Electricity and magnetism

They repel each other

提示The two ends are alike, and alike ends will not sit together.

为什么Unlike 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

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

为什么Placing 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

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

为什么That 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

提示The tiny negative particles on the outside of an atom.

为什么Rubbing 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

提示Opposite charges pull inwards, exactly as unlike poles do.

为什么Like 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

提示The flow of charge round the loop, read in amps.

为什么An 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

提示The component that glows when charge passes through it.

为什么Circuit 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

提示Nothing is eaten up on the way round the loop.

为什么Charge 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

提示It measures the electrical push a battery gives to each charge.

为什么It 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

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

为什么Every 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

提示Divide the voltage by the current.

为什么Resistance = 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

提示Each extra loop adds its own magnetic effect to the pile.

为什么More 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.