Magnetic Fields (Cambridge O Level Physics)

• All magnets are surrounded by a magnetic field
• A magnetic field is defined as:

The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel)

• Two bar magnets can be used to produce a uniform magnetic field
• Point opposite poles (north and south) of the two magnets a few centimetres apart
• A uniform magnetic field will be produced in the gaps between opposite poles
  • Note: Outside that gap, the field will not be uniform

Magnetic Field Between Two Poles

A uniform field is created when two opposite poles are held close together. Magnetic fields are always directed from North to South

• A uniform magnetic field is one that has the same strength and direction at all points
  • To show that the magnetic field has the same strength at all points there must be equal spacing between all magnetic field lines
  • To show that the magnetic field is acting in the same direction at all points there must be an arrow on each magnetic field line going from the north pole to the south pole

• The magnetic field lines are the same distance apart between the gaps of the poles to indicate that the field strength is the same at every point between the poles
• This field can be determined by using plotting compasses that will point from north to south or by using iron filings

Magnetic Field Lines

• Magnetic field lines are used to represent the strength and direction of a magnetic field
• The direction of the magnetic field is shown using arrows

• There are some rules which must be followed when drawing magnetic field lines. Magnetic field lines:
  • Always go from north to south (indicated by an arrow midway along the line)
  • Must never touch or cross other field lines

Magnetic Field Around a Bar Magnet

• The magnetic field is strongest at the poles
  • This is where the magnetic field lines are closest together

• The magnetic field becomes weaker as the distance from the magnet increases
• This is because the magnetic field lines are getting further apart

Magnetic Field Around a Bar Magnet

The magnetic field around a bar magnet loops outward from the north pole to the south pole

• The strength of the magnetic field is shown by the spacing of the magnetic field lines
  • If the magnetic field lines are close together then the magnetic field will be strong
  • If the magnetic field lines are far apart then the magnetic field will be weak

• Magnetic forces are due to interactions between magnetic fields
  • Two bar magnets can repel or attract

• The field lines will look slightly different for each:

   Magnetic Fields Showing Attraction and Repulsion

Magnetic field lines for attracting and repelling bar magnets

• Therefore, the magnetic field lines around different configurations of two bar magnets would look like:

Magnetic Field Patterns for Different Orientations of Bar Magnets

Magnetic field lines between two bar magnets