Magnetic Field Around Wires & Solenoids
- When a current flows through a conducting wire a magnetic field is produced around the wire
- A conducting wire is any wire that has current flowing through it
- The shape and direction of the magnetic field can be investigated using plotting compasses
- The compasses would produce a magnetic field lines pattern that would like look the following
Magnetic Field Around a Current-Carrying Wire
The magnetic field pattern around a current-carrying wire is a series of concentric circles
- The magnetic field is made up of concentric circles
- A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
- As the distance from the wire increases the circles get further apart
- This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
- The right-hand thumb rule can be used to work out the direction of the magnetic field
Right-Hand Thumb Rule
The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire
- Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field
Different Views of Magnetic Field Lines Around a Current-Carrying Wire
Side and top view of the current flowing through a wire and the magnetic field produced
- If there is no current flowing through the conductor there will be no magnetic field
- Increasing the amount of current flowing through the wire will increase the strength of the magnetic field
- This means the field lines will become closer together
Magnetic 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
Field Lines Around Loops of Wire
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 Produced by a Solenoid
Magnetic field around and through a solenoid
- The magnetic field inside the solenoid is strong and uniform
- 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
If the current is travelling in a clockwise direction around the end of the solenoid, the induced pole is a south pole and vice versa
- A solenoid can be used as an electromagnet by adding a soft iron core
- 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
- The magnetic field produced by the electromagnet can be switched on and off
- When the current is flowing there will be a magnetic field produced around the electromagnet
- When the current is switched off there will be no magnetic field produced around the electromagnet
Structure of Electromagnet
An electromagnet consists of a solenoid wrapped around a soft iron core
- Changing the direction of the current also changes the direction of the magnetic field produced by the iron core
Factors Affecting Magnetic Field Strength
- 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 strength of an electromagnet can be changed by:
- Increasing the current will increase the magnetic field produced around the electromagnet
- Decreasing the current will decrease the magnetic field produced around the electromagnet
Examiner Tip
When trying to figure out how an electromagnetic device works:
- Look for a coil / solenoid - this is going to act as an electromagnet
- Look for a piece of iron - this will be attracted to the solenoid