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MECH1310Chapter7Homework

True/False
Indicate whether the statement is true or false.
 

 1. 
A permanent magnet will retain its magnetism for a long period of time.
 

 2. 
Unlike magnetic poles repulse one another.
 

 3. 
Magnetic flux is another name for a magnetic field.
 

 4. 
A conductor with no current through it will still have a magnetic field around it.
 

 5. 
Aluminum, brass, and iron make good magnets.
 

 6. 
Two north magnetic poles attract one another.
 

 7. 
A magnetic field around a conductor is a good indication that current is flowing.
 

 8. 
Soft iron, copper, and silver make good magnets.
 

 9. 
The speed of movement of the coil of a generator affects the direction of current produced.
 

 10. 
A solenoid converts electrical energy into mechanical energy.
 

Multiple Choice
Identify the choice that best completes the statement or answers the question.
 

 11. 
A solenoid consists of ________.
a.
a burglar alarm relay
c.
a permanent magnet
b.
a coil of wire wound around a core
d.
two plates separated by an insulator
 

 12. 
A coil of wire with current flowing through it is called a(n) ________.
a.
capacitor
c.
electromagnet
b.
north pole
d.
south pole
 

 13. 
If two north magnetic poles are brought close to each other, ________.
a.
a force of attraction pulls them together
c.
an electromagnet is created
b.
a force of repulsion pushes them apart
d.
a current is induced
 

 14. 
How does the magnetic field change around a conducting wire if the current increases?
a.
There is no magnetic field around a conducting wire.
b.
The field grows weaker.
c.
The field grows stronger.
d.
It is impossible to predict.
 

 15. 
Permeability is ________.
a.
the ability of a magnetic material to conduct a current
b.
the shielding effect of a material
c.
the opposition to being magnetized
d.
the ease of a material to be magnetized
 

 16. 
Which one of the statements below does NOT apply to the voltage induced into a wire when it is moved through a magnetic field?
a.
It depends on the strength of the magnetic field.
b.
It depends on the current flowing in the wire.
c.
It depends on the length of moving wire in the field.
d.
It depends on the speed of wire motion.
 

 17. 
Unlike magnetic poles ________ and like poles ________.
a.
attract, repel
c.
repel, attract
b.
attract, attract
d.
repel, repel
 

 18. 
According to the left-hand rule for the magnetic field around a conductor, what points in the direction of current flow?
a.
thumb
c.
meter
b.
fingers
d.
index finger
 

 19. 
In which one of the following applications is electromagnetism used?
a.
relays
c.
loudspeakers
b.
tape recorders
d.
all of these
 

 20. 
If the cross-sectional area of a magnetic field decreases, the flux density ________.
a.
decreases
c.
remains the same
b.
increases
 

 21. 
If a basic one-loop, dc generator rotates at 60 revolutions each second, how many times each second does its dc output voltage reach a peak value?
a.
120
c.
60
b.
0
d.
240
 

 22. 
A ________ is a type of electromagnetic device that has a movable iron core called a plunger.
a.
relay
c.
solenoid
b.
analog meter
d.
speaker
 

 23. 
If a relay picks up and makes a contact but does not stay picked up, what's the trouble?
a.
The magnetic field is too weak.
b.
The magnetic field is too strong.
c.
The relay coil is open.
d.
There is no trouble. The relay is operating normally.
 

 24. 
If a tape recorder's output voltage increases and decreases when you shake the recorder, a possible problem might be ________.
a.
an open circuit in the speaker
b.
too high a source voltage
c.
a loose circular magnet around the speaker's voice coil
d.
an open capacitor
 

 25. 
If the contacts fail to open when a relay is de-energized, what is the trouble?
a.
The return spring tension is too large.
b.
There is too little current in the relay coil.
c.
Dirt is causing the relay armature to stick.
d.
There is no trouble. The relay is operating normally.
 

 26. 
How will two permanent bar magnets react if they are brought close to each other?
a.
They will repel if the north and south poles are together.
b.
They will attract if their north poles are together.
c.
They will attract if the north and south poles are together.
d.
They will attract if their south poles are together.
 

 27. 
The quantity of lines of force per unit of area is known as ________.
a.
permeability
c.
magnetic flux
b.
ampere-turns
d.
flux density
 

 28. 
The voltage produced by a magnetic field cutting through a conductor depends on the ________.
a.
direction of motion
c.
speed of motion
b.
length of the conductor
d.
all of the above
 

 29. 
The polarity of an induced voltage depends on the ________.
a.
time that the conductor remains stationary in a magnetic field
b.
direction of motion of a conductor in a magnetic field
c.
amount of current flowing
d.
length of a conductor in a magnetic field
 

 30. 
The magnetic term that is equivalent to voltage is ________.
a.
flux density
c.
magnetomotive force
b.
reluctance
d.
current
 

 31. 
The permeability of air is ________.
a.
7
c.
22
b.
19.2
d.
1
 

 32. 
The magnetic field around an electromagnet is produced by ________.
a.
a permanent magnet
c.
the laminated iron core
b.
a current flowing through the winding
d.
the conductor's permeability
 

