AC and DC

AC and DC are the two kinds of electric current. In alternating current (AC), the current changes direction many times a second. India's mains supply completes 50 full cycles a second, so the current reverses 100 times a second. In direct current (DC), the current moves steadily in one direction. Batteries and solar panels give DC, while the sockets in homes and the power grid use AC.

Here is why both are needed. AC is easy to raise and lower in voltage with a transformer, so it is used to carry power over long distances at high voltage and then reduce it for homes. DC is what electronics and batteries need. So something has to convert between the two. A charger or adapter converts AC from the socket to DC for a phone. An inverter converts DC from a battery or solar panel to AC for the home or the grid. Each conversion loses a small part of the energy as heat.

The reason AC is used for long lines is the loss in the wire. The power lost as heat in a wire grows with the square of the current. For the same power, a higher voltage means a lower current, so the loss falls. A transformer can raise AC to hundreds of thousands of volts for a long line and lower it again near homes. It cannot do this for DC. DC and AC are both dangerous to touch. A DC switch needs special design, because DC does not fall to zero many times a second, so the spark across a switch does not die out by itself.

Take, for example, a laptop charger. It takes 230 volts AC from the wall and gives about 20 volts DC to the laptop. Consider a rooftop solar plant. Its panels give DC, and an inverter turns it into AC so the house and the grid can use it; see solar inverters and microinverters. Imagine an electric car. It charges its battery with DC, so a fast charger converts the grid's AC outside the car, while a slow charger lets the car's own converter do it; see EV chargers.

Long power lines sometimes carry high-voltage DC, because it loses less energy over very long distances. The grid itself is described in the grid, and the battery is described in battery cells, modules and packs.

How is it different from voltage? Voltage is how strong the push of the current is. AC and DC describe the direction in which the current moves.