Download 14 Magnets and Electromagnetism

14 Magnets and Electromagnetism
Answers to Questions
Q1
Q2
Q3
Q4
Q5
Q6
Q7
Q8
Q9
Q10
Q11
Q12
Q13
Q14
Q15
Q16
Q17
Q18
Q19
Q20
Q21
Q22
Q23
Q24
Q25
The second magnet will be repelled according to the rule that like magnetic poles repel one another.
No. Decreasing the distance between the two south poles will quadruple the repulsive force between them.
The force law for magnetic poles, like that for electric charges, varies inversely with the square of the
distance.
Both are central forces with the property that like repels like and unlikes attract.
No. The existence of an isolated magnetic pole has never been confirmed.
Yes. In the absence of magnetic materials a compass needle will align itself with the Earth's magnetic field.
The existence of ferromagnetic materials such as in plumbing, structural members in buildings, etc. can affect
its alignment.
The Earth's magnetic pole near the geographic North Pole is actually a south magnetic pole. The Earth's
magnetic north pole is near the south geographic pole. The North Pole of a bar magnet or compass comes
from the expression "north-seeking" when freely suspended.
Because the South Magnetic Pole of the Earth attracts North-seeking magnets, which are North magnetic
poles.
Yes. With the current in the wire along a north to south direction the compass needle will be deflected away
from north.
No. The magnetic field due to the current in the wire will produce a field at the position of the compass that is
tangential to the compass needle and thus will cause no torque.
The wires carrying a current are electrically neutral and therefore there is no electrostatic effect. The force of
one current-carrying wire on another is a magnetic effect.
The magnetic force on the charge is directed down according to the right hand rule.
There is no magnetic force on a charge when there is no component of velocity perpendicular to the magnetic
field.
No. There is only a force on the negative charge if its velocity has a component perpendicular to the
direction of the magnetic field.
The direction of a current is defined as the direction of the motion of a positive charge carrier. When this
definition was originally devised it had not yet been determined that charge carriers were actually negative
(i.e., electrons).
The magnetic field due to the current in the loop will be perpendicular to the loop and down at the center.
The magnetic field line associated with this loop goes down through the loop and back around the outside, so
that the effective North Pole for the loop would be pointing down.
The loop will rotate counterclockwise. The forces on the long arms are outward and, because they do not
share a common line of action, impart a counterclockwise torque on the loop.
Yes. The forces combine to rotate the loop until its plane is perpendicular to the field, at which time the net
torque is zero.
There are effective current loops in an atom arising from the orbital motion and spin of electrons. The origin
of ferromagnetism in certain materials is due to unbalanced electron spins. The electron may be visualized
as a small ball of charge spinning on its axis that would produce an effective magnetic field.
Like the electric motor, the ammeter has a permanent magnet to provide a magnetic field that interacts with
a current provided by an external source of Emf.
No. The alternating current is a current flow that automatically reverses direction (polarity).
A steady state current will not induce a current in the secondary regardless of the number of turns. Only
when the flux through the secondary coil is changing is there an induced current.
No. Magnetic flux is the product of magnetic field times an area perpendicular to the field direction. If one
draws magnetic field lines such that the field strength is the number of lines crossing unit area, then the flux
across a surface is simply the number of lines.
The current will be induced in a clockwise direction as viewed from the top looking down. Since the flux is
increasing upward, according to Lenz's Law the current will be induced in a direction to put lines of flux
through the loop in the opposite direction, opposing the increase.
A certain voltage is induced for each turn, so the overall voltage is proportional to the number of turns since
they are all in series. The coil with twice as many turns will have twice the induced voltage.
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Q26
From 0 to 1s the flux is changing the most rapidly and during this time the induced voltage will be the largest.
Q27
Q28
Q29
Q30
Q31
Yes. Although the magnetic field is constant, because the coil is rotating, the flux through the coil is
continuously changing. Flux is the product of the area times the component of the field perpendicular to it. It
is easier to visualize in terms of lines of force. The field lines are constant, and as the generator coil rotates
the number of lines crossing the coil goes from zero to a maximum and back.
No. The simple generator can produce either a pulsating direct current or an alternating current depending on
the electrical connections. Used as a d.c. generator the current varies from a maximum to zero but is
monodirectional.
No. The function of the motor is to convert electrical energy into mechanical energy. The function of the
generator is to convert mechanical energy into electrical energy. In the process of running, a motor does
generate a back or counter emf that reduces the current that it would draw based strictly on the resistance of
its windings.
No, it will not work as shown in the diagram. If one contact of the battery and the primary were to be
continuously opened and closed, this would produce a variable flux and then the transformer would work.
No. A transformer cannot violate conservation of energy. As the voltage is stepped up, the current is stepped
down in proportion.
Answers to Exercises
E1
2.5 N
E2
3.2 x 10-5 N/m
E3
F2 = 1/3 F1
E4
5 cm
E5
20 C
E6
18 N
E7
0.3 N
E8
4.0 T
E9
1.92 T·m2
E10
0.012 T·m2
E11
24 V
E12
a. 1.8 T·m2
b. 9 V
E13
a. Step up
b. 440 V
E14
1.5 A
E15
15 turns
E16
33.3 turns
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Answers to Synthesis Problems
SP1
a.
b.
c.
d.
e.
2.0 x 10-4 N/m
Wires repel one another
6.0 x 10-5 N
2 x 10-5 T
Perpendicular to plane of page and into page
SP2
a.
b.
c.
d.
e.
5N
Upwards, perpendicular to both the direction of motion and the direction of the magnetic field.
No. The force will impart a change of direction only.
200 m/s2
200 m
SP3
a.
b.
c.
d.
e.
0.0018 m2
0.043 T• m2
Zero
0.25 s
0.1728 V
SP4
a. 80 turns
b. 25 A
c. No. As the primary wire gets warmer, it will develop a thermal resistance to the motion of the electrons
thus causing less current to flow in I1 (I1 gets smaller); and since I1 is proportional to I2, the current in I2
will also decrease.
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