What are electric current and voltage?

Everything is made of atoms, and atoms hold tiny particles with electric charge: positive protons and negative electrons. In a metal wire some electrons are free to wander. Charge is measured in coulombs (C). One coulomb is the charge of about 6.24 billion billion electrons.

Current is how much charge flows past a point each second: I = Q ÷ t. Its unit is the ampere (A), one coulomb per second. An ammeter measures it, placed in the loop so the flow passes through it. A torch bulb takes about 0.3 A.

Voltage is the push: the energy each coulomb carries from the battery to the bulb. Its unit is the volt (V), one joule per coulomb. A voltmeter measures it across a part, one lead on each side. Each AA cell gives 1.5 V; put two in a row and each coulomb picks up 3 joules. How much current that push drives depends on the resistance, which is Ohm’s law (see OhmsLawClear).

Two surprises. The electrons crawl: about 0.1 mm per second in a torch wire, slower than a snail. But the wire is already full of them, and the push races round the loop at nearly the speed of light, so the bulb lights at once. And the arrows on circuit diagrams point the “wrong” way. Conventional current flows from + to −, a choice Benjamin Franklin made in the 1740s, long before anyone knew electrons exist. Electrons, being negative, really drift from − to +.

Think of water in a pipe: the battery is a pump, voltage is the pressure, current is the flow. It helps, but it breaks down. Cut a pipe and water pours out; cut a wire and nothing leaks, the current just stops. And the pipe does not need to be a loop, but a circuit does.

There are two ways to connect several things to one battery. In series they sit one after another in a single loop, like beads on a string. In parallel each one gets its own path between the battery’s two ends, like the rungs of a ladder.

Series: the same current flows through every bulb, but they share the battery’s voltage. Two bulbs get half each, three get a third, so each glows far dimmer. Worse, if one bulb breaks, the loop is broken and all go dark. Old fairy lights were wired like this: about 50 little bulbs sharing 230 V, and one dead bulb meant hunting along the whole string.

Parallel: every bulb gets the full voltage and shines at full brightness, and one broken bulb does not affect the others. The currents add up: four bulbs take four times the current from the battery. Your house is wired in parallel, so every socket gets the full 230 V and each switch controls only its own branch.

That adding-up is why we need fuses. A fuse is a thin wire in the main line that melts if the current gets too big, breaking the loop before the wiring overheats. A short circuit, a thick wire straight across the battery with no bulb in the way, drives a huge current. Chapter 3 shows the MCBs that do this job in a home.

The sockets in an Indian home supply 230 V of alternating current (AC) at 50 hertz: the push flips direction 50 times a second, so the electrons in the wires just jiggle back and forth. In the USA it is 120 V at 60 Hz. A battery gives direct current (DC), a steady push one way. An inverter turns a battery’s DC into AC for your home during power cuts (see UPSClear). AC won because a transformer can easily step its voltage up for long-distance lines and back down for homes (see FaradayClear).

Three wires come to each socket. Live (brown, or red in older Indian homes) carries the push. Neutral (blue, or black) is the return path. Earth (green, or green and yellow) is a safety wire bolted to metal cases and to a rod in the ground. Everything is wired in parallel (chapter 2), so every appliance gets the full 230 V.

Each appliance draws I = P ÷ V. A ceiling fan takes about 0.3 A, an AC about 7 A and a 2 kW geyser 8.7 A. That’s why a geyser needs a big 16 A socket: its current would overheat a small 6 A one. The same 2 kW in the USA takes 17 A, so big American heaters run on 240 V instead.

In the distribution board, each circuit has an MCB (miniature circuit breaker): a switch that trips when too much current flows, like a fuse you can reset. The RCCB watches for current leaking to earth. If live and neutral differ by just 30 mA, current is going somewhere it shouldn’t, maybe through a person, and it cuts the power in a few hundredths of a second. What you pay for is energy: one unit on the bill is one kilowatt-hour (see WorkClear).

A cell turns chemical energy into electrical energy. It has two different materials, the electrodes, in a chemical called the electrolyte. Their reaction pushes electrons out of the negative electrode, round the circuit and into the positive one. Inside the cell the loop is closed by ions, charged atoms drifting through the electrolyte. A battery is several cells joined together.

The chemistry sets the voltage: 1.5 V for an alkaline AA cell, about 3.7 V for a lithium-ion cell, 2.1 V for each lead-acid cell. Join cells in series and the voltages add: six lead-acid cells make a 12 V car battery, and about 96 lithium cells make an electric car’s pack of nearly 400 V (see CarClear).

How long it lasts depends on its capacity. A phone battery might say 5,000 mAh: it could give 5 A for one hour, or 0.5 A for ten. Multiply by the voltage and you get the energy it stores in watt-hours: 3.7 V × 5 Ah = 18.5 Wh. An electric car stores about 30,000 Wh, the same as 1,600 phones.

The rate of using energy is power: P = V × I, in watts (see WorkClear). Divide the energy by the power and you get the running time. Your body runs on electricity too: nerve cells fire tiny voltage pulses of about a tenth of a volt (see NervousClear).

A bird on a power line is safe. Both its feet are on the same wire, so there is almost no voltage between them: about a thousandth of a volt. With no difference in push, almost no current flows through the bird. If a big bird’s wing touches the wire and the earthed pole at the same time, it bridges thousands of volts and is killed. This really happens to vultures and eagles on Indian power poles.

What harms you is the current through your body, and that depends on the voltage and your resistance: I = V ÷ R. Dry skin resists strongly. Wet skin resists about a hundred times less, so the same 230 V drives a hundred times the current. About 1 mA can be felt, about 10 mA makes your muscles clench so you can’t let go, and about 100 mA across the chest can stop the heart. That’s why RCCBs trip at 30 mA, and why you never touch switches with wet hands.

Lightning is the extreme: about 30,000 A and hundreds of millions of volts, for a few millionths of a second. A lightning rod gives it a thick metal path to the ground. In India lightning kills nearly 3,000 people a year: in a storm, go indoors, and stay away from lone trees and open fields.

Myth: “a battery stores electrons.” It doesn’t. It stores chemical energy. Every electron that leaves the − end comes back in at the + end, so a flat battery has just as many electrons as a full one (chapter 4).

Myth: “higher voltage always means more current.” A 230 V geyser takes 8.7 A, but a car’s 12 V starter motor takes about 150 A. Current depends on the resistance too (see OhmsLawClear).

Stay safe: never touch mains wiring or open sockets, never poke anything into a socket, and never go near a fallen power line. For any electrical fault, switch off at the board and call a qualified electrician.