Electric Circuits

What is a circuit?

A circuit is a path that lets electric charge flow around and back to where it started. A steady current needs two things: a complete loop, and a source of energy that keeps pushing the charges around it.^[1] Break the loop anywhere, for example by opening a switch, and the current stops everywhere in it.

The source: batteries and emf

A battery is a two-terminal device that keeps one terminal at a higher electric potential (voltage) than the other.^[1] Its “push” is called the electromotive force, or emf. Despite the name, emf isn’t a force at all: it is the work done on each unit of charge, measured in volts. The name stuck for historical reasons.^[1]

Every real battery also has some internal resistance, so the voltage at its terminals drops a little when it delivers more current. Internal resistance grows as a battery runs down.^[1] Car batteries (lead-acid) and the lithium-ion batteries in phones and laptops all behave this way.^[1]

Voltage, current and resistance

Three quantities describe what happens in a circuit:

• Current (I) is the rate at which charge flows, measured in amperes.^[4]
• Voltage (V) is the difference in electric potential that drives it, measured in volts.^[5]
• Resistance (R) is how strongly a component opposes the current, measured in ohms.^[5]

For many components they are linked by Ohm’s law, V = I × R.^[5] The power a component uses is P = I × V, and the electrical energy it uses ends up as heat in resistors and wires.^[6]

Series and parallel

Components can be connected in two basic ways.

In series, one after another, the current that leaves the first component flows into the next, so the current is the same in each.^[2] The total resistance is the sum of the individual resistances, and the supply voltage is shared between them.^[2]

In parallel, side by side, all the leads on one side are connected together, and all the leads on the other side too. Each component then has the same voltage across it, and the total current splits between the branches.^[2] The combined resistance is always smaller than the smallest single resistance.^[2]

The math (optional)

For resistors in series: R = R₁ + R₂ + R₃ + …

For resistors in parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ + …^[2]

Example: two 100-ohm resistors in series make 200 ohms. The same two in parallel make 50 ohms.

That is why a car’s headlights, radio and other systems are wired in parallel: each gets the full voltage of the battery and can work completely independently.^[2] Household wiring follows the same idea.^[2]

Staying safe

This page explains how circuits work. It is not a wiring guide. Mains electricity can kill, and electrical work must follow your local electrical code and be done by a qualified electrician.

There are two main hazards.^[3] For workplaces, NIOSH (part of the US Centers for Disease Control and Prevention) lists electrocution, electric shock, burns and arc blasts, and falls from height, often involving ladders and overhead power lines, as the main ways electricity injures people.^[7]

• Thermal hazard: too much current can overheat wires and start a fire inside a wall.
• Shock hazard: current passing through a person. Shocks range from painful but harmless to fatal.

The body is very sensitive to current. About 1 milliamp is the threshold of feeling, and around 5 milliamps is treated as the maximum harmless shock. Between 5 and 30 milliamps, muscles can contract so that a person can’t let go. A current above 300 milliamps through the heart can be fatal.^[3]

Buildings are protected in several ways:^[3]

• Fuses and circuit breakers limit excessive currents, stopping wires from overheating.
• A ground (earth) wire holds an appliance’s metal case at zero volts, so it can’t give a shock if a live wire inside touches it.
• A ground fault circuit interrupter (GFCI) compares the currents in the live and neutral wires and trips when they are not equal, which means current is leaking somewhere it shouldn’t.

For people who work with electricity, NIOSH’s advice starts with the basics: de-energize (switch off) circuits before doing any work, use lockout devices so they can’t be switched back on, and leave live equipment to qualified people using proper protective equipment.^[7] The electricity delivered to homes is itself stepped down to a lower voltage, which is safer to use.^[8]

Going further

Build your own circuits safely in the free simulation below, then read Ohm’s Law for the rule that ties voltage, current and resistance together.