Welcome! In a scrapyard, a crane swings over a pile of old cars, picks one up with a giant magnet – then drops it exactly where it wants. In this lesson you’ll learn how electromagnets work and then design the magnet for a scrapyard crane. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Electromagnets – Build a Scrapyard Crane.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. describe the magnetic field around a wire and a solenoid
2. explain how to make an electromagnet stronger
3. describe the motor effect and (Higher) use F = BIl.
Step 1: Starter (5 minutes)
A scrapyard could use a giant permanent magnet (like a fridge magnet, but huge) instead of an electromagnet. What would go wrong? Write down your ideas.
Show answer
A permanent magnet can’t be switched off, so the crane could never drop the car! An electromagnet is only magnetic while a current flows, so it can be switched on to pick things up and off to let them go.
📝 Copy into your book
A PERMANENT magnet produces its own magnetic field all the time.
An ELECTROMAGNET is only magnetic when a current flows – it can be switched on and off.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What is a solenoid?
- Give three ways to make an electromagnet stronger.
- What is the motor effect?
📖 Read: Magnetism from electricity
When a current flows through a wire, it produces a magnetic field around the wire, shaped like circles around it. The field is stronger with a bigger current and closer to the wire.
If the wire is wound into a coil, called a solenoid, the fields of each turn add together. Inside the solenoid the field is strong and uniform, and outside it has the same shape as the field around a bar magnet. Adding an iron core inside the solenoid makes the field much stronger. This is an electromagnet. It can be made stronger by increasing the current, increasing the number of turns on the coil, or adding an iron core. Soft iron is used because it loses its magnetism as soon as the current is switched off.
If a wire carrying a current is placed in a magnetic field, the two magnetic fields interact and the wire feels a force. This is called the motor effect, and it’s how electric motors work. The force is bigger if the current is bigger, the magnetic field is stronger, or more wire is inside the field.
Show answers
(a) A coil of wire.
(b) Increase the current; increase the number of turns; add an iron core.
(c) When a wire carrying a current is in a magnetic field, it experiences a force.
📝 Copy into your book
To make an electromagnet STRONGER:
1. increase the CURRENT
2. increase the number of TURNS on the coil
3. add a soft IRON CORE.
Step 3: The motor effect (8 minutes)
Copy the equation, then try the questions.
📝 Copy into your book
(Higher) force = magnetic flux density × current × length F = B I l
F in N, B in tesla (T), I in A, l in m
(for a wire at right angles to the magnetic field)
Q1 A 0.20 m length of wire carries a current of 4.0 A at right angles to a magnetic field of 0.50 T. Calculate the force on the wire.
Show answer
F = B I l = 0.50 × 4.0 × 0.20 = 0.40 N
Q2 What would happen to the force if the current was reversed?
Show answer
The force would be the same size but in the opposite direction. (Reversing the magnetic field would do the same.)
Q3 Give two ways to make the force on the wire bigger.
Show answer
Any two: increase the current; use a stronger magnetic field; put a longer length of wire in the field.
✅ Score out of 3.
Step 4: Main task – Build a Scrapyard Crane (15 minutes)
You’re the engineer for a new scrapyard. The crane’s electromagnet must be strong enough to lift a car, and it must drop the car cleanly when switched off. The supplier offers four designs:
| Design | Turns on coil | Current | Core | Cost |
|---|---|---|---|---|
| W | 500 | 50 A | None (air) | £4000 |
| X | 2000 | 100 A | Soft iron | £9000 |
| Y | 2000 | 100 A | Steel | £8500 |
| Z | 500 | 100 A | Soft iron | £6000 |
Your task: write a recommendation to the scrapyard owner.
- Rank the designs from strongest to weakest, explaining your reasoning using the three factors.
- Explain why design Y would be a problem, even though it’s strong. (Hint: steel keeps its magnetism – it becomes a permanent magnet.)
- Recommend one design and justify your choice.
Show a model recommendation
1. Ranking: X and Y are strongest (most turns, biggest current, and a core). Z is next – same current and iron core, but fewer turns. W is weakest – fewest turns, smallest current and no core.
2. Problem with Y: steel stays magnetised after the current is switched off. The crane wouldn’t drop the car cleanly – it would stay stuck, or be let go at the wrong moment when it’s shaken off.
3. Recommendation: design X. It is one of the strongest because it has the most turns, the biggest current and an iron core. Soft iron loses its magnetism as soon as the current is switched off, so the car drops exactly when the operator wants. It costs more, but a magnet that’s too weak 
or won’t let go (Y) would be useless or dangerous.
✅ Check: did you use all three factors, and explain why the core material matters?
Step 5: Exam practice (14 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) Give two advantages of using an electromagnet instead of a permanent magnet in a scrapyard crane.
Show model answer
It can be switched off to drop the load (1). Its strength can be changed by changing the current (1).
Q2 (3 marks) Describe how a student could investigate how the number of turns on a coil affects the strength of an electromagnet.
Show model answer
Wrap a set number of turns (e.g. 10) of wire around an iron nail and connect it to a power supply; count how many paper clips it can pick up (1). Repeat with 20, 30, 40 and 50 turns (1). Keep the current (and the nail and paper clips) the same each time, and repeat each reading to find a mean (1).
Q3 (3 marks – Higher) A wire of length 0.05 m carries a current of 2.0 A at right angles to a magnetic field. The force on the wire is 0.015 N. Calculate the magnetic flux density.
Show model answer
F = B I l (1)
0.015 = B × 2.0 × 0.05 (1)
B = 0.015 ÷ 0.10 = 0.15 T (1)
✅ Add up your marks out of 8 (or out of 5 if you skipped the Higher question). Write the score in your book.
Step 6: Exit ticket (10 minutes)
Spot them! Electromagnets and motors are everywhere – door bells, speakers, fans, electric toothbrushes. List as many things at home as you can that use one. Then finish:
- The core of an electromagnet should be soft iron because…
- A wire in a magnetic field moves when…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Waves and magnetism mixed equation practice.
