C4 Electromagnetism

• O - current flowing out

  X - current flowing in
• Right-hand Grip Rule : to determine direction of current flow.
• Fleming's left-hand rule: to determine force

  • thumb - direction of force
  • index - magnetic field (N to S)
  • middle - current in
• Fleming's right-hand rule: to determine direction of induced current

  • thumb - direction of force
  • index - magnetic field (N to S)
  • middle - induced current
• Catapult field: resultant magnetic field due to interaction between magnetic field of current carrying conductor and magnetic field of magnet.
• Brushed dc motor:

  • uses carbon or metal brushes
  • rotating coil
  • stationary magnet
  • friction between brush and commutator causes wear out
  • sparking at commutator
  • loud operating noises
• Brushless dc motor:

  • no carbon or metal brush
  • stationary coil
  • rotating magnet
  • moving components not in contact, no wear out
  • no sparking
  • reduced operational noise
• Magnetic flux: number magnetic field lines passing through a surface.
• Number of magnetic field lines per unit area directly proportional to the strength of the magnetic field.
• Electromagnetic induction is the process by which a changing magnetic field induces a current in a wire.
• Faraday's Law: Magnitude of induced emf is directly proportional to the rate of change of magnetic flux
• Magnitude of induced emf higher, when:

  • speed of relative motion increase
  • strength of magnetic field increase
  • number of turn in solenoid increase (for solenoid only)
• Lenz's Law: Induced current always in the direction opposing change of magnetic flux producing it.
• Moving North pole towards solenoid:

  • galvanometer needle deflects opposing direction of magnet (towards magnet)
  • induced emf and induced current produced due to cutting of magnetic flux
• Magnet held stationary:

  • galvanometer needle stays in the centre (no deflection)
  • no induced emf or induced current as magnetic field lines are not cut
• Moving North pole away from solenoid:

  • galvanometer needle deflects opposing direction of magnet (away from magnet)
  • induced emf and induced current produced through cutting of magnetic flux
• ac and dc generators apply electromagnetic induction to produce induced emf.
• When coil is vertical, induced current/emf is min (zero).

  when coil is horizontal, induced current/emf is at max.

  When coil goes from vertical to horizontal, induced current/emf goes from minimum to maximum.

  Split ring commutator keeps current in the galvanometer in the same direction as the coil rotates.
• Similarities between brushed and brushless dc motor:

  • have coil and magnet
  • converts electrical energy into rotational kinetic energy
  • force produced by catapult field
• Similarities between ac and dc generator:

  • electric current generated by electromagnetic induction
  • coil rotated by external force
  • coil cuts magnetic flux
  • emf and current induced in the coil
• Direct current (dc) generator:

  • output is direct current
  • connected by split ring commutator
  • split ring commutator and carbon brushes exchange contact every half rotation
• Alternating current (ac) generator:

  • output is alternating current
  • connected to two slip ring commutators
  • commutators are connected to the same carbon brushes for every rotation
• Transformers applies electromagnetic induction to increase or decrease voltage of an ac current.
• Step-up transformer:

  • Np < Ns
  • Vp < Vs
  • x-ray machine
  • defibrillator
• Step-down transformer:

  • Np > Ns
  • Vp > Vs
  • handphone / laptop charger
  • welding machine
• Efficiency of transformer = ( output power / input power ) x 100%
• Ideal transformer:

  • efficiency = 100%
  • no energy loss
  • output power / input power = 1
  • Vp x Ip = Vs x Is
• Cause and Effect of energy loss:

  • heating of coils - resistance in wires produces heat
  • eddy current - change in magnetic flux causes eddy current which produces heat
  • leakage of magnetic flux - alternating current not fully cut by the secondary coil
  • magnetic hysteresis - continuous change in magnitude produces heat energy
• Ways to reduce energy loss:

  • resistance of coil - use thicker copper wires to lower resistance
  • eddy currents - use laminated soft iron core
  • leakage of magnetic flux - wind secondary coil over primary coil
  • magnetic hysteresis - use soft iron core material
• Electrical power loss, P = I^2 x R