Magnetism & electromagnetism (intro)
The idea
Magnets push and pull without touching — you have felt that strange grip since you first stuck one to a fridge. Every magnet has two poles, north and south; like poles repel and opposite poles attract, and the invisible region of influence around a magnet is its magnetic field. Only a few metals respond strongly — iron, nickel, and cobalt — which corrects a common belief that magnets grab all metal: an aluminum can or a copper coin will not stick no matter how strong the magnet.
The deeper surprise is that electricity and magnetism are two faces of the same thing. Any electric current creates a magnetic field around its wire, and coiling the wire concentrates that field. Wrap the coil around an iron core and you have an electromagnet: a magnet you can switch on and off, reverse, and strengthen at will — more current or more turns of wire means a stronger pull. That controllability is why electromagnets, not permanent magnets, run scrapyard cranes, electric motors, and doorbells.
Worked example
An electromagnet made with 50 turns of wire around an iron nail lifts about 10 paper clips. Using the same battery and nail, roughly how many clips should a 150-turn version lift?
- Identify the relationship: with the same current, an electromagnet's strength grows roughly in proportion to its number of turns, because each loop of wire adds its own contribution to the field.
- Find the scaling factor: 150 turns ÷ 50 turns = 3, so the new coil has three times the turns.
- Scale the result: about 10 clips × 3 = 30 paper clips.
- Sanity-check the realism: this is an estimate, not a guarantee — extra wire adds a little resistance, which trims the current slightly — but the direction is reliable: more turns, stronger magnet, and a real test should land near 30 clips.
Answer. The 150-turn electromagnet should lift roughly 30 paper clips — about three times as many.
Check your understanding
- Why would a magnet stick to a steel door but not to an aluminum window frame, even though both are metal?
- What are two different changes you could make to an electromagnet to strengthen it, and why does each one work?
- How would you use a compass to detect whether a current is flowing through a nearby wire?
- Why are electromagnets chosen over permanent magnets for a scrapyard crane, and what would go wrong with a permanent one?
Build the foundations first
Magnetism & electromagnetism (intro) builds on these concepts. If any feel shaky, start there.