Electricity and magnetism

Seeing it repel one pole of another magnet

提示Attraction happens with any piece of iron or steel, so which behaviour cannot be faked?

为什么Iron and steel are pulled towards both poles of any magnet, so attraction tells you nothing about the object doing the pulling. Repulsion happens only between two like poles, so it is the single reliable test that an object carries a field of its own.

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.

The rod pushes electrons to the far side, leaving the near face positive

提示In a metal the free negatives can shift, so work out which end they run to.

为什么The rod's field drives the sphere's free electrons to the far side, so the near face is left positive and the far face negative. Electrostatic force weakens with distance, and the positive near face is closer to the rod, so the pull beats the push and the sphere moves. This is charging by induction, and it is why a charged balloon sticks to a neutral wall.

0.9 A

提示Charge cannot be created or destroyed at a junction, so the branches share out what leaves the cell.

为什么Charge is conserved where wires meet, so the reading in the main wire is the sum of the branches: 0.4 + 0.5 = 0.9 A. An ammeter moved into one branch would read only that branch. The 12 V is not needed for this calculation, though it does tell you both branches have the full 12 V across them.

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.

Turns, current, core — all three boost the field

提示Think coil, flow, and what you could slide up the middle.

为什么Each works on the same field: extra loops stack up more magnetism, a stronger flow drives it harder, and iron concentrates it. The advantage over a bar magnet is that switching off the electricity switches the magnetism off, which is how a scrapyard crane drops a car.