Lesson Explainer: Electromagnetic Induction in Generators Physics • 9th Grade

In this explainer, we will learn how to describe electromagnetic induction in devices such as generators and dynamos.

Electromagnetic induction is the term for the production of an electric current in a conductor when the conductor is moving near a magnet.

The following figure shows a bar magnet at rest near to a coil of conducting wire.

Only one line of the magnetic field of the bar magnet is shown. The field lines actually radiate from the north pole of the magnet to the south pole symmetrically in all directions.

No current is induced in the coil if neither the magnet nor the coil moves.

The following figure shows the north pole of a bar magnet being mechanically moved either toward or away from the face of a coil of conducting wire.

A current is produced in the coil by electromagnetic induction.

We see that the direction of the current produced in the coil depends on whether the magnet is moved toward or away from the coil, but that in either case, a current is produced.

A current would also be produced if the magnet remained at rest and the coil was moved toward or away from the magnet. This current would have the same magnitude as if the magnet was moved.

If a magnet is moved through a conducting coil, from one side to the other, this means that the magnet is first moved toward the coil and then moved away from the coil. The current in the coil, therefore, reverses direction when this happens.

A generator is a device that uses electromagnetic induction to produce electric current.

The following figure shows the most basic components of a generator.

Diagrams of generators usually show only the poles of the magnet used.

The coil of a generator must be connected to a circuit, as shown in the following figure.

The circuit can contain any electrical device that we wish to use the generator to produce a current in.

To generate a current, forces must be applied to rotate the coil, as shown in the following figure.

Forces are applied to make the coil rotate.

It is very important to understand the following points about the forces causing the coil to rotate:

  • The forces causing the rotation of the coil are mechanical, applied forces.
  • The forces causing the rotation are not electric nor magnetic forces.
  • The forces causing the rotation do not depend on the existence of a magnetic field.
  • The rotation of the coil due to these forces is basically the same as the mechanical rotation of any object around an axis and is not an electromagnetic phenomenon.

When the coil rotates, a current is produced in the coil by electromagnetic induction. The circuit is connected to the coil, so there is also a current in the circuit.

It is important to notice that the circuit has not been included in the diagram showing the forces producing the rotation of the coil. A rotating coil can be connected to a circuit using different methods.

One of these methods is using slip rings. This is shown in the following figure.

It is useful to look at the rings themselves, shown in the following figure, in more detail.

We see that the ends of the coil are in contact with the rings.

The rings rotate, and they rotate at the same speed that the coil rotates at.

The rings are connected to the ends of the circuit by brushes that rub against the rotating rings. This connects the coil to the circuit.

For a coil that has a constant rotational speed, the change in current with time in such a circuit connected to a generator by slip rings is shown in the following figure.

The graph shows the change in current for one complete rotation of the coil.

We see that the current reverses direction during a rotation of the coil. For one half of the rotation, the current is in one direction through the circuit, and for the other half of the rotation, the current is in the opposite direction. This is alternating current.

Another method of connecting a generator to a circuit is using a commutator. This is shown in the following figure.

The commutator is shown in detail in the following figure.

We see that as the coil rotates, the two halves of the commutator also rotate. The speed of rotation is the same for the coil and the commutator.

Each of the two halves of the commutator connects to a brush that connects to a different end of the circuit.

When the commutator has rotated through half of one complete rotation, the end of the circuit to which each half of the commutator connects changes. This means that the end of the coil to which each half of the circuit connects also changes.

For a coil that has a constant rotational speed, the change in current with time in such a circuit connected to a generator by a commutator is shown in the following figure.

The graph shows the change in current for one complete rotation of the coil.

We see that the direction of the current is the same throughout a rotation but that the magnitude of the current changes. This is rectified alternating current.

The following figure shows how the current variation with time for one full rotation of a coil for connection using slip rings compares with that for connection using a commutator.

Let us now look at an example concerning the current produced by slip-ring design and commutator design generators.

Example 1: Understanding Generators

The diagram shows two designs for simple generators. The first design uses slip rings to conduct the induced current to an external circuit. The second design uses a commutator to conduct the induced current to an external circuit.

The graph below shows the potential difference against time for four different sources.

  1. Which line on the graph corresponds to the potential difference produced by the commutator design generator?
  2. Which line on the graph corresponds to the potential difference produced by the slip-ring design generator?

Answer

Part 1

The potential difference produced by a generator is directly proportional to the current produced by the generator. What can be said about the change in current with time for a generator can also be said about the change in potential difference with time for the generator.

The current produced by a generator that uses a commutator to connect to a circuit always has a single direction. This is not true for line 𝑆, so line 𝑆 can be eliminated.

The current produced by a generator that uses a commutator to connect to a circuit varies with time. This is not true for line 𝑄, so line 𝑄 can be eliminated.

The current produced by a generator that uses a commutator to connect to a circuit is zero at two instants during one complete rotation of the coil. This is not true for line 𝑃, so line 𝑃 can be eliminated.

Line 𝑅 shows a current that always has the same direction, varies with time, and has a value of zero at regular intervals. Line 𝑅 is the correct option.

