OCR B: Advancing Physics Chapter 15

  • Created by: Caraa
  • Created on: 23-05-13 13:15
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  • Electromagnetic Machines
    • Drawing and interpreting field diagrams
      • Field lines
        • Go from north to south
        • Closer together means stronger field
      • Right hand rule
        • To work out the direction on the field around  a wire
        • Thumb points in direction of current, fingers show field
      • Left hand rule
        • Thumb is the force, second finger is field, middle finger is the current
        • The direction of the force is also that that motion takes place
        • A current carrying wire in a magnetic field causes a force on the wire
        • F = BIl          B represents magnetic field strength/ flux density
      • Combining fields
        • Add the vectors
    • Types of electromagnetic machines
      • Motors
        • Where electricity is used to move the parts
        • Coil and magnet
        • Current carrying loop placed in magnetic field. This causes a force on the side arms causing it to rotate. Current reversed every half turn so the loop rotates steadily.
        • E.g a loud speaker where its fed with electricity which makes the speaker part movce
      • Transformers
        • Used to step up or down current or voltage
        • 2 coils, laminated iron core
        • E.g a adapter/ charger which needs to step down the voltage from the mains
        • Vs/Vp= Ns/Np (Where s is secondary and p primary)
        • A.C fed into primary coil which produces a magnetic flux. Magnetic field passes through iron core to the secondary coil where it induces a alternating voltage.
      • Generators
        • Creates a voltage/ current
        • Coil and magnet
        • Alternator is a generator of A.C
          • The voltage a current change direction every half rotation of the coil.
    • Electromagnetic induction
      • Faraday's Law
        • The induced e.m.f is directly proportional to the rate of change of flux
        • e.m.f = -N.d (flux) /dt
      • Lenz's law
        • The induced e.m.f is always in such a direction as to oppose the charge that caused it
        • Shown by the minus sign in the equation
      • e.m.f = vlB
      • An e.m.f is induced when there is relative motion between a conductor and magnet. Either one can move.
      • Cutting flux always induces e.m.f but will only induce current if the circuit is complete
    • Key terms/ quantities
      • Flux
        • Measure of how many field lines are cut
        • Symbol = phi
        • Units = Weber (Wb)
      • Flux linkage
        • Symbol = N.phi
        • Units = Weber (Wb)
      • Flux Density
        • Symbol = B = phi/A
        • Units = Tesla (T)
      • E.m.f
        • Symbol =  epsilon
        • Units = Volts (V)
      • Power
        • Symbol = P
        • Units = Watts (W)
      • Force
        • Symbol = F
        • Units = Newtons (N)
    • Properties of magnetic circuits
      • permeance
        • permeance = permeability x (A/L) = flux/ (NI)
        • Needs to be as high as possible for the idea transformer


Mrs Jones


A great resource to make your own.  The left hand rule could be explained more clearly: First finger = Field direction N➡️S, seCond finger = Current ➕➡️➖ and the thuMb points in the direction of Movement.  I'd add your own diagrams and more detail on transformers.

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