Chapter 15- Electromagnetic Machines

A summary of A2 OCR Advancing Physics Chapter 15

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Magnetic Fields and Motors

  • Magnetic field- a region where a force is exerted on magnetic materials. Represented by field lines going from north to south. The closer the lines, the stronger the field
  • If you put a current-carrying wire into a magnetic field, the fields will interact. The field lines from the magnet shorten and straighten to move to a lower energy level, creating a force to catapult the wire out of the field
  • The direction of the force is perpendicular to both the current direction and the magnetic field
  • Magnetic field strength- the force on one metre of wire carrying a current of one amp at right angles to the magnetic field. Also called flux density. Measured in teslas, T
  • If a current-carrying loop is placed in a magnetic field, the forces on the side arms will make it rotate. By using a split-ring commutator, the current can be reversed each time the loop becomes vertical (half-turn)
  • This allows the loop to rotate steadily- becomes a motor
  • Induction motors operate by altering the magnetic field around a coil of wire that is free to move, which induces a current in the wire, causing it to rotate
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Electromagnetic Induction

  • Magnetic flux density- strength of magnetic field per unit area
  • When you move a coil in a magnetic field, the size of the emf induced depends on the magnetic flux passing through the coil, and the number of turns
  • If a conducting rod moves through a magnetic field, its electrons will experience a force and will accumulate on one end of the rod. This induces e.m.f. (electromotive force) across the ends.
  • E.m.f. is induced wherever there is relative motion between a conductor and a magnet. It is produced whenever lines of flux are cut.
  • Flux cutting always induces e.m.f. but will only induce current if the circuit is completed
  • Faraday's Law- The induced emf is directly proportional to the rate of change of flux linkage
  • Lenz's Law- The induced emf is always in such a direction as to oppose the change that caused it. 
  • This agrees with the conservation of energy
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Transformers and Alternators 1

  • Transformers- make use of electromagnetic induction to change the size of voltage for an alternating current. An alternating current in the primary coil produces flux.
  • The magnetic field is passed through the iron core to the secondary coil, inducing a voltage of the same frequency.
  • Step-up transformers increase voltage by the secondary coil having more turns, and step-down transformers decrease voltage by having more turns on the primary coil
  • Permeance- the amount of flux induced in a material for a given number of turns that surround it. The higher the permeance, the greater the amount of flux induced
  • Permeance (like conductance) is inversely proportional to the length, and proportional to cross-sectional area
  • A good transformer has high permeance, so is short, fat and made from a high permeability material eg. iron
  • You also want the conductance of the copper coils to be as high as possible to limit energy loss. Need right number of turns with shortest piece of wire possible
  • Even though there is an air 'gap' in the iron core, flux still flows. As air has a very low permeability compared to iron, so the amount of flux in the circuit would be dramatically lower than without the air gap
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Transformers and Alternators 2

  • Transformers are not 100% inefficient. There are small losses of power, mostly in the form of heat
  • Heat is produced by eddy currents in the iron core- current induced by the changing magnetic flux in the core. The effect is reduced by laminating the core with layers of insulation
  • Heat is also generated by resistance in the coils- minimised by using thick copper wire with a low resistance
  • Electricity from power stations is sent round the national grid at a low current, as loses due to resistance in the cables are proportional to I2. Low current = high voltage
  • Generators- convert KE into electrical energy, They induce an electric current by rotating a coil in a magnetic field
  • An alternator looks like a motor, but with slip rings and brushes instead of a split-ring commutator. The output voltage and current change direction with every half-turn, producing alternating current
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