Current is a flow of charge

An electric current is a flow of electric charge around a circuit. It only flows if there is a complete loop with no gaps, driven by a cell or battery.

• Current is measured in amperes (amps), symbol A.
• You measure current with an ammeter, connected in series (in the loop).
• The bigger the current, the more charge flows past each point every second.

Watch out: current is not used up by a lamp. The same current flows all the way round a single loop — the lamp transfers energy, but it does not "eat" the current.

What is current?

Which way does current go?

Inside a metal wire the tiny particles that actually move are electrons, and they carry a negative charge. But long before electrons were discovered, scientists agreed on a direction to draw current in.

We use conventional current: it flows from the positive (+) terminal of the cell, round the circuit, to the negative (−) terminal.

Common mistake: conventional current and electron flow point in opposite directions. For KS3 circuits, always draw conventional current from + to −.

The symbols you must know

We draw circuits with standard symbols so anyone can read them:

Tip: an ammeter is a circle with an A inside; a voltmeter is a circle with a V inside. The lamp is a circle with a cross through it.

Match the component to its symbol

Tap a component on the left, then tap its correct symbol description on the right.

Potential difference

Potential difference (p.d.), also called voltage, tells you how much energy the cell gives to each bit of charge flowing round the circuit, and how much energy the charge passes on to each component.

• Potential difference is measured in volts, symbol V.
• You measure it with a voltmeter, connected in parallel with (across) the component.
• A bigger p.d. across the same component drives a bigger current through it.

Measuring voltage

Battery and bulb ratings

A cell is labelled with its voltage rating, such as 1.5 V. Join cells + to − to make a battery: their voltages add.

A lamp (bulb) is labelled with the voltage it is designed for, such as 3 V. Match it to the battery:

• Same voltage: the lamp glows normally.
• Battery voltage too low: the lamp is dim.
• Battery voltage too high: it is too bright and may blow (its thin wire overheats and breaks).

Match the lamp

Resistance slows the flow

Resistance is how much a component opposes the flow of current. It is measured in ohms (Ω).

• The higher the resistance, the smaller the current (for the same voltage).
• Long thin wires have more resistance than short thick wires.
• A resistor gets warm because it transfers energy as current flows through it.

Think of it like a slide: the p.d. is like the height of the slide (how much energy each person gets on the way down), resistance is a rough, narrow slide that slows you down, and current is how many people get down per second.

Resistance and current

Calculating resistance

Resistance is the ratio of potential difference to current. Measure the p.d. across a component and the current through it, then divide:

Common mistake: dividing the wrong way round. It is volts ÷ amps, never amps ÷ volts.

Work out the resistance

Low resistance, high resistance

Conductors, such as copper and other metals, have a very low resistance, so current flows through them easily. Insulators, such as plastic, rubber and glass, have a very high resistance, so almost no current flows.

• 1 m of copper connecting wire: less than 0.1 Ω.
• A plastic ruler, end to end: more than 1 000 000 000 Ω (a thousand million ohms).

Why cables are copper inside plastic: the copper carries the current; the plastic stops it reaching you.

Series: one single loop

In a series circuit all the components are joined in one single loop, so there is only one path for the current.

• The current is the same at every point in the loop.
• The cell's voltage is shared between the components.
• Add more lamps → each is dimmer, because the voltage is split further.
• If one lamp breaks, the loop is broken and they all go out.

Common mistake: the current is not "used up" as it goes round a series circuit — it is exactly the same everywhere in the single loop.

Parallel: separate branches

In a parallel circuit the components are on separate branches, so the current has more than one path to take.

• Each branch gets the full voltage of the cell, so lamps stay bright.
• The current splits at a junction. Where the branches meet again, the branch currents add up to the current from the cell.
• If one branch breaks, the others keep working.

Why homes use parallel: you can switch one light off without the rest going out, and each device gets full mains voltage.

Currents add where branches meet

In a parallel circuit, the current from the cell splits at a junction. Where the branches meet again, the branch currents add up to the current in the main line.

Common mistake: the current does not always split equally. Here the two lamps have different resistances, so their branch currents are different.

Find the missing current

Series or parallel?

Read each description and tap whether it is a series or parallel circuit.

What's different?

Does a lamp use up current?

How does a scrapyard crane drop a car on command?

Charge by rubbing

Static electricity is a build-up of charge on an object that isn't flowing. When you rub two insulators together, tiny electrons are transferred from one to the other.

• The object that gains electrons becomes negatively charged.
• The object that loses electrons is left positively charged.
• Rubbing does not make new charge — it just separates positive and negative charges.

Like charges repel

Charged objects push and pull on each other without touching:

• Like charges repel — two + charges (or two − charges) push apart.
• Opposite charges attract — a + and a − charge pull together.

The region around a charge where it can push or pull is called an electric field. We draw it with field lines pointing away from + and towards −.

Common mistake: "opposites attract" is right, but like charges repel — two negatives do not attract each other.

Charged balloons

Poles: attract and repel

Every magnet has two ends called poles: a north (N) pole and a south (S) pole. The rule is like static charge:

• Unlike poles attract — N and S pull together.
• Like poles repel — N and N (or S and S) push apart.
• Magnets attract magnetic materials: iron, steel, nickel and cobalt.

Common mistake: just like charges, like poles repel. Two north poles do not attract each other.

The field around a bar magnet

A magnetic field is the region around a magnet where it can affect magnetic materials or other magnets. We draw it with field lines that always go out of the N pole and into the S pole.

The Earth is a giant magnet

The Earth has its own magnetic field, as if a huge bar magnet sat inside it. A compass contains a tiny magnet that is free to turn.

• The needle lines up with the Earth's field, so its N end points roughly north.
• A compass is a handy way to detect and map a magnetic field.
• Near a bar magnet, the compass needle points along the field lines.

Magnetism from electricity

When a current flows through a wire it makes a magnetic field. Wind the wire into a coil around an iron core and you get an electromagnet — a magnet you can switch on and off.

You can make an electromagnet stronger by: adding more turns of wire, using a bigger current, or adding an iron core.

Uses: scrapyard cranes (switch off to drop the metal), electric bells, loudspeakers, motors, and maglev trains.

How a motor turns

A wire carrying a current makes its own magnetic field. Put the wire between the poles of a magnet and the two fields interact: the wire feels a force.

In a DC motor, a coil of wire sits between the poles of a magnet. Current flows one way along one side of the coil and the opposite way along the other. So one side is pushed up, the other down, and the coil turns.

Keeping it spinning: DC (direct current) flows one way only. Every half turn, a switching part reverses the current in the coil, so it keeps turning the same way.

Scrapyard crane

Explain in one line

The big ideas

You've covered the big ideas of KS3 electricity and electromagnetism. Press Finish to see your score.