Chapter 13 - Electromagnetism
TRUE/FALSE
1. The N-pole of a magnet will repel the N-pole of another magnet.
ANS: T
2. A compass can be used to detect the direction of a magnetic field.
ANS: T
3. The magnetic field of a bar magnet is weakest at its poles.
ANS: F
4. The closer the field lines are, the stronger the magnetic field.
ANS: T
5. Michael Faraday discovered that the current in a wire could cause a compass needle to move.
ANS: F
6. The magnetic field around a conductor becomes weaker as the distance from the conductor
increases.
ANS: T
7. If the direction of the current in a coil is reversed, the poles of the magnetic field in the coil will also
be reversed.
ANS: T
8. The magnetic field of a conductor is made weaker when it is twisted into a loop.
ANS: F
9. The strength of an electromagnet may be increased by decreasing the current in the conductor.
ANS: F
10. Removing some coils from an electromagnet will make the magnet stronger.
ANS: F
11. When using the right hand rule for the motor principle, your thumb represents the direction of the
force on the conductor.
ANS: F
12. A motor converts electrical energy to kinetic energy.
ANS: T
13. An ammeter measures the direction of electric current.
ANS: F
MULTIPLE CHOICE
1. Lodestone
a. is a type of iron ore
b. can attract small pieces of iron
c. will rest in a north-south direction when allowed to pivot freely
d. has all of the above characteristics
e. has none of the above characteristics
ANS: D
2. Which statement about the N-pole of a magnet is true?
a. It will repel an S-pole.
d. It is a north-seeking pole.
b. It is stronger than an S-pole.
e. It is a south-seeking pole.
c. It is weaker than an S-pole.
ANS: D
3. The N-pole of a bar magnet will
a. attract a positive charge and repel a negative charge
b. repel a positive charge and attract a negative charge
c. attract positive and negative charges
d. repel positive and negative charges
e. not affect a charged object
ANS: E
4. Which substance would not be attracted by a magnet?
a. copper
d. steel
b. iron
e. cobalt
c. nickel
ANS: A
5. Which statement about the magnetic north pole of Earth is true?
a. Its location never changes.
b. It corresponds to the N-pole of a bar magnet.
c. It corresponds to the S-pole of a bar magnet.
d. It is at the same location as the geographic north pole of Earth.
e. both B and D
ANS: C
6. Magnetic field lines
a. show the strength of a magnetic field
b. show the direction of a magnetic field
c. become more widely spaced as the magnetic force weakens
d. all of the above
e. none of the above
ANS: D
7. The magnetic field lines around a bar magnet
a. cluster mostly around the N-pole
b. travel from the negative pole to the positive pole
c. leave the S-pole and travel to the N-pole
d. travel from the positive pole to the negative pole
e. none of the above
ANS: E
8. Which statement about magnetic fields is NOT true?
a. The spacing of the field lines indicates the strength of the field.
b. There is no magnetic field inside a magnet.
c. Field lines do not cross one another.
d. The field is three-dimensional.
e. The field weakens as the distance from the magnet increases.
ANS: B
9. The circle between the ends of the horseshoe magnet shown below represents a compass. If the
pole on the left is the N-pole, in which direction will the needle of the compass point?
a. to the left
b. to the right
c. up
d. down
e. in circles
ANS: B
10. A bar magnet is placed on a flat surface, with its N-pole on the right side. If a compass is placed
near the N-pole, the compass needle should point
a. to the left
b. to the right
c. up
d. down
e. in circles
ANS: B
11. Two equal bar magnets are placed end-to-end with opposite poles a small distance apart. If a small
piece of iron is placed exactly halfway between the magnets, the iron will
a. be attracted to the N-pole
d. move upward
b. be attracted to the S-pole
e. move downward
c. stay in the middle
ANS: C
12. A bar magnet is placed on a flat surface, with its N-pole on the right. If a compass is placed just
above the middle of the magnet, the compass needle should point
a. to the left
b. to the right
c. up
d. down
e. in circles
ANS: A
13. What will most likely happen when a bar magnet made of iron is heated?
a. The magnet will be permanent.
b. The magnet will become stronger.
c. The magnet will become weaker.
d. The poles of the magnet will reverse.
e. The magnet will become ferromagnetic.
