What is alternating current?

A remote-controlled car runs on batteries. A fridge runs on electricity from a wall socket. Both use electric current, but there is a surprising difference between them.

The current from a battery always flows the same way. The current from a socket keeps turning around!

One way: direct current

A battery has a minus end (−) and a plus end (+). It pushes electrons out of its minus end, around the circuit and back into its plus end. They always go the same way.

A current that always flows in one direction is called direct current, or DC for short. Batteries and solar panels make direct current.

Back and forth: alternating current

The electricity in a wall socket works differently. Its push keeps swapping sides. First it pushes the electrons one way, then the other way, then back again, over and over.

So the electrons in the wires do not travel around the circuit. They rock back and forth on the spot, by less than the width of a hair. A current that keeps changing direction like this is called alternating current, or AC.

In direct current, the electrons drift one way. In alternating current, they rock back and forth, so the current keeps changing direction.

Switch between direct and alternating current and watch the electrons and the graph. The swinging is slowed right down so you can see it.

A kettle heats up with either kind of current. Its electrons bump into atoms whichever way they move, and every bump makes heat. That is what resistance does.

What comes out of a socket?

In Europe, the push in a wall socket, the voltage, is about 230 volts. It swings back and forth 50 times every second. We say it has a frequency of 50 hertz (Hz). One hertz means once every second.

Other countries use different numbers. In North America, sockets give 120 volts at 60 hertz. Many chargers work with both, but some devices only work in the countries they were made for.

Why do we use alternating current?

Power stations are often far away from the towns that need electricity. The electricity travels there through long power lines. On the way, the wires waste some energy as heat, because of their resistance.

How much energy is wasted depends on the current. A big current heats the wires a lot, and a small current heats them only a little.

Power is push times flow. So engineers send the same power with a huge push and a small flow: hundreds of thousands of volts. Much less energy is wasted on the way.

But such a high voltage would be deadly in a home. So it must be lowered again, and alternating current makes that easy.

A machine called a transformer can raise or lower the voltage of alternating current. It has no moving parts, only two coils of wire wound around a block of iron. A transformer does not work with direct current.

At the power station, a transformer raises the voltage for the long journey. Near your home, other transformers lower it again, step by step, to 230 volts.

Chargers turn AC into DC

Phones, laptops and most gadgets need a small, steady direct current. So their chargers do two jobs. They lower the voltage from 230 volts to a few volts, for example 5 volts for many phones. And they turn alternating current into direct current.

TVs and computers have similar parts built inside.

The war of the currents

About 140 years ago, people argued fiercely about which kind of current to use. Thomas Edison built one of the first power networks, in New York in 1882. It used direct current, which could only reach homes less than about 1.5 kilometres from the power station.

Nikola Tesla, an inventor, and George Westinghouse, a businessman, bet on alternating current instead. With transformers, it could travel much farther. Edison warned that its high voltages were dangerous.

In 1893, alternating current lit up a huge world fair in Chicago. Soon it had won this “war of the currents”. You can read the whole story in the history of electricity.

Go deeper: The shape of alternating current for grown-ups and the extra curious

In a European socket, the voltage does not jump between two values. It rises and falls smoothly, following a curve called a sine wave.

The voltage in a European socket over time. It swings between about +325 and −325 volts, and one full swing takes 20 milliseconds.

One full swing is called a cycle. At 50 Hz, one cycle takes $\frac{1}{50}$ of a second, which is 20 milliseconds. The current changes direction twice in every cycle, so 100 times a second.
The voltage at any moment is $v(t) = V_\text{peak} \cdot \sin(2\pi f t)$ . Here $v(t)$ is the voltage at time $t$ (in seconds), $V_\text{peak}$ is the highest voltage and $f$ is the frequency (50 Hz).
Why do we say 230 volts, when the peak is about 325? The 230 V is a kind of average called the RMS value (root mean square). An AC voltage of 230 V heats a kettle exactly as much as a steady DC voltage of 230 V would. For a sine wave, the peak is $\sqrt{2} \approx 1.41$ times bigger: $230\ \text{V} \times \sqrt{2} \approx 325\ \text{V}$ .

Go deeper: How a transformer works for grown-ups and the extra curious

A transformer has two coils of wire wound around the same iron core. Alternating current in the first coil makes a magnetic field that keeps growing, shrinking and flipping. This changing field creates a voltage in the second coil. Michael Faraday discovered this effect, called electromagnetic induction, in 1831. The voltages are in the same ratio as the numbers of turns:

\frac{V_2}{V_1} = \frac{N_2}{N_1}

Here $V_1$ and $V_2$ are the voltages of the first and the second coil, and $N_1$ and $N_2$ are their numbers of turns. A steady direct current makes a steady field, which creates no voltage at all. The power stays almost the same: when the voltage goes up, the current goes down.

Go deeper: How much does a high voltage save? for grown-ups and the extra curious

The power wasted as heat in a line is $P_\text{loss} = I^2 \cdot R$ , where $I$ is the current and $R$ is the resistance of the line (see electric power). Imagine sending one million watts (1 MW):

at 10,000 V, the current is 1,000,000 ÷ 10,000 = 100 A;
at 400,000 V, the current is 1,000,000 ÷ 400,000 = 2.5 A.

The current is 40 times smaller, so the heat wasted in the same wire is $40^2 = 1{,}600$ times smaller.

Direct current is making a comeback, though. Some very long power lines and undersea cables use high-voltage direct current (HVDC). Modern electronics can now change the voltage of DC too, and over very long distances, especially under the sea, DC works better than AC.

Go deeper: Where the saw picture goes wrong for grown-ups and the extra curious

A saw is a good picture of the rocking, but not of where the energy comes from. The electrons in your kettle never came from the power station: they stay in the kettle’s wire and only rock back and forth. What travels along the power lines is the push, and it moves almost as fast as light. So energy flows one way, from the power station to the kettle, even though the electrons go nowhere.

Check yourself

Why do power lines carry electricity at a very high voltage?

Show the answer

Less energy is wasted as heat on the way — With a big push, the same power needs only a small current. A small current wastes much less energy heating the wires.