
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 Npole approaches, the e.m.f. (or current) is in one direction.

When Npole 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 viceversa.

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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