Magnetic Fields (OCR A Level Physics): Revision Note

Defining Magnetic Fields

• 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

Representing Magnetic Fields

• 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