Back emf (electromotive force)

The video tutorial explains the workings of electric motors, focusing on the concept of magnetic flux and induced EMF. It discusses how the magnetic flux changes as a current-carrying loop rotates in a magnetic field, leading to an induced EMF that opposes the change. The concept of back EMF is introduced, explaining how it affects the current in electric motors and its practical implications, such as dimming lights when motors start. The tutorial also covers the design of motors to maintain constant back EMF using multiple loops and a split ring commutator.

What happens to the magnetic flux through a loop when it rotates 90 degrees in a magnetic field?

• It remains constant.

• It decreases.

• It increases.

• It becomes zero.

According to Lenz's Law, what is the direction of the induced magnetic field in a loop?

• It is parallel to the magnetic flux.

• It opposes the change in magnetic flux.

• It supports the change in magnetic flux.

• It is perpendicular to the magnetic flux.

What is the effect of back EMF on the current in an electric motor?

• It has no effect on the current.

• It decreases the current.

• It reverses the current direction.

• It increases the current.

Why do lights dim when an electric motor starts up?

• Because the motor draws more current.

• Because the back EMF is constant.

• Because the motor draws less current.

• Because the back EMF is high.

What happens to back EMF when an electric motor suddenly stops?

• It decreases to zero.

• It increases.

• It remains constant.

• It reverses direction.

Why does back EMF remain constant when a motor rotates at a constant angular velocity?

• Because the magnetic flux is constant.

• Because the motor has a single loop.

• Because of the split ring commutator.

• Because the torque is variable.

What is the role of a split ring commutator in an electric motor?

• It maintains a constant magnetic flux.

• It decreases the back EMF.

• It reverses the current every 180 degrees.

• It increases the speed of the motor.