# OCR B: Advancing Physics Chapter 15

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• Created by: Caraa
• Created on: 23-05-13 13:15
• 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
• 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
• 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
• 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)
• 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