CIE A Level Physics复习笔记20.1.1 Representing Magnetic Fields

Magnetic Field Definition

  • A magnetic field is a field of force that is created either by:
    • Moving electric charge
    • Permanent magnets
  • Permanent magnets are materials that produce a magnetic field
  • A stationary charge will not produce a magnetic field
  • A magnetic field is sometimes referred to as a B-field
  • A magnetic field is created around a current carrying wire due to the movement of electrons
  • Although magnetic fields are invisible, they can be observed by the force that pulls on magnetic materials, such as 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
  • The direction of the magnetic field into or out of the page in 3D is represented by the following symbols:
    • Dots (sometimes with a circle around them) represent the magnetic field directed out of the plane of the page
    • Crosses represent the magnetic field directed into the plane of the page


The magnetic field into or out of the page is represented by circles with dots or crosses

Exam Tip

The best way to remember which way around to draw magnetic fields in 3D is by imagining an arrow coming towards or away from you

  • When the head of an arrow is coming towards you, you see the tip as a dot representing the arrow coming ‘out’ of the page
  • When an arrow is travelling away from you, you see the cross at the back of the arrow representing the arrow going ‘into’ the page