Answers

rawfrogpondUrban and Civil

Nov 16, 2013 (3 years and 11 months ago)

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4.7
) Electromagnetism


1)

You have a magnet and coil attached to a sensitive galvanometer.

a)

What happens as
a

magnet is moved in and out of the coil
?


The needle swings back and forth because a current is induced in the coil.

b)

What
will happen

if the coil
and

the magnet are moved together? the needle will

A)

move more

B) move less

C) move the same

D) not move

It is the relative motion
of the coil and the magnet
that matters.

2)

A magnet is dropped down a copper tube.

What happens to its speed? It accelerates at

A)

9.8 m/s
2

B) less than 9.8 m/s
2


C) greater than 9.8 m/s
2

D)
a decreasing rate

We know that a moving magnet will induce a current. That current will generate a magnetic field
that will either repel or attract the magnet. It must repel the magnet or else
th
e magnet would gain
extra kinetic energy from nowhere.
It will have a

lower kinetic energy

than from freefall and the
missing energy is found in
electrical energy
of

the induced current

in the tube
.
From the previous
demonstration, we know that a greater s
peed has a greater effect

and so as the magnet speeds up the
opposing force

should increase and the acceleration decrease.

It clearly takes longer to exit the tube
.
Measurements need to be made on tubes of different lengths to show that the acceleration is

not
constant
.

These will show that terminal velocity is reached very early.

3)

Draw freebody diagrams of the magnet at the top, middle and end of the tube.

Gravity
stays constant. The magnetic force is zero at the start and equal to gravity at the
end
.

4)

Wher
e does the gravitational energy go?

Into electrical energy of the current in the tube.

5)

How and why is this used for brakes on the Drop Zone?

These brakes will work even if there is a sudden power failure.

6)

A copper ring surrounds the iron core of an electr
omagnet.

a)

What will happen if

the current is turned on?

The increased magnetic field will induce a current in the ring. The rings magnetic field is repelled by
the electromagnetic field and it will be pushed up.

Whether it rises slightly or jumps up depend
s on
its mass and resitance.

b) How is this similar to an induction stove?

The induced current encounters resistance in the ring and the ring heats up. There are no elements in
an induction stove. Instead the stove induces a current in the pots and the pot
s heat up.
This saves
energy and is safer.

7)

Attach the Lego motors
together with a wire
.

a
) What happens as you turn one? Why?

b
) Compare motors
to

generators
.

Both contain permanent and electromagnets. The motor converts electrical energy into kinetic ene
rgy
because the coil is repelled by the magnet. The generator converts kinetic energy into electrical energy
because moving the coil in a magnetic field induces a current.


8)

A radio has had the wires leading to the speakers cut. The speaker leads have been
attached to a coil of
wire and the radio leads have been attached to another coil of wire.


When the radio is turned on you will hear nothing

A)

because the circuit to the speakers h
as been broken.

B)

unless the coils are placed on top of each other.

C)

unless the coils are placed on top of each other and are touching.

D)

unless the coils are placed close beside each other.

E)

unless the coils are placed close beside each other and are touching.

Explain:

The changing current in the radio coil produces a changing magnetic field. This changing field can
induce a changing current (and therefore sound) in the speaker coil. This will be hard to hear unless the
coils are on top of each other, so that th
e strongest magnetic fields are aligned. They do not need to be
touching.


9)
Go to
http://phet.colorado.edu/en/simulation/faraday

and select
Transformer
.
There is no current going
through the l
ight bulb. This is because

A) the coils are too far apart




B) the voltage is too low


C) there aren’t enough loops




D) something else

Explain:


A current is induced by a changing magnetic field and this one is constant.

Switch to AC.


For the following

questions use the simulation and the real apparatus.


10)
You will decrease the effect if the coil is moved

A) further away





B) so the centers are not aligned

C) both will reduce the effect




D) neither will have an effect

Explain:


Both. The field w
eakens with distance and off axis.


11)
You can increase the effect by

A) increasing the amplitude of the AC signal


B) increasing the frequency of the AC signal

C) increasing both





D) neither will have an effect

Explain:

Both. Increasing the amplitud
e of the signal changes the maximum strength of the magnet. Increasing
t
he frequency

increases the rate of change of the magnetic field.
A frequency of zero is a DC source and
has no effect.
This is hard to show with the radio


the higher frequencies shou
ld be easier to hear than
the lower, but it is not clear. A frequency generator and sound meter are needed.




12)
You can increase the effect by maximizing the number of loops attached to

A)
source


B)
pick up

C) both

D) neither will have an effect

Expl
ain:

Both. Increasing the loops for the input increases the maximum magnetic field strength. Increasing the
number of output coils

increases how effectively this is picked up.


13)
You will decrease the effect the most by

A) halving the diameter of the lo
ops



B) doubling the distance

C) they will have equal effects



D) they will have no effect

Explain:

If the pickup coil is aligned and up close, it will pick
up the entire

field
as long as the pickup

coil is as
big as the source coil.
However, doubling t
he distance will always drastically reduce the effect.


Use just the radio for the next two questions.

14)
The sound will be reduced if you place the following materials between the coils

A) iron


B) aluminum


C) aluminum and iron

D) paper

E) all

of the above

Explain:

Any conducting material will have a current induced in it and therefore reduce the current in the coil. A
thicker piece will have a greater effect.


15)
The sound will be increased if you place the following materials inside the coi
ls

A) iron


B) aluminum


C) aluminum and iron

D) paper

E) all of the above

Explain:

Iron will help if it concentrates more field into the pickup coil. It is more effec
tive for a small diameter
coil which has a field that curves more inside.








16)

Maxwell summarized all of electricity and magnetism into four laws.
Explain in simple words
and
diagrams
what each of
Maxwell`s Equations
means and give an example of how you can demonstrate it.


a)


E

=

/




Charges produce radial electric fields.

Hair and balloon.


b)



B = 0



Magnetic fields form loops. There are no monopoles.

Solenoid.


c)


X
E

=
-

dB/dt



An electric field is generated by a changing magnetic field
.




Transformers, generators.


d)


X
B
=




I

+






d
E

/dt

A magnetic field is generated by a current.

Electromagnet.








17)

The last
term of the last equation was added by Maxwell
for reasons of symmetry. He
used it to predict
that electric an
d magnetic fields can
form a self
-
propagating loop
whenever a charge is
accelerated
.

a)

Which equation is changed by accelerating a charge

and what happens after?

Accelerating a charge means you have produced a new magnetic field and so the third equation sa
ys
that the will produce a field. This new magnetic field, according to the second part of the fourth
equation will induce a new electric field and so on.

b)

The
disturbance moves
in a vacuum at a speed of sqrt (

/




) where;




4


x 10
-
7

Tm/A


the
magnetic permeability

of free space,
and



=ㄯ1

k


the
electric permittivity

of free space.
Calculate
the speed

of the disturbance
.

v = 3.00 x 10
8

m/s.

c) You can see this modelled at
http://phet.colorado.edu/en/simulation/radio
-
waves



18)

Hot spots are formed in microwave ovens and so a rotating tray is used to heat more evenly. You can
locate the hot spots by placing a layer of chocolate, cheese or mar
shmallows in the oven. Measure the
separation of the hot spots and use the speed of light to calculate the frequency of the microwaves.

The separation is the distance between two nodes of a standing wave. Therefore it is ½ of a wavelength.
f = c/



The

frequency will in the right order of magnitude when compared to the given frequency
on the back, but not much better.



Textbook:

8.5


p. 418
#

1
-

3,
p. 419 #
1
-
8
, p.
429
#

20