Moving Wires and Motors
Introduction

Electrical energy is probably the most useful form of energy on earth. One reason for this is that electricity can very easily be converted into motion. Loudspeakers, hairdryers, vacuum cleaners, and electric motors are a few everyday examples of appliances which turn electricity into movement. They all use an effect known as the motor effect.
The motor effect
When a current flows through a wire it produces a
magnetic field
Around every magnet there is an invisible magnetic field.
magnetic field
.

When a magnet is placed in a magnetic field, it experiences a
force
A force is a push or a pull. A force can lift an object, change its shape, or accelerate it.
force
.

What do you think happens when a current-carrying wire is placed in a magnetic field?

Investigate what happens using the experiment in Fig.1 below:

Figure 1.   A wire in a magnetic field.
Display as fullscreen


With no current flowing, the wire does not move at all.

Push the top button to set the current flowing towards you. Which way does the wire move?

Now set the current so it flows away from you. Which way does the wire move now?

Use your findings to complete the table below.

  • Direction of magnetic field Direction of current Direction of wire's motion
    Left to right
    Left to right

  • Click here to mark the question


If the magnetic field were reversed, the movement would change direction too. This movement of a current-carrying wire placed in a magnetic field is called the motor effect. The full table of field, current and motion for the
motor effect
When a current flows along a wire in a magnetic field, the wire experiences a force. This is known as the motor effect.
motor effect
is shown below.


Direction of magnetic field Direction of current Direction of wire's motion
Left to right Towards you Upwards
Left to right Away from you Downwards
Right to left Towards you Downwards
Right to left Away from you Upwards

Fleming's left-hand rule
The direction of the current, the direction of the magnetic field and the direction of movement are at right angles to each other. The way to remember how the wire moves is by using
Fleming's left-hand rule
Fleming's left-hand rule can be used to work out the direction of the force when a current flows in a magnetic field.
Fleming's left-hand rule
. Hold your left hand as shown in the picture in Fig.2. Follow these three steps to learn how to predict which way the wire will move:

  1. Rotate your hand so the First finger is in the direction of the magnetic Field.

  2. Point your seCond finger in the direction of the Current.

  3. Your thuMb is now pointing in the direction of the Movement.

Figure 2.   Fleming's left-hand rule.
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Let's put this into practice. For example, when the magnetic field points from left to right, point the first finger of your left hand to the right. The current flows out of the screen, so point your second finger towards you, out of the screen. Now your thumb is pointing upwards. This is the direction of motion, and agrees with what you found in the experiment.

Practise using the left-hand rule to check your other answers.

If the magnetic field pointed from top to bottom and the current flowed into the screen, which way would the wire move?
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The force exerted on the wire depends on two factors:
  • the strength of the magnetic field

  • the current flowing through the wire


A larger magnetic field produces a larger force, and so does a larger current.

The moving-coil loudspeaker
Loudspeakers make use of the motor effect. An electrical coil is placed inside a magnetic field. When the current through the coil changes, it moves backwards and forwards. The coil is connected to a cone, which vibrates and creates sound. This is called a moving-coil loudspeaker.
Click on the figure below to interact with the model.

 Figure 3.  The moving-coil loudspeaker.

Coils and motors
In Fig.4 a coil of wire has been placed in a magnetic field. Current can flow around the coil. The (conventional) current will flow anticlockwise around the coil (viewed from above) to make the top of the coil into the north pole. The north pole of the coil will be attracted towards the south pole of the magnet. This means that the coil will rotate. Before you turn the current on, think about whether the coil will rotate clockwise or anticlockwise.

In which direction will the coil move when you turn the current on?
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Figure 4.   An electric motor. Watch how the coil rotates when a current flows around the coil.
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The motor is designed so that even when the coil turns upside down, the current still flows anticlockwise around the coil. This means that the coil continues to turn in the same direction.

Click on the 'Apply/reverse current' button to change the direction of current flow. It should now be flowing clockwise around the coil. Watch what happens to the motion of the coil.

In which direction does the coil move when the current flows clockwise around the coil?
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You can use Fleming's left-hand rule to predict which way the coil will turn. Check that this rule correctly predicts the direction in which the coil rotates.

These are a few ways to make an electric motor more powerful:
  • Use a larger current flowing through the coil.

  • Increase the number of turns of wire around the coil.

  • Use a soft-iron core in the middle of the coil.

  • Use stronger magnets, such as electromagnets either side of the coil.

Summary


When a current flows along a wire in a magnetic field, the wire experiences a force. This is known as the motor effect.

The direction of the force depends on the direction of the current flow and the magnetic field.

The direction of the force can be worked out using Fleming's left-hand rule.

Exercises
1. Use Flemings's left-hand rule to fill in the blanks in the table below.

  • Direction of magnetic field Direction of current Direction of wire's motion
    West Down
    West Up
    North West
    West North
    North Down
    Up South

  • Click here to mark the question
2. A current is flowing through a wire in a magnetic field. Do the changes below increase the size of the exerted force?
  • Changing the direction the current is flowing in
    Increasing the current
    Increasing the resistance of the wire
    Increasing the strength of the magnetic field
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3. If you want to make a loudspeaker make a noise, what type of current should you pass through it?
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4. What happens to an electric motor if you reverse the direction of current flow?
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5. Decide what effect each of the changes below would have on the
power
Power is the rate of doing work, or the rate of energy transfer. The unit of power is the watt, W.
power
of an electric motor.
  • Reduce the number of turns of wire around the coil
    Increase the strength of the magnets either side of the coil
    Increase the current flowing through the coil
    Change the direction the current is flowing in
  • Click here to mark the question
Well done!
Try again!