3.15 How the Electric Motor Works

Question 1

What does a d.c. motor consist of?

Answer 1

A rectangular current - carrying coil sitting in a uniform magnetic field between two poles

Question 2

Why will a current-carrying coil in an electric motor experience forces in opposite directions?

Answer 2

Because the current is flowing in different directions on each side of the coil, and each side of the coil is perpendicular to the magnetic field

Question 3

What do electric motors use to swap the contacts every half turn to keep the motor rotating in the same direction?

Answer 3

Split-ring commutator

Question 4

How can the direction of the electric motor be reversed? (2)

Answer 4

• By swapping the polarity of the d.c. supply (reversing the current)
• By swapping the magnetic poles over (reversing the field)

Question 5

As electric motors start running, what happens to the current?

Answer 5

The current through it decreases from its initial value. As the current flows, it dissipates some energy which heats up the coil (and so increases the resistance).

Question 6

How do electric motors behave like generators?

Answer 6

As it moves, it creates a current in the opposite direction to the current originally flowing through it.

Question 7

What are three places that electric motors can be found with explanations? (3)

Answer 7

• Factories: many machines contain large electric motors to allow them to move. Motors are also used to drive conveyor belts which transport goods between machines.
• Electric vehicles: the driving force for these cars is provided by electricity instead of petrol or diesel.
• Household appliances: almost all household appliances that contain moving parts contain an electric motor - hair dryers, vacuum cleaners, blenders, fans etc.

Question 8

What are three ways to speed up an electric motor?

Answer 8

• Increasing the current
• Adding more turns to the coil
• Increasing the magnetic field strength

Question 9

How the split ring commutator ensures the coil rotates in one direction (4)

Answer 9

• The split-ring commutator swaps the positive and negative connections every half turn.
• Therefore, the direction of the current through the coil reverses every half turn.
• This means that the forces acting on the coil will always be acting in the same direction.
• This causes the coil to rotate continuously in one direction.