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Magnets and Magnetic Fields
Magnets
• Magnets attract iron-containing objects.
• Magnets have two distinct poles called the north pole
and the south pole. These names are derived from a
magnet’s behavior on Earth.
• Like poles of magnets repel each other; unlike poles
attract each other.
Magnets and Magnetic Fields
Magnetic Domains
• Magnetic Domain
A region composed of a group of atoms whose
magnetic fields are aligned in the same direction is
called a magnetic domain.
• Some materials can be made into permanent
magnets.
– Soft magnetic materials (for example iron) are
easily magnetized but tend to lose their
magnetism easily.
– Hard magnetic materials (for example nickel) tend
to retain their magnetism.
Magnets and Magnetic Fields
Magnetic Fields
• A magnetic field is a region in which a magnetic
force can be detected.
• Magnetic field lines can be drawn with the aid of a
compass.
Magnets and Magnetic Fields
Magnetic Field of a Bar Magnet
Magnets and Magnetic Fields
Magnetic Fields, continued
• Earth’s magnetic field is similar to that of a bar
magnet.
• The magnetic south pole is near the Geographic
North Pole. The magnetic north pole is near the
Geographic South Pole.
• Magnetic declination is a measure of the difference
between true north and north indicated by a
compass.
Magnets and Magnetic Fields
Earth’s Magnetic Field
Magnetism from Electricity
Magnetic Field of a Current-Carrying Wire
• A long, straight, current-carrying wire has a cylindrical
magnetic field.
• Compasses can be used to shown the direction of
the magnetic field induced by the wire.
• The right-hand rule can be used to determine the
direction of the magnetic field in a current-carrying
wire.
Magnetism from Electricity
The Right-Hand Rule
Magnetism from Electricity
Magnetic Field of a Current-Carrying Wire
Magnetism from Electricity
Magnetic Field of a Current-Carrying Wire
Negative Current
Zero Current
Positive Current
Magnetism from Electricity
Magnetic Field of a Current Loop
• Solenoids produce a strong magnetic field by
combining several loops.
• A solenoid is a long, helically wound coil of insulated
wire.
Magnetic Force
Charged Particles in a Magnetic Field
• A charge moving through a magnetic field experiences
a force proportional to the charge, velocity, and the
magnetic field.
B
Fmagnetic
qv
magnetic force on a charged particle
magnetic field =
(magnitude of charge)(speed of charge)
Magnetic Force
Charged Particles in a Magnetic Field,
continued
• The direction of the magnetic force on a moving
charge is always perpendicular to both the magnetic
field and the velocity of the charge.
• An alternative right-hand rule can be used to find the
direction of the magnetic force.
• A charge moving through a magnetic field follows a
circular path.
Magnetic Force
Alternative Right-Hand Rule: Force on a
Moving Charge
Magnetic Force
Sample Problem
Particle in a Magnetic Field
A proton moving east experiences a force of 8.8 
10–19 N upward due to the Earth’s magnetic field.At
this location, the field has a magnitude of 5.5  10–5 T
to the north. Find the speed of the particle.
Magnetic Force
Sample Problem, continued
Particle in a Magnetic Field
Given:
q = 1.60  10–19 C
B = 5.5  10–5 T
Fmagnetic = 8.8  10–19 N
Unknown:
v=?
Magnetic Force
Sample Problem, continued
Particle in a Magnetic Field
Use the definition of magnetic field strength.
Rearrange to solve for v.
B
Fmagnetic
v
Fmagnetic
qv
qB
8.8  19 –19 N

(1.60  19 –19 )(5.5  10 –5 )
v  1.0  105 m/s
Magnetic Force
Magnetic Force on a Current-Carrying
Conductor
• A current-carrying wire in an external magnetic field
undergoes a magnetic force.
• The force on a current-carrying conductor
perpendicular to a magnetic field is given by:
magnitude of magnetic force = (magnitude of magnetic field) 
(current)  (length of conductor within B)
Magnetic Force
Force on a Current-Carrying Wire in a
Magnetic Field
Magnetic Force
Magnetic Force on a Current-Carrying
Conductor, continued
• Two parallel current-carrying wires exert a force on
one another that are equal in magnitude and
opposite in direction.
• If the currents are in the same direction, the two
wires attract one another.
• If the currents are in opposite direction, the wires
repel one another.
• Loudspeakers use magnetic force to produce
sound.
