Physics@OLY Review Sheet for Electromagnetic Induction Name ...

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Nov 16, 2013 (3 years and 6 months ago)

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Physics@OLY Review Sheet for Electromagnetic Induction Name _______________ P____ 3/27/13

Concepts:

Electromagnetic induction is the inducing of voltage by changing the magnetic field near a conductor.



According to Faraday’s law, the induced voltage in

a coil is proportional to the product of the
number of loops and the rate at which the magnetic field within the loops changes.



A generator uses electromagnetic induction to convert mechanical energy to electric energy.



A transformer uses electromagnetic
induction to induce a voltage in the secondary that is different
from that in the primary coil.

Electromagnetic induction may be described in terms of fields.



A changing magnetic field induces an electric field.



A changing electric field induces a magnetic

field.



Electromagnetic waves are composed of vibrating electric and magnetic fields that regenerate each
other.



All electromagnetic waves travel at the same speed, the speed of light.

Terms

Electromagnetic induction (EM induction)

Faraday’s law

Generator

Lenz’s Law

Motor

Transformer (ac to ac & ac to dc)

Review questions

1. What did Michael Faraday and Joseph Henry discover?___

2. How can voltage be induced in a wire the help of a magnet? ___

3. A magnet moved into a coil of wire will induce voltage in the

coil. What is the effect of moving a magnet
into a coil with more loops? ___

4. Why is it more difficult to move a magnet into a coil of more loops that is connected to a resistor? ___

5. Current, as well as voltage, is induced in a wire by EM induction.
Why is Faraday’s Law expressed in
terms of induced voltage, and not induced current? ___

6. How does the frequency of a changing magnetic field compare with the frequency of the alternating
voltage that is induced?____

7. What is a generator, and how does
it differ from a motor? ___

8. Why is alternating voltage induced in the rotating armature of a generator? ___

9. The armature of a generator

must rotate in order to induce voltage and current. What causes the
rotation? ___

10. A motor is characterized by
3 main ingredients: magnetic field, moving charges, and magnetic force.
What are the 3 main ingredients that characterize a generator? ___

11. How can a change in voltage in a coil of wire (the primary) be transferred to a neighboring coil of wire
(the sec
ondary) without physical contact? ___

12. Why does an iron core that extends inside and connects the primary and secondary coils intensify EM
induction? ___

13. What does a transformer actually transform

voltage, current, or energy? ___

14. What does a ste
p
-
up transformer step up

voltage, current, or energy? ___

15. How does the relative number of turns on the primary and the secondary coil in a transformer affect
the step
-
up or step
-
down voltage factor? ___

16. If the number of secondary turns is 10 times
the number of primary turns, and the input voltage is 6
volts, how many volts will be induced in the secondary coil? ___

17. In a transformer, how does the power input to the primary coil compare with the power output of the
secondary coil? ___

17b. How do
es the product of the voltage and current in the primary compare to the product of the voltage
and current

in the secondary? ___

18. Why is it advantageous to transmit electrical power long distances at high voltages? ___

19. What fundamental quantity unde
rlies the concepts of voltages and currents? ___

20. Distinguish between Faraday’s law expressed in terms of fields and Maxwell’s counterpart to Faraday’s
law. How are the 2 laws symmetrical?___

21. How do the wave speeds compare for high
-
frequency and low
-
frequency and low
-
frequency EM
waves? ___

22. What is light? ___

23. Draw a photon in terms of Magnetic & Electric Field variations.



Think & Explain

2. Why is a generator armature more difficult to rotate when it is connected to and supplying electric
c
urrent to a circuit? ___

3. Some bicycles have electric generators that are made to turn when the bike wheel turns. These
generators provide energy for the bike’s lamp. Will a cyclist coast farther if the lamp connected to the
generator is turned off? ___

4. An electric hair drier running at normal speed draws a relatively small current. But if somehow the
motor shaft is prevented from turning, the current dramatically increases and the motor overheats. Why?
___

5. When a piece of plastic tape
coated with i
ron oxide that is magnetized more in some parts than others is
moved past a small coil of wire, what happens in the coil? ____ What is a practical application of this? __

6. When a strip of magnetic material, variably magnetized, is embedded in a plastic c
ard that is moved past
a small coil of wire, what happens in the coil? _____ What is a practical application of this? _____

7. If a car is made of iron and steel moves over a wide closed loop of wire embedded in a road surface, will
the magnetic field o
f the Earth in the loop be altered? ____Will this produce a current pulse? ___ What is a
practical application of this? ___

8. How could you move a conducting loop of wire through a magnetic field without inducing a voltage in
the loop? ___

9. Why does a
transformer require alternating voltage? ___

10. Can an efficient transformer step up energy? _____ Why? ____

11. What is wrong with this scheme? To generate electricity without fuel, arrange a motor to run a
generator that will produce electricity tha
t is stepped up with a transformer so that the generator can run
the motor while furnishing electricity for other uses.


Think & Solve

1. A transformer has an input of 9 volts and an output of 36 volts. If the input is changed to 12 volts, what
will the ou
tput be? ___

2. A laptop computer
requires 24 volts to operate properly. A transformer nicely allows the device to be
powered from a 120
-
volt outlet. If the primary has 500 turns, how many turns should the secondary have?
___

3. A model electric train requ
ires a low voltage to operate. If the primary coil of its transformer has 400
turns, and the secondary has 40 turns, how many volts will power the train when the primary is
connected to a 120
-
volt household circuit? ___