Part 2

The potential difference produced by a generator is directly proportional to the current produced by the generator. What can be said about the change in current with time for a generator can also be said about the change in potential difference with time for the generator.

The current produced by a generator that uses slip rings to connect to a circuit varies with time. This is not true for line 𝑄, so line 𝑄 can be eliminated.

The current produced by a generator that uses slip rings to connect to a circuit reverses direction during each rotation of the coil. This is not true for lines 𝑃 and 𝑅.

Line 𝑆 shows a current that always repeatedly reverses direction. Line 𝑆 is the correct option.

The current produced by generators varies with time and is zero at two instants during a complete rotation of a coil. It is important to understand why the current varies in this way.

A current produced by electromagnetic induction in a moving wire is proportional to the force on the wire perpendicular to the magnetic field that the wire moves through.

The following figure shows the direction of current due to electromagnetic induction in the coil of a generator with the coil in two positions.

We see that when the force is perpendicular to the magnetic field of the generator, a current is produced perpendicular to both the motion of the wire and the magnetic field.

We see that when the force is parallel to the magnetic field of the generator, there is no force perpendicular to the magnetic field and no current is produced.

Let us now look at an example concerning the current produced by a slip-ring design generator at different positions of the rotation of the generator coil.

Example 2: Understanding Slip-Ring Generators

Parts (a), (b), (c), and (d) of diagram (A) show a setup for a simple alternating current generator. A single loop of copper wire rotates in a uniform magnetic field provided by two permanent magnets. The four parts of the diagram show the loop in four different positions as it rotates.

Diagram (B) is a graph of the current outputted by this generator over time.

  1. Which position of the loop in diagram (A) does point 𝑃 in diagram (B) correspond to?
  2. Which position of the loop in diagram (A) does point 𝑅 in diagram (B) correspond to?

Answer

Part 1

Point 𝑃 is the point at which the magnitude of the current in the loop is greatest.

Position (c) of the loop corresponds to zero current, as the force acting on the loop is parallel to the magnetic field of the generator. Position (c) can be eliminated.

When the force loop is parallel to the magnetic field, the current produced is zero. When the loop is perpendicular to the magnetic field, the current with the greatest magnitude is produced. This is at position (a).

Part 2

Point 𝑅 is the point at which the current in the loop is zero. The current in the loop is zero when the force on the loop is parallel to the magnetic field. This is at position (c).

The coil used in a generator can have one or more turns, as shown in the following figure.

Although the turns of a coil may look like they are separate, closed loops of wire, the turns are in fact a single length of wire. The ends of the coil are the ends of one wire. This means that there is only one path for current through the coil. This means that the turns of the coil can be considered to be connected in series.

When the coil of a generator rotates, the same potential difference is produced in each turn of the coil. The potential differences in each coil are combined in series, as the emfs of a group of batteries connected in series would be.

Let us now look at an example concerning the number of turns in a generator coil.

Example 3: Understanding Slip-Ring Generators

The diagram shows two designs of generators. Both designs use fixed permanent magnets to create a magnetic field and slip rings to conduct the induced current to an external circuit. Design (a) has a single loop of wire in the magnetic field, whereas design (b) has 5 loops. What is the advantage of design (b) over design (a)?

  1. Design (b) produces a higher output voltage than design (a).
  2. Design (b) produces a higher frequency output voltage than design (a).
  3. Design (b) produces a lower output voltage than design (a).
  4. Design (b) produces a lower frequency output voltage than design (a).
  5. Design (b) is cheaper to produce than design (a).

Answer

The only difference between the designs is the number of loops in the generator coil (this is also called the number of turns in the coil).

Design (b) uses more turns. This cannot be cheaper than using fewer turns, so option E is incorrect.

The frequency of the output depends only on the frequency of the rotation of the coil. Option B and option D are then incorrect.

Each turn in the coil of a generator has the same potential difference across it, and the turns of the coil are connected in series, as there is only one path across the coil for current.

Potential differences combined in series are summed. More turns must then correspond to a greater potential difference.

Design (b) uses more turns, so it must produce a greater output voltage. Option A states this and is the correct option.

Another issue concerning generator design is that the brushes used in generators become worn down by friction as they scrape the moving surfaces of slip rings or commutators. Generators that avoid the need for brushes by rotating the magnet of a generator, as shown in the following figure, do exist.

The magnets are mounted on a rotating disk. The coil does not turn, so it requires no brush contacts to an external circuit . The disadvantage of this design is that the magnet in a generator has a much greater mass than the coil of the generator, so greater force is required to rotate the magnets.

Let us now summarize what has been learned in this explainer.

Key Points

  • A generator uses electromagnetic induction to produce a current in a conducting coil.
  • The coil of a generator is rotated within a magnetic field.
  • A generator can be connected to a circuit using slip rings to produce an alternating current.
  • A generator can be connected to a circuit using a commutator to produce a rectified alternating current.
  • The current in a generator coil is greatest when the force on the coil is perpendicular to the magnetic field of the generator.
  • The current in a generator coil is zero when the force on the coil is parallel to the magnetic field of the generator.
  • The more turns in a generator coil, the greater the current produced in the coil.
  • Brushes in generator contacts wear out with use.

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