Magnetic fields

  • Created by: Catriona
  • Created on: 02-05-14 20:23


  • A magnet has a magnetic field around it, the field is a region where the magnetic force can be felt.
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Lines of flux

  • Lines of flux show the direction and strength of the magnetic field.
  • The direction of the lines of flux show the direction of the force that an n-pole would feel. So lines of flux always go north to south (n-pole would be repelled from north and attracted to south).
  • The more closely packed the lines, the stronger the field.
  •  When two magnetic fields cancel out and the resultant field strength is zero it is called a neutral point (no lines of flux in these areas).


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Magnetic flux density

  • The stronger the field, the more densely packed the line of flux, therefore we describe the strength of the magnetic field by magnetic flux density, B.
  • This is the quantity of flux passing through unit area. It is measured in tesla (T).
  • Flux density can be measured using a Hall probe.
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The magnetic field around a long straight wire

  • An electric current is always surrounded by a magnetic field.
  • Lines of flux are circles around the wire.
  • Circles of flux are more widely separated the further from the wire, weaker field.
  •  The right hand grip rule gives a simple way to remember the direction of the field. Imagine gripping the wire, so that your right thumb points in the direction of the current, your fingers then curl in the direction of the lines of flux (current goes plus to minus).


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The magnetic field of a flat coil

  • Close to the wire, the lines of flux are circles.
  • In the centre of the coil the fields due to the sides of the coil are in the same direction and they combine to give a strong magnetic field.


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The magnetic field of a solenoid

  • A solenoid is a long coil with a large number of turns of wire.
  • The magnetic field outside the solenoid has the same shape as the field around a bar magnet.
  • Inside the solenoid lines of flux are close together, parallel and evenly spaced. Flux density is constant for most of the length of the solenoid. The field is uniform and strong.
  • Right hand grip rule can be used to show direction of field. Curl fingers in direction of current, your thumb now points along the direction of lines of flux inside the coil.


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Magnetic materials

  • Electron spins in atoms act like a tiny electric current and so the atom produces a very tiny magnetic field.
  • In some atoms magnetic effects cancel out in others they do not, so each atom is like a tiny magnet.
  • In ferromagnetic materials these tiny magnets line up to produce a strong magnetic field.
  • Inside a solenoid the atomic magnets line up along the lines of flux, so the core becomes magnetised.
  • A steel core stays magnetised.
  • An iron core demagnetises, because the atomic magnets have enough vibrational energy to turn in random directions. A magnet that can be switched of is an electromagnet.
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Magnetic force

  • A wire carrying a current in magnetic field feels a force.
  • You can use Flemming’s left hand rule to predict the direction. First finger = field (N to S), second finger = current (+ to -), thumb gives direction of force.

 What affects the size of the force?

  •  Force is directly proportional to:
     magnetic flux density
     length (of conductor in field)
  • F=ILB


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Magnetic force on a moving charge

  • Charged particles feel a force when they move through a magnetic field.

What affects the size of the force?

  •  The force on the particle is directly proportional to:
     magnetic flux density
     charge on the particle
     velocity of the particle
  • F=qvB


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