A magnet you can switch
A crane's electromagnet lifts iron. It drops the iron when the current stops.
- A coil, no current. No field at all.
- Current flows. Field lines run straight inside, all one way.
- Outside, lines curve end to end, like a bar magnet.
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The rule
A coil of many turns is a solenoid. A turn is one loop of the wire. Its field is strong and even inside. Even means the same all through. Outside, it looks like a bar magnet's field. More turns or more current makes the field stronger. It is a magnet only while current flows.
Soft iron placed inside the coil becomes a magnet. Soft iron stops being a magnet soon after the current stops. The result is an electromagnet. The iron's field adds to the coil's field. So the electromagnet is stronger than the coil alone.
| Change | Effect |
|---|---|
| More turns in the same length | Stronger field |
| More current | Stronger field |
| A soft iron core inside | Stronger magnet |
| Current off | No field, no magnet |
Worked example
Two coils have the same length and the same current. One has 20 turns. The other has 40. The 40-turn coil is the stronger magnet. That is because each turn adds to the field.
Look at one end of a coil, face on. If the current goes anticlockwise there, that end is a north pole. If it goes clockwise, it is a south pole. Reverse the current and the two ends swap.