Course: Combined Science + Separate Physics (electromagnetic devices are S only) | Tier: Foundation + Higher | Lesson: Build a Scrapyard Crane
Notes
The magnetic effect of a current
When a current flows through a wire, a magnetic field is produced around it. The field lines are concentric circles around the wire. You can show this by placing compasses around a vertical wire and switching the current on.
✏️ Copy into your book
- A current-carrying wire produces a magnetic field around it.
- The field is stronger with a bigger current and closer to the wire.
- Reversing the current reverses the direction of the field.
- Shaping a wire into a coil (a solenoid) increases the strength of the field.
- Inside a solenoid the field is strong and uniform. Outside, it has a similar shape to the field around a bar magnet.
- Adding an iron core to a solenoid increases the field strength. A solenoid with an iron core is an electromagnet.
Making an electromagnet stronger
- Increase the current.
- Add more turns to the coil.
- Add an iron core.
The big advantage of an electromagnet over a permanent magnet is that it can be switched on and off, and its strength can be changed.
Electromagnetic devices (S only)
Electric bell: when the switch is pressed, current flows and the electromagnet attracts an iron armature, so the hammer hits the bell. This breaks the circuit, the electromagnet switches off and a spring pulls the armature back, which completes the circuit again. This repeats, so the bell rings continuously.
Scrapyard crane: an electromagnet picks up iron and steel when switched on, and drops it when switched off.
Questions
Q1 (F) Describe the shape of the magnetic field around a straight current-carrying wire.
Show answer
Concentric circles around the wire.
Q2 (F) What is a solenoid?
Show answer
A coil of wire. When a current flows through it, it produces a magnetic field.
Q3 (F) Give three ways to make an electromagnet stronger.
Show answer
Increase the current, add more turns to the coil, add an iron core.
Q4 (F) Why is an electromagnet used in a scrapyard crane instead of a permanent magnet?
Show answer
It can be switched off, so the crane can drop the scrap metal where it’s needed.
Q5 (F/H) What happens to the magnetic field around a wire if the current is reversed?
Show answer
The field reverses direction (compasses around the wire point the opposite way).
Q6 (F/H) Describe the magnetic field inside and outside a solenoid.
Show answer
Inside, the field is strong and uniform (parallel, evenly spaced field lines). Outside, it is like the field of a bar magnet.
Q7 (S only) Explain how an electric bell rings continuously when the switch is held down.
Show answer
Current flows, the electromagnet attracts the iron armature and the hammer hits the bell. This breaks the contact, so the current stops and the electromagnet switches off. The spring pulls the armature back, the circuit is completed again, and the cycle repeats.