Magnetic Force
Force Between Parallel Conducting Wires
Magnetic Force
Sample Problem
Force on a Current-Carrying Conductor
A wire 36 m long carries a current of 22 A from east
to west. If the magnetic force on the wire due to
Earth’s magnetic field is downward (toward Earth)
and has a magnitude of 4.0  10–2 N, find the
magnitude and direction of the magnetic field at this
location.
Magnetic Force
Sample Problem, continued
Force on a Current-Carrying Conductor
Given:
I = 22 A
Fmagnetic = 4.0  10–2 N
Unknown:
B=?
Magnetic Force
Sample Problem, continued
Force on a Current-Carrying Conductor
Use the equation for the force on a current-carrying
conductor perpendicular to a magnetic field.
Rearrange to solve for B.
Magnetic Force
Sample Problem, continued
Force on a Current-Carrying Conductor
Using the right-hand rule to find the direction of B,
face north with your thumb pointing to the west (in the
direction of the current) and the palm of your hand
down (in the direction of the force). Your fingers point
north. Thus, Earth’s magnetic field is from south to
north.
Multiple Choice
1. Which of the following statements best describes the
domains in unmagnetized iron?
A. There are no domains.
B. There are domains, but the domains are smaller
than in magnetized iron.
C. There are domains, but the domains are oriented
randomly.
D. There are domains, but the domains are not
magnetized.
Multiple Choice, continued
1. Which of the following statements best describes the
domains in unmagnetized iron?
A. There are no domains.
B. There are domains, but the domains are smaller
than in magnetized iron.
C. There are domains, but the domains are oriented
randomly.
D. There are domains, but the domains are not
magnetized.
Multiple Choice, continued
2. Which of the following statements is most correct?
F. The north pole of a freely rotating magnet points north
because the magnetic pole near the geographic North Pole is
like the north pole of a magnet.
G. The north pole of a freely rotating magnet points north
because the magnetic pole near the geographic North Pole is
like the south pole of a magnet.
H. The north pole of a freely rotating magnet points south
because the magnetic pole near the geographic South Pole is
like the north pole of a magnet.
J. The north pole of a freely rotating magnet points south
because the magnetic pole near the geographic South Pole is
like the south pole of a magnet.
Multiple Choice, continued
2. Which of the following statements is most correct?
F. The north pole of a freely rotating magnet points north
because the magnetic pole near the geographic North Pole is
like the north pole of a magnet.
G. The north pole of a freely rotating magnet points north
because the magnetic pole near the geographic North Pole is
like the south pole of a magnet.
H. The north pole of a freely rotating magnet points south
because the magnetic pole near the geographic South Pole is
like the north pole of a magnet.
J. The north pole of a freely rotating magnet points south
because the magnetic pole near the geographic South Pole is
like the south pole of a magnet.
Multiple Choice, continued
3. If you are standing at Earth’s magnetic north pole and
holding a bar magnet that is free to rotate in three
dimensions, which direction will the south pole of the
magnet point?
A. straight up
B. straight down
C. parallel to the ground, toward the north
D. parallel to the ground, toward the south
Multiple Choice, continued
3. If you are standing at Earth’s magnetic north pole and
holding a bar magnet that is free to rotate in three
dimensions, which direction will the south pole of the
magnet point?
A. straight up
B. straight down
C. parallel to the ground, toward the north
D. parallel to the ground, toward the south
Multiple Choice, continued
4. How can you increase the strength of a magnetic
field inside a solenoid?
F. increase the number of coils per unit length
G. increase the current
H. place an iron rod inside the solenoid
J. all of the above
Multiple Choice, continued
4. How can you increase the strength of a magnetic
field inside a solenoid?
F. increase the number of coils per unit length
G. increase the current
H. place an iron rod inside the solenoid
J. all of the above
Multiple Choice, continued
5. How will the electron move once
it passes into the magnetic field?
A. It will curve to the right and
then continue moving in a
straight line to the right.
B. It will curve to the left and then
continue moving in a straight line
to the left.
C. It will move in a clockwise
circle.
D. It will move in a
counterclockwise circle.
Multiple Choice, continued
5. How will the electron move once
it passes into the magnetic field?
A. It will curve to the right and
then continue moving in a
straight line to the right.
B. It will curve to the left and then
continue moving in a straight line
to the left.
C. It will move in a clockwise
circle.
D. It will move in a
counterclockwise circle.
Multiple Choice, continued
6. What will be the magnitude of
the force on the electron once
it passes into the magnetic
field?
