Electromagnetic Induction

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Electromagnetic Induction

• Chapter 28

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• When the magnetic field passing through a coil
  changes, an emf appears in the coil. This
  phenomenon is called electromagnetic induction.
• We use the quantity of magnetic flux , to
  calculate the size of this emf. Suppose a plane
  loop of wire of area A has a magnetic field at
  right angles to it. Suppose the magnetic flux
  density at a point on the loop is B, then the
  magnetic flux density passing through area A is
  defined by
• Magnetic Flux through Area A Magnetic flux
  density x Area
• i.e. ? B A
• The unit of magnetic flux is the weber (Wb). One
  weber is defined as the flux through the area if
  the magnetic flux density over an area of 1m2 is
  1tesla.

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Michael Faraday
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Royal Institution Christmas Lectures
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• Faradays law of electromagnetic induction states
  that the size of the emf is directly proportional
  to the rate of change of flux.

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• Lenzs law states that the direction of an
  induced current is always such as to oppose the
  charge producing it.
• An electric generator is a device that converts
  mechanical energy to electrical energy. It does
  this by the process of electromagnetic induction.
  Examples include electric power stations,
  alternators in cars and dynamos in bicycles.

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• An alternating current is one where the direction
  of the electrons changes their directions
  regularly, e.g. we get a sine graph so the graph
  is called a sinusoidal graph. To view this we use
  an oscilloscope. The national grid supplies AC.
• When we compare a value of alternating current to
  a value of direct current we find that we will
  need a higher value of alternating current to
  give the same heating power as a similar value of
  direct current. This is because alternating
  current varies over time. If we had 5A d.c., we
  need a higher value of a.c., we call this the rms
  value.

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• Similarly for alternating voltages
• In a resistor we have the power of a.c. flow is
• given by

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• If a changing magnetic field in one coil causes
  an induced emf to appear in a second nearby coil
  there is said to be mutual induction between the
  two coils, e.g. transformers.
• Whenever the current passing through a coil
  changes, the magnetic field surrounding that coil
  changes. This changing magnetic field induces an
  emf in the coil that opposes the changing current
  (i.e. a back emf), e.g. inductors. This
  phenomenon is known as self induction.

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• A coil (an inductor) opposes the flow of direct
  current (d.c.) with its resistance (in ohms).
• A coil (an inductor) opposes the flow of
  alternating current (a.c.) with its resistance
  (in ohms) and the back emf induced in it.
• A charged capacitor blocks d.c. A capacitor
  conducts a.c. since it charges and discharges as
  the a.c. changes direction.
• A transformer is a device that is used to change
  the value of an alternating voltage.

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• If Ns is greater than Np then Vo is greater than
  Vi and it is called a step up transformer.
• If Ns is less than Np then Vo is less than Vi and
  it is called a step down transformer.
• Transformers are used in electricity distribution
  to increase (when leaving power stations) voltage
  on the national grid and to reduce (at
  substations, before reaching the consumer). Click
  here
• Computers, televisions, radios, mobile phone
  chargers all contain transformers to supply the
  necessary voltage to the various parts.

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Not a transformer