The Field around a Solenoid
- When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it
Diagram showing the magnetic field around a flat circular coil
- To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid
- The magnetic field around the solenoid is similar to that of a bar magnet
Magnetic field around and through a solenoid
- The magnetic field inside the solenoid is strong and uniform
- Inside a solenoid (an example of an electromagnet) the fields from individual coils
- Add together to form a very strong almost uniform field along the centre of the solenoid
- Cancel to give a weaker field outside the solenoid
- One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole
- To work out the polarity of each end of the solenoid it needs to be viewed from the end
- If the current is travelling around in a clockwise direction then it is the south pole
- If the current is travelling around in an anticlockwise direction then it is the north pole
- If the current changes direction then the north and south poles will be reversed
- If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid
Poles of a Solenoid
- The strength of the magnetic field produced around a solenoid can be increased by:
- Increasing the size of the current which is flowing through the wire
- Increasing the number of coils
- Adding an iron core through the centre of the coils
- The iron core will become an induced magnet when current is flowing through the coils
- The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall
Examiner Tip
Remember the term ‘uniform field’ means a field which has the same strength and direction at all points. This is represented by parallel field lines.When discussing the strength of an electromagnet, avoid saying “add more coils”:
The coil describes the overall object – the individual loops of wire should be referred to as turns.
The correct phrase to use is “add more turns to the coil”.