Home » Handbook of O Level Physics » UNIT 23: ELECTROMAGNETIC EFFECTS

# 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