 33. 
A weber is a unit of ________.
a.
magnetic flux
c.
permeability
b.
reluctance
d.
magnetomotive force
 

 34. 
A tesla is a unit of ________.
a.
permeability
c.
reluctance
b.
magnetic flux density
d.
magnetomotive force
 

 35. 
The term reluctance in magnetic circuits is like the term ________ in electrical circuits.
a.
resistance
c.
voltage
b.
capacitance
d.
current
 

 36. 
Soft iron has a high ________.
a.
permeability
c.
resistance
b.
mmf
d.
reactance
 

 37. 
Magnetic lines that are close to each other are said to have a high ________.
a.
inductance
c.
current
b.
flux density
d.
reluctance
 

 38. 
Reversing the current through an electromagnet ________ the magnetic field.
a.
reverses
c.
collapses
b.
decreases
d.
increases
 

 39. 
An ampere-turn is a unit of ________.
a.
reactance
c.
resistance
b.
permeability
d.
magnetomotive force (mmf)
 

 40. 
If the number of turns of wire on an electromagnet increases, the magnetomotive force mmf ________.
a.
decreases
c.
increases
b.
remains the same
 

 41. 
The amount of voltage that is induced in a conductor depends on the ________.
a.
direction of motion of the conductor in the magnetic field
b.
length of the conductor in the magnetic field
c.
direction of the current flowing
d.
length of time a conductor remains stationary in the magnetic field
 

 42. 
Increasing the speed at which a magnet moves through a coil of wire causes the induced voltage to ________.
a.
remain the same
c.
increase
b.
decrease
 

 43. 
If more turns of wire are added to a coil and then that coil is moved through a magnetic field, the voltage produced will ________.
a.
be less than the original coil
c.
be more than the original coil
b.
be the same as the original coil
 

 44. 
The ________ electromagnet cannot be further magnetized.
a.
open
c.
saturated
b.
capacitive
d.
shorted
 

 45. 
An iron core electromagnet has a higher ________ than an air core electromagnet.
a.
current
c.
resistance
b.
reactance
d.
permeability
 

 46. 
Applying an ac voltage to an electromagnet's coil will produce a ________.
a.
north magnetic field
c.
changing magnetic field
b.
south magnetic field
d.
steady magnet field
 

 47. 
If some turns of wire are removed from an electromagnet, the magnetic field will ________.
a.
be the same as the original electromagnet
c.
be more than the original electromagnet
b.
be less than the original electromagnet
 

 48. 
Like magnetic poles ________.
a.
have no effect on each other
c.
have the same current through them
b.
attract each other
d.
repel each other
 

 49. 
A simple electric doorbell probably uses ________ to ring the bell.
a.
a permanent magnet
c.
transistor
b.
a DPDT switch
d.
an electromagnet
 

 50. 
A relay is a device that ________.
a.
isolates the actuating signal from the control signal
b.
uses an electromagnet to open and close contacts
c.
has a coil to actuate some contacts that control other circuits
d.
all of these
 

 51. 
The principal difference between a solenoid and a relay is that:
a.
relays use magnets, solenoids do not.
b.
solenoids do mechanical work, relays do electrical work.
c.
solenoids do electrical work, relays do mechanical work.
d.
relays relay electricity, solenoids do not.
 

 52. 
The movable part of the solenoid is the:
a.
the plunger.
c.
magnetic field.
b.
electromagnetic coil.
d.
all of these.
 

 53. 
The movable part of the relay is the:
a.
electromagnetic coil.
c.
the armature.
b.
magnetic field.
d.
all of these.
 

 54. 
Hysteresis is a characteristic of magnetic materials wherein a change in magnetization:
a.
occurs exactly with the application of a magnetic force.
b.
occurs before the application of a magnetizing force.
c.
varies according to the current flow.
d.
occurs after the application of a magnetic force.
 

 55. 
Electromagnetic induction is best defined as the process by which a voltage is produced in a conductor when there is:
a.
energy produced by the rotating coils.
b.
motion between the magnetic poles.
c.
motion between the conductor and the magnetic field.
d.
motion around the magnetic fields.
 

 56. 
Faraday's law states that the voltage induced across a coil of wire is proportional to these two factors.
a.
the diameter of the conductor and the rate of change of the magnetic flux
b.
the size of the coil and the rate of change of the magnetic flux
c.
the size of the magnets and the rate of change of the magnetic flux
d.
the number of turns in the coil and the rate of change of the magnetic flux
 

 57. 
Faraday's law and Lenz's law are related in that they deal with:
a.
an induced voltage.
c.
a resulting current flow.
b.
varying magnetic field.
d.
all of these.
 

 58. 
The purpose of a commutator in a dc generator is to:
a.
maximize the current generated.
c.
provide filtration.
b.
keep current flowing in one direction.
d.
concentrate flux density.
 

 59. 
For a high current switching application where it is not practical to route heavy wiring to the switch location which device would be used?
a.
electromagnet
c.
generator
b.
solenoid
d.
relay
 

 60. 
If a higher voltage is needed from a generator, what must be done?
a.
Turn the coil faster.
b.
Decrease the number of coils.
c.
Increase the distance between the coil and magnetic field.
d.
both B and C.
 



 
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