F. qvB
G. –qvB
H. qB/v
J.
Multiple Choice, continued
6. What will be the magnitude of
the force on the electron once
it passes into the magnetic
field?
F. qvB
G. –qvB
H. qB/v
J.
Multiple Choice, continued
7. An alpha particle (q = 3.2  10–19 C) moves at a
speed of 2.5  106 m/s perpendicular to a magnetic
field of strength 2.0  10–4 T. What is the magnitude of
the magnetic force on the particle?
A. 1.6  10–16 N
B. –1.6  10–16 N
C. 4.0  10–9 N
D. zero
Multiple Choice, continued
7. An alpha particle (q = 3.2  10–19 C) moves at a
speed of 2.5  106 m/s perpendicular to a magnetic
field of strength 2.0  10–4 T. What is the magnitude of
the magnetic force on the particle?
A. 1.6  10–16 N
B. –1.6  10–16 N
C. 4.0  10–9 N
D. zero
Multiple Choice, continued
Use the passage below to answer questions 8–9.
A wire 25 cm long carries a 12 A current from east to
west. Earth’s magnetic field at the wire’s location has
a magnitude of 4.8  10–5 T and is directed from
south to north.
8. What is the magnitude of the magnetic force on the
wire?
F. 2.3  10–5 N
G. 1.4  10–4 N
H. 2.3  10–3 N
J. 1.4  10–2 N
Multiple Choice, continued
Use the passage below to answer questions 8–9.
A wire 25 cm long carries a 12 A current from east to
west. Earth’s magnetic field at the wire’s location has
a magnitude of 4.8  10–5 T and is directed from
south to north.
8. What is the magnitude of the magnetic force on the
wire?
F. 2.3  10–5 N
G. 1.4  10–4 N
H. 2.3  10–3 N
J. 1.4  10–2 N
Multiple Choice, continued
Use the passage below to answer questions 8–9.
A wire 25 cm long carries a 12 A current from east to
west. Earth’s magnetic field at the wire’s location has
a magnitude of 4.8  10–5 T and is directed from
south to north.
9. What is the direction of the magnetic force on the
wire?
A. north
B. south
C. up, away from Earth
D. down, toward Earth
Multiple Choice, continued
Use the passage below to answer questions 8–9.
A wire 25 cm long carries a 12 A current from east to
west. Earth’s magnetic field at the wire’s location has
a magnitude of 4.8  10–5 T and is directed from
south to north.
9. What is the direction of the magnetic force on the
wire?
A. north
B. south
C. up, away from Earth
D. down, toward Earth
Multiple Choice, continued
• Wire 1 carries current I1 and
creates magnetic field B1.
• Wire 2 carries current I2 and
creates magnetic field B2.
10. What is the direction of the
magnetic field B1 at the
location of wire 2?
F. to the left
G. to the right
H. into the page
J. out of the page
Multiple Choice, continued
• Wire 1 carries current I1 and
creates magnetic field B1.
• Wire 2 carries current I2 and
creates magnetic field B2.
10. What is the direction of the
magnetic field B1 at the
location of wire 2?
F. to the left
G. to the right
H. into the page
J. out of the page
Multiple Choice, continued
• Wire 1 carries current I1 and
creates magnetic field B1.
• Wire 2 carries current I2 and
creates magnetic field B2.
11. What is the direction of the
force on wire 2 as a result of
B1?
A. to the left
B. to the right
C. into the page
D. out of the page
Multiple Choice, continued
• Wire 1 carries current I1 and
creates magnetic field B1.
• Wire 2 carries current I2 and
creates magnetic field B2.
11. What is the direction of the
force on wire 2 as a result of
B1?
A. to the left
B. to the right
C. into the page
D. out of the page
Multiple Choice, continued
B2I 2
2
• Wire 1 carries current I1 and
creates magnetic field B1.
• Wire 2 carries current I2 and
creates magnetic field B2.
12. What is the magnitude of the
magnetic force on wire 2?
Multiple Choice, continued
B1 I 1
1
B1 I 1
2
B1 I 2
2
B2 I 2
2
• Wire 1 carries current I1 and
creates magnetic field B1.
• Wire 2 carries current I2 and
creates magnetic field B2.
12. What is the magnitude of the
magnetic force on wire 2?
Short Response
13. Sketch the magnetic field lines around a bar
magnet.
Short Response, continued
13. Sketch the magnetic field lines around a bar
magnet.
