Triple Physics Topic 10 - Motors

The Motor Effect

You may well have seen a demonstration using equipment set up like this: This experiment demonstrates the motor effect

When there is no current, the carbon rod does not move.

 Question 1 Why do we not use a steel rod? When the current is turned on in the way shown in the picture, the carbon rod rolls along the rails from left to right as shown.

 Question 2 What would happen if the current were reversed? Question 3 What would happen to the rod if the current were flowing as before, but the magnet were turned upside down? Question 4 What would happen to the rod, if both the current and the magnetic field were reversed? Question 5 What would happen to the carbon rod if the magnet were not there? This experiment tells us that if a magnetic field interacts with an electric current, there is a force, which results in the carbon rod moving.

The reason behind this is that whenever an electric current flows, it ALWAYS makes a magnetic field.  It doesn't matter that the material the current flows through is not magnetic, a magnetic field is formed.  Carbon is certainly non-magnetic, but a field is formed around the carbon rod when the current flows.  This interacts with the field from the magnet.

If the current were larger the force would be larger.

 Question 6 What would happen to the force if the magnetic field were stronger? Now we move the magnetic field through ninety degrees.  The magnet itself is not shown for clarity. The magnetic field is now parallel to the current.  We find that the rod does NOT move at all, however strong the current.

In order for there to be a force on a wire:

• there has to be a current;

• there has to be a magnetic field;

The force is maximum when the angle between the magnetic field and the current is 90 degrees.  It is zero when the current and the magnetic field are parallel.

The force can be increased by using a coil of wire instead of a single wire.

Fleming's Left Hand Rule

For the maximum force, the current and magnetic field are at right angles, and the motion is at right angles as well.  This is summed up in Fleming's Left Hand Rule: The motor effect is used in:

• Electric motors;

• Moving coil voltmeters and ammeters (the old fashioned type with a needle);

• Loudspeakers. The Simple Electric Motor

The electric motor uses the interaction of a magnetic field on a current to produce a force.  This is true from the smallest toy motor like this: to a large traction motor like this: We will look at how the motor effect is used in a simple motor.  You are not expected to know the detailed working of the electric motor, but you should know how the principles above are applied to the electric motor.

An electric motor is made up like this. The main parts of the motor are:

• the magnets that provide the magnetic field.  In this motor they are permanent magnets, but in many motors they are electromagnets.

• the armature, which is a coil of wire free to turn.  The armature in this motor is made of a single coil of wire.  It has two poles.

• The commutator, which connects the armature to the brushes.  The current passes to the armature by the commutator.

• The brushes that bring the current to the commutator.

When the current is turned on, the force from the current and the magnetic field makes the left hand side of the armature go up, while the right hand side goes down.  The motor turns clockwise. Question 7 Would the force be a maximum or minimum at this point?  Why? Now the armature has moved through 90 degrees. Question 8 Can you suggest two reasons why the force is zero? Although the force is zero, the armature has sufficient momentum to keep it going.  The action of the commutator makes the current flow anticlockwise all the time, so there is an upwards force on the left and a downwards force on the right, so the motor keeps spinning clockwise.

 Question 9 Suggest two different changes that would make the motor turn anticlockwise. Question 10 What would happen if you applied the changes in Question 9 at the same time? Question 11 What would happen if you tried to start the motor with the armature at the vertical position as shown in the picture above? The force which a simple motor can produce can be increased by:

• increasing the current through the armature;

• increasing the number of turns in the coils on the armature;

• increasing the magnetic field strength.  In a motor where the magnetic field strength is provided by electromagnets, the magnetic field strength can be changed quite easily.

This is a common problem with two pole motors, which have a single coil of wire as shown above.  Often a two pole motor needs a spin to get it going.  Most toy motors have three poles.  Therefore they can start whatever position the poles are.  Larger motors have many more.

Electric motors can be made more efficient by having more poles, and circular magnets, which means that the magnetic field remains at 90 degrees all the time.

(Extension only)  Click here to read about the generator effect.  It is NOT on the syllabus.

 Summary The motor effect involves the interaction of a magnetic field with an electric current to produce a force. The magnetic field must be at 90 degrees to the current to get a maximum force. The direction of the force is at 90 degrees to the field and the current. The stronger the current or the magnetic field, the stronger the force. This effect is used in electric motors.