Magnetic Fields (OCR A Level Physics)

• A magnetic field is a field of force that is created either by:
  • Moving electric charge
  • Permanent magnets

• A magnetic field is sometimes referred to as a B-field
• Permanent magnets are materials that produce a magnetic field
• A magnetic field is created around a current carrying wire due to the movement of electrons
  • A stationary charge will not produce a magnetic field
• Although magnetic fields are invisible, they can be observed by the force that pulls on magnetic materials
  • Examples include iron or the movement of a needle in a plotting compass

• Magnetic fields are represented by magnetic field lines
  • These can be shown using iron filings or plotting compasses

• Field lines are best represented on bar magnets, which consist of a north pole on one end and south pole on the other
• The magnetic field is produced on a bar magnet by the movement of electrons within the atoms of the magnet
  • This is a result of the electrons circulating around the atoms, representing a tiny current and hence setting up a magnetic field
• The direction of a magnetic field on a bar magnet is always from north to south

Magnetic field lines are directed from the north pole to the south pole

• When two bar magnets are pushed together, they either attract or repel each other:
  • Two like poles (north and north or south and south) repel each other
  • Two opposite poles (north and south) attract each other

Two opposite poles attract each other and two like poles repel each other

• The key aspects of drawing magnetic field lines:
  • The lines come out from the north poles and into the south poles
  • The direction of the field line shows the direction of the force that a free magnetic north pole would experience at that point
  • The field lines are stronger the closer the lines are together
  • The field lines are weaker the further apart the lines are
  • Magnetic field lines never cross since the magnetic field is unique at any point
  • Magnetic field lines are continuous

• A uniform magnetic field is where the magnetic field strength is the same at all points
  • This is represented by equally spaced parallel lines, just like electric fields