Answer:
Short Response, continued
14. Describe how to use the right-hand rule to
determine the direction of a magnetic field around a
current-carrying wire.
Short Response, continued
14. Describe how to use the right-hand rule to
determine the direction of a magnetic field around a
current-carrying wire.
Answer: Imagine wrapping the fingers of your right hand
around the wire and pointing your thumb in the
direction of the current. The magnetic field lines form
concentric circles that are centered on the wire and
curve in the same direction as your fingers.
Short Response, continued
15. Draw a diagram showing the path of a positively
charged particle moving in the plane of a piece of
paper if a uniform magnetic field is coming out of the
page.
Short Response, continued
15. Draw a diagram showing the path of a positively
charged particle moving in the plane of a piece of
paper if a uniform magnetic field is coming out of the
page.
Answer:
Magnetism from Electricity
Magnetic Field of a Current Loop
Electricity from Magnetism
Electromagnetic Induction
• Electromagnetic induction is the process of
creating a current in a circuit by a changing magnetic
field.
• A change in the magnetic flux through a conductor
induces an electric current in the conductor.
• The separation of charges by the magnetic force
induces an emf.
Electricity from Magnetism
Electromagnetic Induction in a
Circuit Loop
Electricity from Magnetism
Electromagnetic Induction,
continued
• The angle between a magnetic field and a circuit
affects induction.
• A change in the number of magnetic field lines
induces a current.
Electricity from Magnetism
Characteristics of Induced
Current
• Lenz’s Law
The magnetic field of the induced current is in a
direction to produce a field that opposes the
change causing it.
• Note: the induced current does not oppose the
applied field, but rather the change in the applied
field.
Electricity from Magnetism
Characteristics of Induced
Current, continued
• The magnitude of the induced emf can be predicted
by Faraday’s law of magnetic induction.
• Faraday’s Law of Magnetic Induction
 M
emf  – N
t
average induced emf = –the number of loops in the circuit 
the time rate of change in the magnetic flux
•
The magnetic flux is given by M = ABcosq.
Electricity from Magnetism
Sample Problem
Induced emf and Current
A coil with 25 turns of wire is wrapped around a
hollow tube with an area of 1.8 m2. Each turn has
the same area as the tube. A uniform magnetic field
is applied at a right angle to the plane of the coil. If
the field increases uniformly from 0.00 T to 0.55 T in
0.85 s, find the magnitude of the induced emf in the
coil. If the resistance in the coil is 2.5 Ω, find the
magnitude of the induced current in the coil.
Electricity from Magnetism
Sample Problem, continued
Induced emf and Current
1. Define
Given:
∆t = 0.85 s
A = 1.8 m2
N = 25 turns
R = 2.5 Ω
Bi = 0.00 T = 0.00 V•s/m2
Bf = 0.55 T = 0.55 V•s/m2
Unknown:
emf = ?
I=?
q = 0.0º
Electricity from Magnetism
Sample Problem, continued
Induced emf and Current
1. Define, continued
Diagram: Show the coil before and after the change
in the magnetic field.
Electricity from Magnetism
Sample Problem, continued
Induced emf and Current
2. Plan
Choose an equation or situation. Use Faraday’s
law of magnetic induction to find the induced emf in
the coil.
  AB cosq 
 M
emf  – N
 –N
t
t
Substitute the induced emf into the definition of
resistance to determine the induced current in the
coil.
emf
I
R
Electricity from Magnetism
Sample Problem, continued
Induced emf and Current
2. Plan, continued
Rearrange the equation to isolate the unknown.
In this example, only the magnetic field strength
changes with time. The other components (the coil
area and the angle between the magnetic field and
the coil) remain constant.
B
emf  – NA cos q
t
Electricity from Magnetism
Sample Problem, continued
Induced emf and Current
3. Calculate
Substitute the values into the equation and
solve.

V•s  
0.55
–
0.00
2 

m


emf  –(25)(1.8 m2 )(cos0.0º ) 
 –29 V
(0.85 s)
–29 V
I
 –12 A
2.5 Ω
emf  –29 V
I  –12 A
Electricity from Magnetism
Sample Problem, continued
Induced emf and Current
4. Evaluate
The induced emf, and therefore the induced
current, is directed through the coil so that the
magnetic field produced by the induced current
opposes the change in the applied magnetic
field. For the diagram shown on the previous
page, the induced magnetic field is directed to
the right and the current that produces it is
directed from left to right through the resistor.