ANS: C
14. A magnetic domain is best described as
a. the magnetic field around a magnet
b. an atom that acts as a tiny magnet
c. a group of atoms with their magnetic axes lined up in the same direction
d. a metal that can be magnetized by induction
e. the strength of a magnetic field
ANS: C
15. An iron nail can be turned into a temporary magnet if it is held in a strong magnetic field. This
method of magnetization is called
a. induction
d. charging
b. saturation
e. convection
c. conduction
ANS: A
16. A bar magnet may lose its magnetic ability. This process is called
a. induced magnetism
d. ferromagnetism
b. reverse magnetism
e. demagnetization
c. magnetic saturation
ANS: E
17. Which scientist discovered the relationship between electricity and magnetism?
a. Andre Ampere
d. Hans Oersted
b. Alessandro Volta
e. Albert Einstein
c. Michael Faraday
ANS: D
18. The following diagram shows the current direction in a straight conductor. The magnetic field lines
around the conductor should be drawn as
a.
b.
c.
d.
e.
concentric clockwise circles around the conductor
concentric counterclockwise circles around the conductor
straight lines in the same direction as the current
straight lines in the opposite direction to the current
straight lines radiating out from the conductor
ANS: B
19. The field lines around a straight conductor are drawn in clockwise circles around the conductor.
The current in the conductor must be travelling
a. into the page
d. to the right
b. out of the page
e. in clockwise circles
c. to the left
ANS: A
20. Two separate wires are placed side by side. When a current travels through them, they repel one
another. Which statement best explains this observation?
a. The wires have magnetic fields around them.
b. The currents are flowing in opposite directions.
c. Both currents are flowing in the same direction.
d. both A and C
e. both A and B
ANS: E
21. The magnetic field lines inside a coil
a. are straight
b. point in the same direction
c. are equally spaced
d. all of the above
e. none of the above
ANS: D
22. The strength of the magnetic field of a coil may be increased by
a. increasing the current in the wire
b. wrapping it around an iron core
c. increasing the number of loops in the coil
d. all of the above
e. none of the above
ANS: D
23. An electromagnet has a lifting force of 3.6 N. If the current through the coil is doubled and the
number of coils is tripled, the lifting force will become
a. 0.90 N
d. 14 N
b. 1.8 N
e. 22 N
c. 2.4 N
ANS: E
24. An electromagnet had a lifting force of 4.5 N. When a core was inserted and the current was
doubled, the lifting force was increased to 12.3 N. What was the relative permeability of the core?
a. 6.2
d. 5.5
b. 2.7
e. 11
c. 1.4
ANS: C
25. The diagram below shows a straight conductor between the poles of a permanent magnet. The
direction of the magnetic force on the wire will be
a. up
b. down
c. left
d. right
e. out of the page
ANS: B
26. Which of the following changes would cause the armature of a DC motor to turn faster?
a. increasing the current in the circuit
b. increasing the number of turns in the coil
c. increasing the strength of the field magnets
d. all of the above
e. none of the above
ANS: D
27. The brushes of a DC motor
a. connect the coil to the armature
b. provide a permanent magnetic field
c. allow the armature to spin easily
d. connect the coil to the circuit
e. change direction with every rotation of the armature
ANS: D
28. If the split-ring commutator of a DC motor was replaced with a solid commutator ring, the
armature would
a. rotate more quickly
b. rotate in the opposite direction
c. stop when the coil was at right angles to the magnetic field
d. stop when the coil was parallel to the magnetic field
e. rotate more slowly
ANS: C
COMPLETION
1. A concentrated area of magnetic force is called a(n) ____________________.
ANS: pole
2. The needle of a compass placed near the ____________________ of a bar magnet will point away
from the magnet.
ANS: N-pole
3. A material that can be induced to become a magnet is called ____________________.
ANS: ferromagnetic
4. A group of atomic magnets aligned in the same direction is called a(n) ____________________.
ANS: domain
5. Dropping or heating a bar magnet will cause it to become ____________________.
ANS: demagnetized
6. If the current direction in two parallel wires is the same, the wires will ____________________
one another.
