Electricity and magnetism

They repel each other

HintThe two ends are alike, and alike ends will not sit together.

The whyUnlike poles attract and like poles push apart. Cut a bar magnet in half and you never get a lone north pole — each piece grows a new south, right down to the atoms.

A compass

HintA tiny needle that swings freely and points wherever it is pulled.

The whyPlacing a small plotting one at points around a bar magnet traces field lines that loop out of the north pole and back into the south. Iron filings show the same pattern all at once.

The Earth acts like a giant magnet

HintThe whole planet has a field of its own, made by its molten iron core.

The whyThat field is generated by currents in the liquid iron outer core. Confusingly, the pole up in the Arctic behaves as a magnetic south, which is precisely why a needle is drawn to it.

Electrons

HintThe tiny negative particles on the outside of an atom.

The whyRubbing transfers them from one surface to the other. Whichever material gains them is left negatively charged and the one that loses them is left positive, so the charges always appear as a matching pair.

They attract each other

HintOpposite charges pull inwards, exactly as unlike poles do.

The whyLike charges repel and unlike charges attract, and the force works across a gap with nothing touching. A charged balloon clings to a wall because it tugs the opposite charges in the wall towards it.

The current

HintThe flow of charge round the loop, read in amps.

The whyAn ammeter goes in series so the flow passes straight through it, and it reads in amperes. One amp is an enormous number of electrons drifting past every second.

A lamp

HintThe component that glows when charge passes through it.

The whyCircuit symbols are an international shorthand, so a diagram drawn in Manchester can be built in Munich. The cross is a reminder of the filament inside a bulb.

It is the same

HintNothing is eaten up on the way round the loop.

The whyCharge cannot pile up or vanish, so the reading is identical everywhere in a single loop. What the bulbs share out is the energy each charge carries, not the charge itself.

potential difference

HintIt measures the electrical push a battery gives to each charge.

The whyIt is the energy handed to every unit of charge as it passes, measured in volts. A voltmeter is connected across a component rather than in line with it.

Parallel

HintEach bulb must sit on a branch with a route of its own.

The whyEvery branch gets the full potential difference and has its own path, so breaking one leaves the others working. It is how house lighting is wired, and why one dead bulb does not darken a room.

4 ohms

HintDivide the voltage by the current.

The whyResistance = voltage ÷ current, so 12 ÷ 3 = 4. The unit is written with the Greek letter omega, and it tells you how hard a component makes the charge work to get through.

It gets stronger

HintEach extra loop adds its own magnetic effect to the pile.

The whyMore turns, a bigger current or an iron core all boost the field. The advantage over a bar magnet is that switching the current off switches the magnetism off, which is how a scrapyard crane drops a car.