Generators, Motors, and Mutual
Inductance
Generators and Alternating
Current
• A generator is a machine that converts mechanical
energy into electrical energy.
• Generators use induction to convert mechanical
energy into electrical energy.
• A generator produces a continuously changing emf.
Generators, Motors, and Mutual
Inductance
Induction of an emf in an AC
Generator
Generators, Motors, and Mutual
Inductance
Generators and Alternating Current, continued
• Alternating current is an electric current that
changes direction at regular intervals.
• Alternating current can be converted to direct
current by using a device called a commutator to
change the direction of the current.
Generators, Motors, and Mutual
Inductance
Motors
• Motors are machines that convert electrical energy
to mechanical energy.
• Motors use an arrangement similar to that of
generators.
• Back emf is the emf induced in a motor’s coil that
tends to reduce the current in the coil of a motor.
Generators, Motors, and Mutual
Inductance
Mutual Inductance
• The ability of one circuit to induce an emf in a nearby
circuit in the presence of a changing current is called
mutual inductance.
• In terms of changing primary current, Faraday’s law
is given by the following equation, where M is the
mutual inductance:
M
I
emf  –N
 –M
t
t
AC Circuits and Transformers
Objectives
• Distinguish between rms values and maximum
values of current and potential difference.
• Solve problems involving rms and maximum values
of current and emf for ac circuits.
• Apply the transformer equation to solve problems
involving step-up and step-down transformers.
AC Circuits and Transformers
Effective Current
• The root-mean-square (rms) current of a circuit is
the value of alternating current that gives the same
heating effect that the corresponding value of direct
current does.
• rms Current
Irms 
Imax
2
 0.707 Imax
AC Circuits and Transformers
Effective Current, continued
• The rms current and rms emf in an ac circuit are
important measures of the characteristics of an ac
circuit.
• Resistance influences current in an ac circuit.
AC Circuits and Transformers
Sample Problem
rms Current and emf
A generator with a maximum output emf of 205 V is
connected to a 115 Ω resistor. Calculate the rms
potential difference. Find the rms current through the
resistor. Find the maximum ac current in the circuit.
1. Define
Given:
∆Vrms = 205 V R = 115 Ω
Unknown:
∆Vrms = ? Irms = ? Imax = ?
AC Circuits and Transformers
Sample Problem, continued
rms Current and emf
2. Plan
Choose an equation or situation. Use the equation
for the rms potential difference to find ∆Vrms.
∆Vrms = 0.707 ∆Vmax
Rearrange the definition for resistance to calculate
Irms.
Vrms
Irms 
R
Use the equation for rms current to find Irms.
Irms = 0.707 Imax
AC Circuits and Transformers
Sample Problem, continued
rms Current and emf
2. Plan, continued
Rearrange the equation to isolate the unknown.
Rearrange the equation relating rms current to
maximum current so that maximum current is
calculated.
Irms
Imax 
0.707
AC Circuits and Transformers
Sample Problem, continued
rms Current and emf
3. Calculate
Substitute the values into the equation and solve.
Vrms  (0.707)(205 V)  145 V
145 V
Irms 
 1.26 A
115 Ω
1.26 A
Imax 
 1.78 A
0.707
4. Evaluate The rms values for emf and current
are a little more than two-thirds the maximum
values, as expected.
AC Circuits and Transformers
Transformers
• A transformer is a device that increases or
decreases the emf of alternating current.
• The relationship between the input and output emf is
given by the transformer equation.
N
V2  2 V1
N1
induced emf in secondary =
 number of turns in secondary 
 number of turns in primary  applied emf in primary


AC Circuits and Transformers
Transformers, continued
• The transformer equation assumes that no power is
lost between the primary and secondary coils.
However, real transformers are not perfectly efficient.
• Real transformers typically have efficiencies ranging
from 90% to 99%.
• The ignition coil in a gasoline engine is a transformer.
AC Circuits and Transformers
A Step-Up Transformer in an Auto
Ignition System
Electromagnetic Waves
Propagation of Electromagnetic
Waves
• Electromagnetic waves travel at the speed of light
and are associated with oscillating, perpendicular
electric and magnetic fields.
• Electromagnetic waves are transverse waves; that is,
the direction of travel is perpendicular to the the
direction of oscillating electric and magnetic fields.
• Electric and magnetic forces are aspects of a single
force called the electromagnetic force.
