Magnets on Demand

Why it matters

The junkyard crane lifts a car, swings it across the yard, and lets go on command. No permanent magnet could ever be talked into releasing — but this one is only a magnet while a current says so.

The idea in plain words

The switchable magnet, summarised. Tap the terms.

strength ∝ turns × current (× iron core)

Your crane's magnet is too weak for a truck engine. List the fixes.

Any of the three — or all, for the truck

• the recipe: construction — insulated copper coil wound on a soft-iron core — a solenoid with a heart
• the controls: three knobs — more turns, more current, iron core in — each multiplies the strength
• 10.13's list: uses — cranes, electric bells, relays, telephone earpieces, maglev brakes
• the comparison: vs permanent — switchable, reversible, adjustable — the three advantages steel can't match
• Wind more turns
• Raise the current
• Ensure a fat soft-iron core

Predict first

To make an electromagnet DROP its load instantly, you…

No current, no field, no magnetism — soft iron holds no grudge. (Reversing the current only swaps N and S; the car stays stuck.)

• switch off the current — the soft-iron core demagnetises at once — correct
• reverse the current
• heat the magnet

What you do

Build the strongest electromagnet you can — turns, current, iron core — then prove the air-core version is a weakling.

Check yourself

An electric bell rings CONTINUOUSLY because its electromagnet…

The hammer's own motion breaks the circuit that attracted it — a self-interrupting loop, hundreds of times a minute.

Reversing the current through an electromagnet…

Direction of current sets polarity (right-hand rule); magnitude sets strength. Two independent knobs.

A relay uses a small electromagnet to…

The little coil pulls a contact that closes the heavy circuit — how a doorbell button can start a factory motor.

• switches itself off and on via the vibrating contact — attract, release, repeat — correct
• stays on the whole time
• uses a permanent magnet
• swaps its N and S poles — correct
• switches it off
• doubles its strength
• switch a BIG circuit with a tiny, safe current — correct
• store electricity
• measure current