UNIT 23: ELECTROMAGNETIC EFFECTS

• Electromagnetic induction: It is the phenomenon of inducing an e.m.f. in a circuit due to a changing magnetic field.

• Faraday’s law: The magnitude of induced e.m.f. is proportional to the rate of change of magnetic flux linking the circuit.

• Lenz’s Law: The direction of induced e.m.f. (and hence current) is such as to oppose the change producing it.

• A.C. Generator

  • Types

    • A coil can be rotated next to a magnet. OR

    • A magnet can be rotated next to a coil.

  • Working

    • There is a magnetic field around permanent magnet.

    • As coil rotates, the coil cuts the magnetic flux of the permanent magnet.

    • Therefore, e.m.f. is induced.

    • When N-pole approaches, the e.m.f. (or current) is in one direction.

    • When N-pole goes away, the e.m.f. (or current) is in the opposite direction.

  • Facts to remember

    • Doubling the number of turns on coil doubles the peak voltage.

    • Doubling the frequency (speed) of rotation doubles the peak voltage.

  • You can increase the peak voltage by

    • increasing turns on coil

    • increasing frequency of rotation

    • increasing the strength of magnetic field by

      • using soft iron core

      • using stronger magnet

      • bringing coil and magnet closer

• Transformer: It is a device that changes a high alternating voltage at low current to a low alternating voltage at high current or vice-versa.

  • Construction

    • Two coils (primary and secondary)……………………one has more turns than the other.

    • Soft iron core

    • Use alternating voltage symbol ~ on primary side

  • Types

    • A transformer that steps up voltage.

    • A transformer that steps down voltage.

  • Transformer equation: V_s / V_p= N_s / N_p = I_p / I_s

    where N is number of turns

• Transmission of electrical power over long distances

  • The challenge: To reduce loss of electrical energy in the form of heat on the power lines.

  • Solutions:

    • We can use very thick cables but this increases the cost of cables and supporting structures.

    • We can use a transformer to step up voltage and thereby reduce current.

      Relevant formula of power loss is: P = I² R
      

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