A current is simply charge on the move. Give it a complete path and it will light a bulb, heat a wire or turn iron into a magnet.
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Current flows only in a closed loop. Break the loop anywhere and everything stops.
🔌 What a circuit needs
A source — a cell or battery to push the charge.
A complete path of conducting wire, with no break.
A device such as a bulb or motor to use the energy, and usually a switch to break the path on purpose.
🔌 Conductor or insulator
| Conductors | Insulators |
|---|---|
| copper, aluminium, iron | rubber, plastic |
| silver (the best of all) | wood, glass |
| salty or impure water | dry air, ceramic |
| the human body | pure distilled water |
Why wet hands are dangerous near a switch. Pure water is a poor conductor, but the water on your skin carries dissolved salts, and salty water conducts well. Your body then becomes part of the circuit.
Current always warms the wire it flows through. Some appliances are built entirely around that fact.
🔥 Where the heating effect is used
| Device | How it uses heat |
|---|---|
| Electric iron, heater, geyser | a coil of high-resistance wire gets hot |
| Filament bulb | the filament glows white-hot |
| Toaster, hair dryer | a hot element warms the air or bread |
| Fuse | a thin wire that melts and breaks the circuit if the current is too large |
Why heating elements use nichrome. It has a high resistance and a very high melting point, so it glows hot without melting. Copper, with its low resistance, would barely warm at all — which is exactly why the connecting wires are copper.
A fuse is a deliberate weak point. It is designed to melt first so the rest of the circuit — and the house — is protected. Replacing a fuse with thick wire defeats the whole purpose and can cause a fire. An MCB does the same job by switching off automatically.
A current flowing through a wire creates a magnetic field around it. Wind the wire into a coil around iron and you have an electromagnet.
🧲 Making an electromagnet stronger
| Do this | Effect |
|---|---|
| More turns in the coil | stronger magnet |
| More current | stronger magnet |
| A soft iron core inside | much stronger magnet |
| Switch off the current | the magnetism disappears |
The great advantage of an electromagnet is that it can be switched off. That is why cranes in scrapyards use them to pick up and then drop heavy iron — a permanent magnet could never let go.
The electric bell uses both effects of the chapter: an electromagnet pulls an iron strip to strike the gong, which breaks the circuit, which releases the strip, which closes the circuit again — many times a second.
12 questions on circuits, heating and magnetic effects. 🍀