Electromagnetic Waves
Propagation of Electromagnetic
Waves, continued
• All electromagnetic waves are produced by
accelerating charges.
• Electromagnetic waves transfer energy. The energy
of electromagnetic waves is stored in the waves’
oscillating electric and magnetic fields.
• Electromagnetic radiation is the transfer of energy
associated with an electric and magnetic field.
Electromagnetic radiation varies periodically and
travels at the speed of light.
Electromagnetic Waves
The Sun at Different Wavelengths
of Radiation
Electromagnetic Waves
Propagation of Electromagnetic
Waves, continued
• High-energy electromagnetic waves behave like
particles.
• An electromagnetic wave’s frequency makes the
wave behave more like a particle. This notion is
called the wave-particle duality.
• A photon is a unit or quantum of light. Photons can
be thought of as particles of electromagnetic radiation
that have zero mass and carry one quantum of
energy.
Electromagnetic Waves
The Electromagnetic Spectrum
• The electromagnetic spectrum ranges from very long
radio waves to very short-wavelength gamma waves.
• The electromagnetic spectrum has a wide variety of
applications and characteristics that cover a broad
range of wavelengths and frequencies.
Electromagnetic Waves
The Electromagnetic Spectrum,
continued
• Radio Waves
– longest wavelengths
– communications, tv
• Microwaves
– 30 cm to 1 mm
– radar, cell phones
• Infrared
– 1 mm to 700 nm
– heat, photography
• Visible light
– 700 nm (red) to 400 nm
(violet)
• Ultraviolet
– 400 nm to 60 nm
– disinfection,
spectroscopy
• X rays
– 60 nm to 10–4 nm
– medicine, astronomy,
security screening
• Gamma Rays
– less than 0.1 nm
– cancer treatment,
astronomy
Electromagnetic Waves
The Electromagnetic Spectrum
Multiple Choice
1. Which of the following equations correctly describes
Faraday’s law of induction?
A. emf
B. emf
C. emf
D. emf
( AB tanq )
 –N
t
( AB cos q )
N
t
( AB cos q )
 –N
t
( AB cos q )
M
t
Multiple Choice, continued
1. Which of the following equations correctly describes
Faraday’s law of induction?
A. emf
B. emf
C. emf
D. emf
( AB tanq )
 –N
t
( AB cos q )
N
t
( AB cos q )
 –N
t
( AB cos q )
M
t
Multiple Choice, continued
2. For the coil shown at right, what must be
done to induce a clockwise current?
F. Either move the north pole of a magnet
down into the coil, or move the south pole
of the magnet up and out of the coil.
G. Either move the south pole of a magnet
down into the coil, or move the north pole of
the magnet up and out of the coil.
H. Move either pole of the magnet down
into the coil.
J. Move either pole of the magnet up and
out of the coil.
Multiple Choice, continued
2. For the coil shown at right, what must be
done to induce a clockwise current?
F. Either move the north pole of a magnet
down into the coil, or move the south pole
of the magnet up and out of the coil.
G. Either move the south pole of a magnet
down into the coil, or move the north pole of
the magnet up and out of the coil.
H. Move either pole of the magnet down
into the coil.
J. Move either pole of the magnet up and
out of the coil.
Multiple Choice, continued
3. Which of the following would not increase the emf
produced by a generator?
A. rotating the generator coil faster
B. increasing the strength of the generator magnets
C. increasing the number of turns of wire in the coil
D. reducing the cross-sectional area of the coil
Multiple Choice, continued
3. Which of the following would not increase the emf
produced by a generator?
A. rotating the generator coil faster
B. increasing the strength of the generator magnets
C. increasing the number of turns of wire in the coil
D. reducing the cross-sectional area of the coil
Multiple Choice, continued
4. By what factor do you multiply the maximum emf to
calculate the rms emf for an alternating current?
F. 2
G.
H.
2
1
1
J.
2
2
Multiple Choice, continued
4. By what factor do you multiply the maximum emf to
calculate the rms emf for an alternating current?
F. 2
G.
H.
2
1
1
J.
2
2
Multiple Choice, continued
5. Which of the following correctly describes the composition
of an electromagnetic wave?
A. a transverse electric wave and a magnetic transverse
wave that are parallel and are moving in the same
direction
B. a transverse electric wave and a magnetic transverse
wave that are perpendicular and are moving in the same
direction
C. a transverse electric wave and a magnetic transverse
wave that are parallel and are moving at right angles to
each other
D. a transverse electric wave and a magnetic transverse
wave that are perpendicular and are moving at right
angles to each other
Multiple Choice, continued
5. Which of the following correctly describes the composition
of an electromagnetic wave?
