AQA GCSE Physics contents

Unit 7 · Magnetism & Electromagnetism

Poles of a Magnet & Magnetic Fields

3 min read · free to read

Poles of a magnet

  • The poles of a magnet are the places where the magnetic forces are strongest. Every magnet has a north (seeking) pole and a south (seeking) pole
  • When two magnets are brought close together they exert a force on each other: two like poles repel; two unlike poles attract
  • Attraction and repulsion between magnetic poles are examples of a non-contact force

Permanent and induced magnets

  • A permanent magnet produces its own magnetic field all the time, for example a bar magnet made of steel
  • An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. An iron nail hanging from a magnet is an induced magnet, and can pick up a second nail
  • Induced magnetism always causes a force of attraction: the end of the material nearest the magnet becomes the opposite pole
  • When removed from the magnetic field an induced magnet loses most or all of its magnetism quickly
  • Test for a permanent magnet: it can repel. A magnetic material (iron, steel, cobalt, nickel) is only ever attracted, so repulsion is the only proof that something is a magnet

Magnetic fields

  • The magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel)
  • The force between a magnet and a magnetic material is always one of attraction
  • The strength of the field depends on the distance from the magnet: it is strongest at the poles and gets weaker further away
  • The direction of the field at a point is the direction of the force that would act on another north pole placed there. Field lines run from the north pole to the south pole of the magnet
  • Field lines never cross. Where they are closer together the field is stronger

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magnetic field of a bar magnetNSthe arrows always point fromNORTH round to SOUTHthe lines are CLOSEST TOGETHER atthe poles — that is where the fieldis strongestA magnetic field is the region where another magnet, or a magnetic material, feels a force.The direction of a field line is the direction the north pole of a compass would point if you put itthere — which is why you can plot a field with a small plotting compass.A permanent magnet produces its own field. An induced magnet only becomes magnetic when it isput in a field, and loses it again when it is taken out — and the force between an induced magnetand the magnet causing it is always ATTRACTION.
The magnetic field of a bar magnet: field lines leaving the north pole and curving round to enter the south pole, arrows pointing from N to S, lines closest together at the poles

Plotting a field with a compass

  • A magnetic compass contains a small bar magnet that lines up with the field it is in
  • To plot the field: place the magnet on paper and put a plotting compass near one pole. Mark a dot at each end of the needle, move the compass so its tail is on the dot you marked at its head, and repeat. Join the dots to give one field line, then start again from a different point
  • The Earth has a magnetic field. A compass needle points in the direction of the Earth's field, roughly towards geographic north. This is evidence that the core of the Earth must be magnetic