ANS: attract
7. Increasing the ____________________ in a conductor will increase the magnetic field around the
conductor.
ANS: current
8. A(n) ____________________ is a type of switch that is closed by the action of an electromagnet.
ANS: relay
9. A(n) ____________________ is a pivoting element of a relay, motor, or generator.
ANS: armature
10. The force exerted on a conductor in a magnetic field is perpendicular to both the
____________________ and the ____________________.
ANS: current flow, field direction
MATCHING
Match each definition with the letter of the term it best describes. A letter may used more than once,
or not at all.
a. diamagnetic
d. paramagnetic
b. ferromagnetic
e. nonmagnetic
c. magnetic
1. material that can become a strong magnet
2. material that magnetizes slightly when placed in the magnetic field of a coil
3.
4.
5.
6.
7.
material that can attract small pieces of iron
material that cannot become a magnet
material that causes a decrease in the magnetic field of a coil
plastic is an example of this kind of material
nickel is an example of this kind of material
1.
2.
3.
4.
5.
6.
7.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
B
D
C
E
A
A
B
Match each description to the letter of the item it best describes. A letter may be used more than
once, or not all.
a. commutator
d. field magnet
b. brush
e. coil
c. armature
f. battery
8.
9.
10.
11.
12.
13.
14.
metal ring split into two parts
connects the commutator to the circuit
rotates due to magnetic forces
produces the magnetic field on the armature
creates a constant magnetic field
provides energy to the circuit
allows current direction on the armature to be changed
8.
9.
10.
11.
12.
13.
14.
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
ANS:
A
B
C
E
D
F
A
SHORT ANSWER
1. Do magnetic field lines have a beginning and an end?
ANS:
No, field lines form closed loops.
2. A commercial advertises a "magnetic" glove that you can wear to dust furniture. It is made of a soft
cloth. The glove actually does attract dust. How does it work?
ANS:
It's not really magnetic. The glove attracts dust using electrostatic forces, not magnetic forces.
3. When iron filings were sprinkled between two bar magnets, the pattern below was observed.
Explain what this indicates about the poles of the magnets.
ANS:
Since the field lines continue from pole to pole, they show an attraction between the two poles. The
two poles must be opposite.
4. How are magnetic poles similar to electric charges? How are they different?
ANS:
Similar
-two types of each
-produce three-dimensional force fields between them
-produce fields with a direction
-can produce both repulsive and attractive forces
Different
-Electric charges can be isolated; magnetic poles cannot be isolated
5. Six magnets slide easily in a track, and settle into the arrangement shown in the diagram. The
S-pole of one magnet is labelled. Identify each of the other labelled items as an N-pole or an S-pole.
ANS:
A, B, and D are N-poles. C and E are S-poles.
6. Explain what happens when you break a bar magnet in half and why.
ANS:
Two smaller bar magnets are produced. The domains in each part remain aligned and each piece
has an S-pole and N-pole in the same positions as the original magnet.
7. Two bar magnets are made of the same type of iron; however, one is much stronger than the other.
How can this happen?
ANS:
The strength of the magnet depends on the number of domains aligned in the same direction. The
stronger magnet has more domains with the same polarity. The weaker magnet has more domains
in random directions.
8. When an iron hammer is dropped onto a hard surface, it often becomes magnetized. Explain how
this happens.
ANS:
Normally, the domains in the iron of the hammer are aligned in random directions. When the
hammer is dropped, the domains vibrate and become aligned with Earth's magnetic field.
9. Bank cards and audiotapes often become useless after exposure to a strong magnetic fields.
Suggest reasons why this happens.
ANS:
Both bank cards and audiotapes contain information that is encoded into magnetically sensitive
material. The information is recorded by arranging the domains into specific patterns. A strong
magnetic field will rearrange the domains and the information will be unreadable.
10. If a long, straight wire carrying a current was placed flat on a piece of paper and iron filings were
sprinkled around it, what would happen to the filings?
ANS:
The filings would stand at right angles to the paper on both sides of the wire.
11. The open circles represent compasses. Show the directions of the compass needles.
ANS:
Both compass needles will point to the left.
