sources of direct current include a single cell, a battery of cells in series, a d.c. power pack on the bench, a solar cell, and the low potential difference output of a phone charger.
4. The size of a direct current can be changed, for example by adjusting a variable resistor, while its direction stays the same, so d.c. means one direction rather than one fixed value.
5. Because the direction never changes, a direct supply can drive components that only work one way round, such as an LED or a rechargeable cell being charged.
6. Reversing the connections to a d.c. supply reverses the current in the circuit, which is why the terminals are marked $+$ and $-$.

Alternating current

1. In an alternating current, written a.c., the direction of the current reverses repeatedly, many times every second.
2. The potential difference of the supply alternates, so the terminal that is positive relative to the other for one half of a cycle becomes negative for the next half, and the push on the charge reverses with it.
3. The delocalised electrons in the wire therefore oscillate backwards and forwards about a fixed position instead of drifting steadily one way round the circuit.
4. One complete cycle is a push in one direction followed by a push in the other, which returns the supply to the state it started from.
5. The mains supply that reaches the sockets in a house is alternating, and so is the output of a generator.
6. An alternating current still transfers energy, because the energy dissipated in a resistance depends on the size of the current and not on the direction in which that current happens to be flowing.
7. A mains heater therefore warms up during every half cycle, and it does not cool between one reversal and the next.

The UK mains supply

1. The mains supply in the United Kingdom is an alternating supply of about $230V$ with a frequency of $50Hz$.
2. The $230V$ figure describes the size of the alternating potential difference between the live wire and the neutral wire, and it is this potential difference that drives the current through an appliance.
3. The $50Hz$ figure is the frequency of the supply, meaning that the supply completes $50$ full cycles every second.
4. The current reverses twice in each cycle, so it changes direction $100$ times every second, which is far too fast to be seen as any flicker in a lamp.
5. The period $T$ is the time taken for one complete cycle, and it is linked to the frequency by $T=\frac{1}{f}$, with $T$ in seconds and $f$ in hertz.
6. A supply at $50Hz$ therefore has a period of $\frac{1}{50}=0.020s$, which is $20ms$.
7. The potential of the live wire alternates because the supply it is connected to alternates, and that reversal is passed unchanged along the cables to the socket.

Reading supply graphs

1. A graph of current against time, or of potential difference against time, shows at once which kind of supply is connected.
2. A direct supply gives a trace that stays on one side of the time axis, because the value keeps the same sign throughout.
3. A direct supply whose size varies gives a trace that changes height yet still never crosses the time axis.
4. An alternating supply gives a trace that rises above the time axis and falls below it in turn, crossing the axis at every reversal.
5. Each crossing of the time axis marks an instant when the current is momentarily zero and about to flow the other way.
6. The peak value is the greatest distance of the trace from the time axis, and it is reached twice in every cycle, once above the axis and once below it.
7. The period is read as the time from one peak to the next peak, or from one crossing of the axis to the second crossing after it, and the frequency then follows from $f=\frac{1}{T}$.
8. A flat trace lying exactly along the time axis means there is no current at all, so the switch is open or the supply is off.

Devices and their supply

1. A filament lamp and a heater work on either supply, because the heating effect depends on the size of the current through the resistance and not on its direction.
2. A kettle element connected to the mains therefore transfers energy at the same rate as it would on a direct supply of the same potential difference.
3. Many electronic devices need direct current, because their circuits contain components such as diodes and integrated circuits that only work with a fixed polarity.
4. A laptop, a phone and an LED lamp are supplied through an adapter that converts the alternating mains into direct current at a lower potential difference.
5. Charging a rechargeable cell also needs direct current, because the charging current has to be driven through the cell in one particular direction.
6. A torch running from cells is on direct current from the start, so no conversion is needed inside it.