Electromagnetic Induction Study Guide

By Andrea Cook,2014-05-06 14:32
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Electromagnetic Induction Study Guide

Electromagnetic Induction Review Sheet 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. A door is rigged with a circuit that

    rings a bell when the door is opened.

    Pushing the door open shoves a magnet

    attached to the door toward a solenoid of

    20 coils with a cross-sectional area 2of .0004 m 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.

    a) What is the maximum emf produced?

    b) What is the average emf produced by the generator

    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 average emf 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?

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