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
- Field lines
- 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.
- Motors
- 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
- Faraday's Law
- 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)
- Flux
- Properties of magnetic circuits
- permeance
- permeance = permeability x (A/L) = flux/ (NI)
- Needs to be as high as possible for the idea transformer
- permeance
- Drawing and interpreting field diagrams
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