Faraday's Law and Lenz's Law

Electric Generators and Back emf

By Rachel Wampler

Electric Generators

• A loop of wire is rotated in a magnetic field.

• Since the angle between the loop and the B-field is changing, the flux is changing.

• Since the magnetic flux is changing an emf is induced.​

Electric Generators

Electric Generators

Where:

N = number of loops

B = magnetic field

A = area of each loop

​ω = angular velocity = 2πf (f = frequency)

t = time in seconds​

emf Produced in a Rotating Planar Coil

AC Generators

• According to Lenz's Law, the current will flow the one direction when the angle is increasing and it will flow the opposite direction when the angle is decreasing.

• These generators are often called alternating current generators.

Multiple Choice

You have made a simple generator to power a TV. The armature is attached the rear axle of a stationary bike. For every time you peddle, the rear axel turns 10 times. Your TV needs a V_rms of 110V to operate. If the B-field is 0.2 T, each loop is a circle with r = 3 cm, and you can comfortably peddle 3 times a second; how many loops must you have in your generator so that you can watch TV while you exercise?

• 26 loops

• 4860 loops

• 155 loops

• 1460 loops

Back emf

• When a coils is turned in a B-field an emf is produced

• If an electric motor is running, its coil is turning in a B-field

• By Lenz's Law, this emf will oppose the emf used to turn the motor, called back emf.

• It will reduce the voltage across the motor and cause it to draw less current (Remember, V=IR)

• The back emf is proportional to the speed, so when the motor starts it draws max I, but as it speeds up the I decreases.​