Magnetic Fields (OCR A Level Physics)

Revision Note

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Katie M


Katie M



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

Bar magnetic field lines, downloadable AS & A Level Physics revision notes

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

Single bar magnet field lines, downloadable AS & A Level Physics revision notes

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

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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