12. Show the connections of the wires to the cell so that the indicated poles will be produced.
ANS:
The wire on the left side should be connected to the negative terminal and the wire on the right
should be connected to the positive terminal.
13. The open circles represent cross-sections of the loops in a coil. Show the current direction in each
coil segment so that the indicated poles are produced.
ANS:
Each circle at the top should have an cross (×
×) and each circle on the bottom should have a dot (•).
14. A lifting electromagnet is often made in a U-shape, with two separate coils making up the legs of
the U. If the electromagnet is to be used to lift scrap iron, would it be better to have similar poles at
the bottom of the legs, or dissimilar poles ?
ANS:
It would be better to have opposite poles on the bottom ends. The magnetic field could travel from
one pole, through the scrap metal, to the other pole. This would increase the lifting ability of the
electromagnet. If the poles were similar, the field lines would not be continuous.
15. A student has used up all of the available wire to make an electromagnet. Describe two ways the
student could still increase the strength of the electromagnet.
ANS:
-increase the number of loops (make the coil radius smaller)
-increase the current through the wire
-wrap the wire around a permeable coil
16. The diagram below shows a cross-section of a current-carrying loop between the poles of a
permanent magnet. In which direction will the coil turn?
ANS:
counterclockwise
17. In which direction can an electric charge move in an magnetic field, without having a force exerted
on it?
ANS:
parallel to the field
PROBLEM
1. An electromagnet can exert a lifting force of 12.0 N when it operates from a current of 5.0 A. What
mass can it lift if the current is increased to 8.0 A?
ANS:
F1 = 12.0 N
I1 = 5.0 A
I2 = 8.0 A
g = 9.8 N/kg
m=?
The magnet can lift 2.0 kg.
2. An electromagnet can lift a load of 45.0 kg when it operates with a current of 5.0 A. What current
must the magnet have if it is to lift 150 kg?
ANS:
m1 = 45.0 kg
I1 = 5.0 A
m2 = 150 kg
I2 = ?
The current must be increased to 17 A.
3. A student dressed in a suit of iron armour has a total mass of 210 kg. An electromagnet has just
enough force to hold him suspended in the air. What is the force applied by the electromagnet?
ANS:
m = 210 kg
g = 9.8 N/kg
F=?
The electromagnet exerted a force of 2.1 × 103 N.
4. When a lifting electromagnet was tested, it could lift 375 N when using a current of 5.0 A. In a
second trial, the current was doubled and a core with a relative permeability of 3.4 was inserted.
What force could the electromagnet exert after these modifications?
ANS:
F1 = 375 N
I1 = 5.0 A
K2 = 3.4
I2 = 10.0 A
F2 = ?
The lifting force of the magnet is now 2.6 × 103 N.
5. Determine the effect on the strength of an electromagnet if the current is doubled, the number of
coils is reduced from 650 to 450, and a core with a relative permeability of 1.9 is inserted.
ANS:
N1 = 650
N2 = 450
K = 1.9
F2 = ?
The force will increase by a factor of 2.6.
ESSAY
1. Describe how Oersted discovered the principle of electromagnetism.
ANS:
-Oersted discovered the principle of electromagnetism by accident. He was giving a lecture on
electricity and, while he was demonstrating an electric current, he noticed that a compass under
the wire moved in response to the current.
-When the current was on, the needle moved so it was at a right angle to the conductor. This
indicated that a magnetic field existed around the conductor.
-When the current direction was reversed, the needle pointed at a right angle again, but in the
opposite direction.
-The compass needle was not affected when the current was turned off.
-Oersted realized that when a current flows through a conductor, a magnetic field is created around
it.
2. Both ammeters and voltmeters contain a galvanometer and a resistor. Describe how the
components are assembled in each, and why.
ANS:
-The galvanometer must be protected from large current.
-An ammeter contains a galvanometer in parallel with a low resistance. When the meter is
connected (in series) to a circuit, this allows most of the current to flow through the resistor. The
ratio of current in each branch is known, and is factored into the reading on the ammeter scale.
-A voltmeter contains a galvanometer in series with a high resistance. When the meter is connected
(in parallel) to a circuit, this restricts the amount of current reaching the galvanometer. The known
resistance is factored into the reading on the voltmeter scale.