🧭 Outside the solenoid
The field lines leave one end, curve round and enter the other end, just like those of a bar magnet. They are closest together at the ends, where the field is strongest.
Electromagnetism » Solenoids and Electromagnets
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.9P, 6.10P
- What a solenoid is and the shape of its magnetic field
- How to find which end of a solenoid is the north pole
- How an electromagnet is built and why it uses soft iron
- How to make an electromagnet stronger, and where they are used
This whole lesson is examined in Paper 2 only.
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Unlock This CourseThe magnetic field of a solenoid
In the last lesson you saw that a current makes a magnetic field around a wire, and that bending the wire into one circle makes the field strongest at the centre. Now keep going. Wind the wire into many turns, like a spring, and you have a solenoid.
Key terms:
- Solenoid: a long coil of wire with many turns, carrying a current.
- Electromagnet: a solenoid wound on a soft iron core, which acts as a magnet only while a current flows.
Each turn adds its own field, and the fields of all the turns join together. The result looks very much like the field of a bar magnet.
🔧 Inside the solenoid
The field lines are straight, parallel and evenly spaced. The field is strong and nearly uniform inside, apart from near the ends.
Which end is the north pole?
A solenoid has a north pole at one end and a south pole at the other. The field lines come out of the north pole. To find it, use your right hand again.
Learn this
Grip the solenoid with your right hand so your fingers curl in the direction of the current. Your thumb then points to the north pole of the solenoid.
Reverse the current and the poles swap over. The north pole becomes the south pole.
How to draw the field of a solenoid
- Draw the solenoid as a rectangle with a few turns of wire shown as small slanted lines or circles.
- Inside, draw several straight, parallel lines along the length of the solenoid, evenly spaced.
- Outside, draw curved lines that leave one end and loop round to enter the other end, like a bar magnet.
- Add arrows. Inside they point towards the north pole end. Outside they run from the north pole round to the south pole.
- Label the north and south poles.
Worked example
A solenoid carries a current that goes clockwise when you look at the right-hand end of it. Which pole is at that end?
Step 1: look at the end. The current goes clockwise round it.
Step 2: grip with the right hand so the fingers curl clockwise.
Step 3: the thumb points away from you, into the solenoid, so the field leaves at the other end.
Answer: the end you are looking at is the south pole, because the field lines go into it. The far end is the north pole.
Building an electromagnet
A solenoid on its own is a fairly weak magnet. Put a piece of iron inside it and the effect is far greater, because the iron becomes magnetised by the field of the coil. This is an electromagnet.
The construction has three parts:
- A coil of insulated wire. The insulation stops the current taking a short cut between neighbouring turns, so it must flow round every turn.
- A soft iron core inside the coil.
- A connection to a d.c. supply, such as a battery or a power pack, so the current always goes the same way and the poles stay the same.
Why soft iron?
You met magnetically soft materials in Magnets and Magnetic Materials. Soft iron is easy to magnetise and easily loses its magnetism when the current is switched off. That is just what we want, because an electromagnet can be switched on and off. A magnetically hard material such as steel would stay magnetised after the current stopped, so it would be no good for this job.
Making an electromagnet stronger
An electromagnet can be made stronger in three ways:
- increase the current,
- increase the number of turns of wire in the coil,
- use an iron core (instead of no core at all).
| Change | Effect on strength |
|---|---|
| More current | Stronger |
| More turns | Stronger |
| Iron core added | Much stronger |
| Current switched off | Magnetism lost (soft iron) |
Uses of electromagnets
- Scrapyard cranes: a large electromagnet picks up scrap iron and steel. Switch the current off and the load drops where it is wanted.
- Electric bells: the electromagnet pulls a metal arm so the hammer strikes the gong.
- Relays: a small current in the electromagnet switches on a circuit carrying a much larger current.
Common mistakes
Common mistakes
- Drawing curved lines inside the solenoid. Inside they are straight and parallel.
- Forgetting arrows or the north and south labels. Each can earn a mark.
- Using a steel core. It would stay magnetic when the current is switched off.
- Writing "iron is magnetic" and nothing more. Say that soft iron loses its magnetism when the current is switched off.
- Using the left hand for the grip rule. Always use the right hand.
- Forgetting that the wire must be insulated.
Exam-style question
A student makes an electromagnet by winding insulated wire around an iron nail and connecting it to a battery.
(a) State why the wire must be insulated. [1 mark]
(b) Explain why the core is made of soft iron and not steel. [2 marks]
(c) Describe two ways the student could make the electromagnet stronger. [2 marks]
(d) Describe how to draw the magnetic field pattern of a solenoid carrying a current. [3 marks]
Model answer
(a) So the current flows round every turn of the coil and does not pass between the turns. (1)
(b) Soft iron is easily magnetised. (1) It loses its magnetism when the current is switched off, but steel would stay magnetised. (1)
(c) Increase the current. (1) Add more turns of wire to the coil. (1)
(d) Inside the solenoid, draw straight, parallel lines. (1) Outside, draw curved lines from one end to the other, like a bar magnet. (1) Add arrows leaving the north pole and label the poles. (1)
Exam tip
In a 2-mark "explain" question, give a fact about soft iron and then the consequence: it loses its magnetism when the current stops.