Home » Handbook of O Level Physics » 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: Vs / Vp= Ns / Np = Ip / Is

      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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