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.

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:



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?



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.

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.
   
   

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.



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.

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.

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.



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.



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.