Electromagnetic induction

Generating electricity

In the previous guide, we have seen what an electric motor does; an electric motor converts electrical energy into kinetic energy.

In order to generate electricity, we need a device that will work the opposite; a device that will transform the kinetic energy into electrical energy.

Such kind of a device is called a generator.

There are different designs of generators for different purposes. For example, if you need a generator for powering the headlights of your bicycle, just attach a dynamo to it.

How does a generator work?

All generators have three things in common:

  1. A magnetic field (provided by magnets or electromagnets).
  2. A coil of wire (fixed or moving coil).
  3. Movement (the coil and magnetic field move relative to each other).

The principles of electromagnetic induction

The process of generating electricity from motion is called as electromagnetic induction.

As we have emphasised before, a coil of wire and a magnet moving relative to each other are needed to induce a voltage across the ends of the wire. This is called as the dynamo effect.

When the coil and the magnetic field move relative to each other, a current flows through the coil if it is a part of a complete circuit. This is known as an induced current.

If the generator is not connected to a circuit, there will be an induced electro motive force (e.m.f) across its ends, ready to make a current flow around a circuit.

An a.c generator

An a.c. generator is a device such as a dynamo, used to generate alternating current. In actual, an a.c generator is the same as a d.c. motor working in reverse!

How an a.c generator works?

  • The axle of the a.c. generator is made to spin so that the coil spins over a magnetic field.
  • This causes a current to be induced.
  • The current moves into the slip ring commutators.
  • As the slip ring commutators are connected to a pair of brushes, current flows through them as well.

Note: There is a difference between how an a.c generator and d.c motor is connected to a circuit:

A d.c motor uses split ring commutators; an a.c. generator uses slip ring commutators.

Alternating current produced by the a.c generator has a tendency to move back and forth (unlike direct current which moves only in one direction).

This happens because each time the coil rotates; it first passes through the magnetic North Pole and then the South Pole.

Electromagnetic induction and magnetic field lines

The actual reason why a current is induced is because the rotating coil cuts through the magnetic field lines.

The idea stated above helps us to understand the factors that affect the magnitude and direction of the induced e.m.f:

  • If the magnet is stationary, there is no cutting of field lines and so no e.m.f is induced.
  • If the magnet is further from the wire, the magnetic field lines are further apart as well, and so fewer are cut giving a smaller e.m.f
  • If the magnet is cut quickly, the lines are cut more frequently and quickly, causing a bigger e.m.f to be induced.
  • A coil gives a bigger effect than a single wire, because each turn of the wire cuts the magnetic field lines, and each therefore contributes to an induced e.m.f.

Hence, from these points we can conclude the four ways of increasing the voltage generated by an a.c. generator:

  1. Turn the coil more rapidly.
  2. Use a coil with more turns.
  3. Use a coil with a bigger area.
  4. Use more stronger and powerful magnets.

Direction of induced e.m.f.

All currents have a magnetic field around them. This field always pushes back against the field that is inducing the current.

So for a coil, when the magnet’s north pole is pushed towards the coil, the current flows so as to produce a north pole at the end of a coil nearest to the magnet. These two north poles repel each other hence you need to push the magnet towards the coil and there by do work.

The energy you use in pushing the magnet is transferred to the current that is where the energy carried by a current comes from; the work done a conductor cut through magnetic  field lines.

An induced current always flows in the direction of its magnetic field that opposes the charge causing it.


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