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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.
a) What happens? It accelerates at
A) 9.8 m/s2
B) less than 9.8 m/s2
C) greater than 9.8 m/s2
D) a decreasing rate
We know that a moving magnet will induce a current. That current will generate a magnetic field that
will repel the magnet. From the previous demonstration, we know that a greater speed produces a greater
current and so as the magnet speeds up the opposing force should increase and the acceleration decrease.
b) Draw force 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 probably equal to gravity by the end.
c) Where does the gravitational energy go?Into electrical energy of the current in the tube.
d) How and why is this used for brakes on the Drop Zone? These brakes work in a power failure.
3) A copper ring surrounds the iron core of an electromagnet.
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 jumps up depends on its mass and resistance.
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 pots heat up. This saves
energy and can be safer because the stove does not get hot.
4) Attach the Lego motors together with a wire.
a) What happens as you turn one? Why? When you turn a motor, you turn a magnet sitting in a coil and
that induces a current that travels to the other motor.
b) Compare motors to generators.Both contain permanent and electromagnets. The motor converts
electrical energy into kinetic energy 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.
5) 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 unless the coils are placed
A) on top of each other.
B) on top of each other and are touching.
C) close beside each other.
D) 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 the strongest magnetic fields are aligned.
6) Go to http://phet.colorado.edu/en/simulation/faraday and select Transformer. There is no current going
through the light 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.
7) 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 weakens with distance and off axis.
8) 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 amplitude of the signal changes the maximum strength of the magnet.
Increasing the 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 should be easier to hear than the
lower, but it is not clear. A frequency generator and sound meter are needed.
9) 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
Explain: 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.
10) You will decrease the effect the most by
A) halving the diameter of the loops
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 the distance will always drastically reduce the effect.
11) 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
d)
X B =  I + dE /dt
An electric field is generated by a changing magnetic field.
Transformers, generators.
A magnetic field is generated by a current. Electromagnet.
12) The last term of the last equation was added by Maxwell for reasons of symmetry. He used it to predict
that electric and magnetic fields can form a self-propagating wave whenever a charge is accelerated. See
http://phet.colorado.edu/en/simulation/radio-waves
a) Which equations predict what happens when a charge is accelerated?
Accelerating a charge means a new magnetic field and the third equation says that it will produce a new
electric field which will induce a new magnetic 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  =1/4k – the electric permittivity of free space.
Calculate the speed of the disturbance. v = 3.00 x 108 m/s.
13) 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 marshmallows 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 calculated frequency will be in the right order of magnitude as shown on the back.
Textbook: 8.5
p. 418 # 1 - 3, p. 419 #1-8, p. 429 # 20