Magnet With a Switch
✨ An ordinary iron nail picks up a pile of paper clips and drops every one of them the instant you break the circuit.
🧰 You need
- one 1.5 volt cell
- about one metre of thin insulated copper wire
- an iron nail
- a heap of steel paper clips
- sticky tape
⚠️ Use one 1.5 volt cell only and never a mains socket; the wire gets warm, so disconnect between tries and let it cool.
👀 What you will see
With the circuit connected, the nail lifts a bunch of clips. The moment you disconnect it, nearly all of them drop, though one or two may cling on. More turns of wire lift more clips, and the wire becomes noticeably warm.
🧪 Do it
- Ask an adult to scrape the insulation off the last two centimetres at each end of the wire.
- Wind twenty tight turns around the nail, all in the same direction, and tape the coil so it cannot unwind.
- Hold one bare wire end on each end of the cell for a few seconds only.
- Dip the nail into the paper clips, lift, and count how many come with it.
- Take one wire off the cell and count how many clips fall.
- Add more turns to make forty and then sixty, and repeat the count each time.
💡 Why it happens
An electric current always creates a magnetic field in the space around the wire. Winding the wire into a coil stacks the fields of all the turns so they add up along the middle of the coil. The iron nail makes it far stronger because iron contains tiny magnetic regions that line up with the coil's field, and a coil with an iron core like this is called an electromagnet. Switch the current off and the regions fall out of line, so the magnetism almost vanishes, which is exactly why scrapyard cranes can pick up a car and then release it.
🔁 Now try this
Count the clips lifted with 20, 40 and 60 turns, keeping the same cell and nail, and plot clips against number of turns.
🤔 Think about it
A fridge magnet cannot be switched off, so which jobs need an electromagnet instead?
🧘 Ask the Acharya
“A fridge magnet cannot be switched off, so which jobs need an electromagnet instead?” — ask him and he will explain it from your own chapters.
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