Download Electromagnetic Induction Study Guide

Electromagnetic Induction Study Guide
AP Physics
Students should:
Know what electromagnetic induction is and when it happens
Be able to use Lenz’s law to predict which way an induced current will flow
Be able to calculate magnetic flux through a loop of wire
Be able to calculate the EMF or voltage produced by a changing magnetic flux
Be able apply the ideas of electromagnetic induction to generators, motors and transformers
Be able to calculate the voltage produced by a generator
Know what back emf is and how it affects a motor’s operation
Be able to calculate the back emf in a motor
Know how a transformer works and where it might be used
Be able to calculate the ideal output voltage and current for a transformer
Be able to calculate the number of windings to create the right multiplier for a transformer
Be able to explain how electromagnetic induction explains light as electromagnetic waves
Be able to explain the properties of electromagnetic waves and how they affect our experience of light
Review Questions
1. Pushing a door open shoves a magnet
attached to the door toward a solenoid of
20 coils with a cross-sectional area of
.0004 m2 as shown. The magnetic field
inside the coils changes from 0.05 T to
0.18 T in 1.9 s. (Assume that the magnetic
field is perpendicular to the coils and has
the same strength at all coils)
a) How big is the electromotive force
between the two ends of the solenoid
generated by electromagnetic induction?
b) If the resistance in the bell circuit that is connected to the solenoid is 5 , how large and in what
direction is the current flowing through it?
2. A magnet is pulled away from a 2 cm
radius, 7 coil solenoid at a rate of 4 cm/s.
While the magnet moves 7.5 cm, the
magnetic field drops in strength from 0.8
T to 0.22 T. (Assume that the magnetic
field is perpendicular to the coils and has
the same strength at all coils)
a) Which way will the current through the
solenoid flow?
b) How big is the potential difference
generated by electromagnetic induction
between the two ends of the solenoid?
3. A portable generator runs by spinning 300 square loops of wire that are 10 cm x 10 cm through a
0.106 T magnetic field at a frequency of 60 Hz. What is the amplitude of the emf produced?
4. You have a generator that is made from 450 circular wire loops that are 3 cm in diameter and rotate
in a magnetic field of 1.7 T from permanent magnets. How quickly would the loop have to rotate to
produce an emf with an amplitude of 15 V?
5. When it is turning at operating speed, a motor with 7  of resistance connected to 120 V draws 2 A
of current. What is the back emf of the motor at operating speed?
6. A Jacob’s Ladder (the ubiquitous mad scientist device where an electric spark climbs between two
wires while making a static sound) is based around a transformer.
a) If the transformer has 60 loops in its primary coil and 7200 loops in its secondary coil, what will the
output voltage be when you plug it into a 120 V outlet?
b) If the primary coil has a maximum current of 10 A, what will the maximum current in the secondary
coil be?
7. The electrical lines running through the power distribution station have a potential difference of
230,000 V between them.
a) If they wanted to tap those lines to run a coffee pot in the operating room that required 4.7 A of
current at 120 V and had a transformer with 200 turns in the secondary coil, how many turns would the
primary coil need to have?
b) How much current would be drawn from the high voltage line?