A. a transverse electric wave and a magnetic transverse
wave that are parallel and are moving in the same
direction
B. a transverse electric wave and a magnetic transverse
wave that are perpendicular and are moving in the same
direction
C. a transverse electric wave and a magnetic transverse
wave that are parallel and are moving at right angles to
each other
D. a transverse electric wave and a magnetic transverse
wave that are perpendicular and are moving at right
angles to each other
Multiple Choice, continued
6. A coil is moved out of a magnetic field in order to induce
an emf. The wire of the coil is then rewound so that the
area of the coil is increased by 1.5 times. Extra wire is
used in the coil so that the number of turns is doubled. If
the time in which the coil is removed from the field is
reduced by half and the magnetic field strength remains
unchanged, how many times greater is the new induced
emf than the original induced emf ?
F. 1.5 times
G. 2 times
H. 3 times
J. 6 times
Multiple Choice, continued
6. A coil is moved out of a magnetic field in order to induce
an emf. The wire of the coil is then rewound so that the
area of the coil is increased by 1.5 times. Extra wire is
used in the coil so that the number of turns is doubled. If
the time in which the coil is removed from the field is
reduced by half and the magnetic field strength remains
unchanged, how many times greater is the new induced
emf than the original induced emf ?
F. 1.5 times
G. 2 times
H. 3 times
J. 6 times
Multiple Choice, continued
Use the passage below to
answer questions 7–8.
A pair of transformers is
connected in series, as
shown in the figure below.
7. From left to right, what are
the types of the two
transformers?
A. Both are step-down
transformers.
B. Both are step-up
transformers.
C. One is a step-down
transformer; and one is a
step-up transformer.
D. One is a step-up
transformer; and one is a
step-down transformer.
Multiple Choice, continued
Use the passage below to
answer questions 7–8.
A pair of transformers is
connected in series, as
shown in the figure below.
7. From left to right, what are
the types of the two
transformers?
A. Both are step-down
transformers.
B. Both are step-up
transformers.
C. One is a step-down
transformer; and one is a
step-up transformer.
D. One is a step-up
transformer; and one is a
step-down transformer.
Multiple Choice, continued
Use the passage below to
answer questions 7–8.
A pair of transformers is
connected in series, as
shown in the figure below.
8. What is the output
potential difference from
the secondary coil of
the transformer on the
right?
F. 400 V
G. 12 000 V
H. 160 000 V
J. 360 000 V
Multiple Choice, continued
Use the passage below to
answer questions 7–8.
A pair of transformers is
connected in series, as
shown in the figure below.
8. What is the output
potential difference from
the secondary coil of
the transformer on the
right?
F. 400 V
G. 12 000 V
H. 160 000 V
J. 360 000 V
Multiple Choice, continued
9. What are the particles that can be used to describe
electromagnetic radiation called?
A. electrons
B. magnetons
C. photons
D. protons
Multiple Choice, continued
9. What are the particles that can be used to describe
electromagnetic radiation called?
A. electrons
B. magnetons
C. photons
D. protons
Multiple Choice, continued
10. The maximum values for the current and potential
difference in an ac circuit are 3.5 A and 340 V,
respectively. How much power is dissipated in this
circuit?
F. 300 W
G. 600 W
H. 1200 W
J. 2400 W
Multiple Choice, continued
10. The maximum values for the current and potential
difference in an ac circuit are 3.5 A and 340 V,
respectively. How much power is dissipated in this
circuit?
F. 300 W
G. 600 W
H. 1200 W
J. 2400 W
Short Response
11. The alternating current through an electric toaster
has a maximum value of 12.0 A. What is the rms
value of this current?
Short Response, continued
11. The alternating current through an electric toaster
has a maximum value of 12.0 A. What is the rms
value of this current?
Answer:
8.48 A
Short Response, continued
12. What is the purpose of a commutator in an ac
generator?
Short Response, continued
12. What is the purpose of a commutator in an ac
generator?
Answer:
It converts ac to a changing current in one
direction only.
Short Response, continued
13. How does the energy of one photon of an
electromagnetic wave relate to the wave’s
frequency?
Short Response, continued
13. How does the energy of one photon of an
electromagnetic wave relate to the wave’s
frequency?
