magnetism part 2

Magnetic Fields
 Force On A Current Carrying Wire In A Magnetic Field
You will be familiar with the basic notion of a magnetic field, in which magnetic materials experience a magnetic force.  However it is worth revising some of the basic ideas that you will have come across in early secondary school.

Revision

  • Magnetic fields can be shown by field lines, which go from North to South.
  • The field lines in a strong magnetic field are more closely packed than in a weak field.
  • Unmagnetised materials are attracted to either pole.
  • Like poles repel; unlike poles attract.
  • In the Earth’s magnetic field, the North pole will align itself to point to the North, if the magnet is allowed to swing freely.
  • The Earth has a magnetic field like a bar magnet.  Notice that the S-pole is under the North geographic pole.  Be careful not to be confused by this.

  • We never get single magnetic poles; if there is an N-pole, there must also be an S-pole to go with it.
  • Magnets are thought to result from the action of tiny atomic magnets called domains.  This can be explained by the movement of electrons that represent a tiny electric current that results in magnetism.  In most materials, the currents cancel out.
  • Only iron, cobalt, and nickel and their alloys are magnetic.
  • When a magnetic material is unmagnetised, the domains are all jumbled up.  If some of the domains are lined up, then the material is partially magnetised.  If the domains are fully lined up, the magnet is saturated, and cannot be magnetised further.
  • Some materials like soft iron lose their magnetism quickly.  These are used for temporary magnets.  Permanent or hard magnetic materials do not lose their magnetism.
  • Electric currents always produce a magnetic field, even if the wire itself is not made of a magnetic material.  The magnetic field of a single current carrying wire is like this:

  • The direction of the current is determined by the Screwdriver Rule.
  • The magnetic field of a solenoid is like a bar magnet.
You will be familiar with the motor effect.  If we put a current carrying wire in a magnetic field, we see that there is a force.


As we pass a current through the wire, there is a force that acts on the wire at 90o to the direction of the magnetic field.  This is given by Fleming’s Left Hand Rule with which you will be familiar.


We can work out the force that is exerted on the wire quite simply.  Experiment shows us that the force is proportional to:
  • The current;
  • The strength of the magnetic field ;
  • The length of wire within the magnetic field.
This is summed up in a simple formula:
 
 F = BIl

[B – magnetic field strength; I – current in A; l – length in m]
 
The term B is called the magnetic field strength, or the flux density, and is measured in Tesla, T.  The magnetic flux density can be thought of as the concentration of field lines.  We can increase the force by increasing any of the terms within the equation.  If we coil up the wire, we increase its length within the magnetic field.

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