Electromagnetism

Electromagnetism

1. Magnetic fields

   • Magnetic fields are represented by flux lines which go from north to south
   • The strength of a field depends on how tightly the flux lines are packed together
   • There is a magnetic field around a wire carrying an electric current
   • The direction of the field around a wire depends on the right hand rule
   • A wire carrying a current in a magnetic field will experience a force
   • Flux lines have a tendency to contract and straighten, which causes an electromagnetic force that pushes on the wire
   • If the current is parallel to the flux lines no force will act because the fields are perpendicular
   • The force on a wire is proportional to flux density
   • F = BIL
     • B = magnetic flux density , Teslas
     • I = current
     • L = length of wire

2. Electromagnetic induction

   • If a conducting rod moves perpendicular to a magnetic field, the e- in the rod will experience a force, which causes them to accumulate at one side of the rod
     • This induces an emf across the ends of the rods, and if the rod was in a circuit current would flow - this is em induction
   • An emf is induced whenever there is relative motion between a conductor and magnetic flux
   • An emf is induced whenever lines of flux are cut
   • Flux cutting will always induce an emf but only a current if the circuit is complete
   • Magnetic flux
     • Can be thought as the total number of flux lines in an area, units Wb
     • Φ = BA
     • Flux linkage is the flux multiplied by the number of turns, units are Wb turns
     • ΦN = Φ x N
     • The size of the emf also depends on how quickly the flux and conductor move relative to eachother
   • Faradays law
     • "The induced emf is directly proportional to the rate of change of flux linkage"
     • Induced emf, =
   • Lenz's law
     • "The induced emf is always in such a direction to oppose the change that caused it"
   • You can induce an emf by dropping a magnet through a coil

3. Motors

   • Convert electrical energy into mechanical energy
   • They are based on idea that there is a force on a current carrying wire in a field by F = BIL
   • The direction of the force on a current carrying wire in a field is given by flemmings left hand rule

4. Transformers

   • Transformers change the size of a voltage for an alternating current
   • Induction:
     • 1) Alternating current flowing in the primary coil produces magnetic flux
     • 2) The changing magnetic field passes through the iron core to the secondary coil, where it induces an alternating voltage
   • V1/V2 = N1/N2 = I2/I1
   • Transformers are not 100% efficient
     • If it was 100% efficient power in would = power out
     • There are small losses of energy mainly in the form of heat
     • Eddy currents are currents induced by the changing magnetic flux in the core - this effect can be reduced by laminating the cores with layers of insulation
     • Heat is also generated by the resistance in the coils, which can be reduced by using thick copper wire
   • Permeance
     • The permeance of an object is like conductance for flux lines
     • Its the amount of flux induced in it for a given number of current turns around it
     • Permeance and conductance are inversely proportional to the length of the object and proportional to the area
     • Iron has a very high permeability

5. Dynamos and generators

   • The emf induced is given by faradays law
   • A changing flux linkage is achieved by spinning a magnet in a coil or a coil around a magnet very fast
   • Convert kinetic energy into electrical energy
   • The output voltage and current change direction with every half rotation of the coil, producing an alternating current
   • A split ring commutator is used to change this a.c. current to d.c.

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