• Magnetic field line patterns are all slightly different around:
  • Straight wires
  • Flat circular coils
  • Solenoids

   

Magnetic Field in a Straight Wire

• When a?current?flows through a conducting wire a?magnetic field?is produced around the wire
• The shape and direction of the magnetic field can be investigated using plotting compasses

• 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

The direction of the magnetic field around a wire is given by the right-hand thumb rule

• Reversing the direction in which the current flows through the wire will?reverse the direction?of the magnetic field
• 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 in a Flat Circular Coil

• 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

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

Magnetic Field in 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

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