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Review Test #7: Magnets & Electromagnetism
Reading: Ch. 24 p.p. 642-663, Ch. 25 p.p. 670-677, 682-689
Topics: Magnets, Electromagnets, Magnetic Fields, Compasses, Motors, Generators, Transformers
Labs & Activities: Experiments with Magnets, Making Motors, Right Hand Rules Quiz
Equations:
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F = BIL
F = qvB
P = IV
Vp = Vs
Np Ns
Know what domains are and how they are aligned in a magnet.
Know what materials can become magnets and how.
Know what an electromagnet is.
Know what a compass is and how it works.
Know how to draw and be able to recognize magnetic field lines.
Know how to use RHR1 (lengths of wire), RHR2 (electromagnets) & RHR3 (force on current carrying wire or
Know the parts of and how a motor works.
force on moving charge).
Know how to solve problems involving force on a charged particle or a length of current carrying
wire when they are in a magnetic field.
Know what an induced alternating current is and how it is created.
Know the parts of and how a generator works.
Be able to recognize and explain applications of generators (moving coil microphone, credit card strips,
light on a bicycle)
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Know what step-up and step-down transformers are.
Be able to solve problems involving transformers and power transmission.
1.
Describe why a permanent magnet is magnetized.
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2. Describe why an electromagnet is magnetized.
3. Draw the magnetic field lines for the magnets shown.
a)
b)
c)
d)
4. A number of countries are developing maglev (magnetic levitation) trains. Such trains travel
faster due to the elimination of friction between the train and the track. Explain how a train
could levitate above a track.
5. Some windows in homes and businesses are equipped with magnetic alarms. A magnet is placed
on the frame of a window. Explain what must happen when the window is raised to trigger an
alarm.
6. Junkyard cranes have enormous electromagnets that can be used to lift a car. Due to the
strong magnetic field of the electromagnet the steel of the car becomes magnetized. Even
when the electromagnet is turned off, the car remains attached to the magnet. Why? Explain
why reversing the current in the electromagnet releases the car.
7. A fax machine requires 15 volts but the wall outlet supplies 120 V. Explain what kind of
transformer should be used in the plug of the fax machine. What is the ratio of primary to
secondary coils?
8. Explain why and how electricity is transmitted over long distances with small currents.
9. Explain whether the magnets shown are attracting or repelling.
10. Which way doe the current carrying wire experience a force?
X represents the
direction of the
current in the wire.
11. Which way is current flowing through the length of wire? Which way does the other compass
point?
12. A hair dryer has a transformer attached to its plug, which transforms the incoming 120 V to
the 15 V needed. If the primary coil has 72 loops, how many loops should the secondary have?
13. Label each of the following devices as a motor (M) or a generator (G).
a) electric guitar string
b) fan
c) mixer
d) windmill
14. A power line carrying 100 A of current is suspended between towers located 50 m apart as
shown. The Earth’s magnetic field at this location is 5 x 10 ⁻⁵ T in the direction shown.
Calculate the magnitude and direction of the force on the wire.
15. A helium ion (+1) travels at right angles to a 0.8 T magnetic field with a velocity of 2 x 10 ⁵ m/s.
Find the magnitude of the force on the ion.