Magnetic Fields (WJEC GCSE Physics)

• All magnets are surrounded by a magnetic field
• A magnetic field is defined as:

The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel)

Magnetic Field Lines

• Magnetic field lines are used to represent the strength and direction of a magnetic field
• The direction of the magnetic field is shown using arrows
• The strength of the magnetic field is shown by the spacing of the magnetic field lines
  • If the magnetic field lines are close together then the magnetic field will be strong
  • If the magnetic field lines are far apart then the magnetic field will be weak
• There are some rules which must be followed when drawing magnetic field lines. Magnetic field lines:
  • Always go from north to south (indicated by an arrow midway along the line)
  • Must never touch or cross other field lines

Magnetic Field around a Bar Magnet

• The magnetic field is strongest at the poles
  • This is where the magnetic field lines are closest together
• The magnetic field becomes weaker as the distance from the magnet increases
• This is because the magnetic field lines are getting further apart

Field lines around a bar magnet

The magnetic field around a bar magnet. The lines get closer together closer to the bar magnet itself and always point from north to south.

• Two bar magnets can repel or attract, the field lines will look slightly different for each:

Field lines of attracting and repelling bar magnets

Magnetic field lines for attracting and repelling bar magnets

Different configurations of bar magnets

 

Magnetic field lines between two bar magnets in a variety of combinations

Magnetic Field around a Current-Carrying Wire

• 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 of a current-carrying straight wire

Diagram showing the magnetic field around a current-carrying wire

• 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 for a wire

The right-hand thumb rule shows the direction of current flow through a wire and the direction of the magnetic field around the wire

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

Magnetic field around a single loop of conducting wire

Diagram showing the magnetic field around a flat circular coil, using the right hand rule

• 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 of a solenoid

Magnetic field around and through a solenoid - this can be found by applying the right hand rule

• 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

Poles of a Solenoid and the direction of current at each pole

Magnetic Field Strength Around 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 turns in the coil
  • 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

Electromagnets

• An electromagnet is a solenoid with an iron core
• 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

• The strength of the 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