Answer:
The energy is directly proportional to the wave’s
frequency (E = hf ).
Short Response, continued
14. A transformer has 150 turns of wire on the primary
coil and 75 000 turns on the secondary coil. If the
input potential difference across the primary is 120 V,
what is the output potential difference across the
secondary?
Short Response, continued
14. A transformer has 150 turns of wire on the primary
coil and 75 000 turns on the secondary coil. If the
input potential difference across the primary is 120 V,
what is the output potential difference across the
secondary?
Answer:
6.0  104 V
Extended Response
15. Why is alternating current used for power
transmission instead of direct current? Be sure to
include power dissipation and electrical safety
considerations in your answer.
Extended Response, continued
15. Answer:
For electric power to be transferred over long
distances without a large amount of power
dissipation, the electric power must have a high
potential difference and low current. However, to
be safely used in homes, the potential difference
must be lower than that used for long-distance
power transmission. Because of induction, the
potential difference and current of electricity can
be transformed to higher or lower values, but the
current must change continuously (alternate) for
this to happen.
Extended Response, continued
Base your answers to questions
16. Why must the current enter
16–18 on the information below.
the coil just as someone
comes up to the table?
A device at a carnival’s haunted
house involves a metal ring that
flies upward from a table when a
patron passes near the table’s
edge. The device consists of a
photoelectric switch that activates
the circuit when anyone walks in
front of the switch and of a coil of
wire into which a current is
suddenly introduced when the
switch is triggered.
Extended Response, continued
Base your answers to questions
16. Why must the current enter
16–18 on the information below.
the coil just as someone
comes up to the table?
A device at a carnival’s haunted
house involves a metal ring that
flies upward from a table when a Answer: The change in current
in the coil will produce a
patron passes near the table’s
changing magnetic field,
edge. The device consists of a
which will induce a current in
photoelectric switch that activates
the ring. The induced current
the circuit when anyone walks in
produces a magnetic field
front of the switch and of a coil of
that interacts with the
wire into which a current is
magnetic field from the coil,
suddenly introduced when the
causing the ring to rise from
switch is triggered.
the table.
Extended Response, continued
Base your answers to questions
17. Using Lenz’s law, explain
16–18 on the information below.
why the ring flies upward
when there is an increasing
A device at a carnival’s haunted
current in the coil?
house involves a metal ring that
flies upward from a table when a
patron passes near the table’s
edge. The device consists of a
photoelectric switch that activates
the circuit when anyone walks in
front of the switch and of a coil of
wire into which a current is
suddenly introduced when the
switch is triggered.
Extended Response, continued
Base your answers to questions
17. Using Lenz’s law, explain
16–18 on the information below.
why the ring flies upward
when there is an increasing
A device at a carnival’s haunted
current in the coil?
house involves a metal ring that
flies upward from a table when a
Answer: According to Lenz’s
patron passes near the table’s
law, the magnetic field
edge. The device consists of a
induced in the ring must
photoelectric switch that activates
oppose the magnetic field
the circuit when anyone walks in
that induces the current in
front of the switch and of a coil of
the ring. The opposing fields
wire into which a current is
cause the ring, which can
suddenly introduced when the
move freely, to rise upward
switch is triggered.
from the coil under the
table’s surface.
Extended Response, continued
Base your answers to questions
18. Suppose the change
16–18 on the information below.
in the magnetic field is
A device at a carnival’s haunted
0.10 T/s. If the radius of
house involves a metal ring that
the ring is 2.4 cm and
flies upward from a table when a
the ring is assumed to
patron passes near the table’s
consist of one turn of
edge. The device consists of a
wire, what is the emf
photoelectric switch that activates
induced in the ring?
the circuit when anyone walks in
front of the switch and of a coil of
wire into which a current is
suddenly introduced when the
switch is triggered.
Extended Response, continued
Base your answers to questions
18. Suppose the change
16–18 on the information below.
in the magnetic field is
A device at a carnival’s haunted
0.10 T/s. If the radius of
house involves a metal ring that
the ring is 2.4 cm and
flies upward from a table when a
the ring is assumed to
patron passes near the table’s
consist of one turn of
edge. The device consists of a
wire, what is the emf
photoelectric switch that activates
induced in the ring?
the circuit when anyone walks in
front of the switch and of a coil of
wire into which a current is
Answer: 1.8  10–4 V
suddenly introduced when the
switch is triggered.
Electricity from Magnetism
Ways of Inducing a Current in a
Circuit