Introduction
to Electromagnetism
Materials
2 x Genecon 2
x Alligator patch cords (light weight)
Small
coil with iron rod Wood
block with two screw eyes
Compass Stiff
wire bent in shape of U
Masking
tape Horseshoe
Magnet
Ring
stand with clamp Tangent
Galvonometer
DC
power supply (0-28 V)
Introduction
In this activity we will explore some of
the basic ideas of electromagnetism. Electromagnetism refers to the magnetic field
produced by an electric current.
Procedure
The Magnetic Field of a Solenoid
Begin by plugging the Genecon cable into
the Genecon. Connect the alligator clips
on the Genecon cable to the connectors on the small solenoid. Place the solenoid on the table with the bore
oriented horizontally. Place the small
iron rod so that it is a little more than halfway into the bore of the
solenoid.
1) Turn the crank of the Genecon one time
rapidly while observing the iron rod.
Record what happens to the iron rod.
(If nothing happens notify your instructor to assist you.)
2) Repeat the procedure only this time turn the
crank of the Genecon in the opposite direction.
Record what happens to the iron rod.
3) Keeping in mind that the iron rod is a
temporary magnet, suggest an explanation for what is happening to the iron rod
when you turn the crank.
4) Explain how you might use an iron rod inside
of a solenoid to make a switch.
It appears that when we turn the crank we
produce a magnetic field in the solenoid.
We would like to verify that observation and determine some properties
of the resulting magnetic field. Connect
the alligator clips on the Genecon cord to an alligator clip patch cord. Wrap the patch cord in a line around the
center of the compass so that it makes three
turns around the compass. You may
want to use some tape to secure the cord. Orient the compass so the compass
needle is parallel to the turns of the wire and turn the Genecon handle at a
constant rate. Be careful not to turn the crank handle too fast as this can break the
Genecon.
5) What happens to the compass needle when you
turn the crank?
Wrap
the entire patch cord around the
compass and turn the Genecon handle at the same rate as in the previous
question.
6) Is it easier to produce the same effect as
you described in question 5)?
Turn
the crank of the Genecon in the opposite direction.
7) Record what happens to the compass
needle.
8) How does this observation differ from the
previous?
Orient
the compass so that the compass needle is perpendicular to the coil of
wires.
9) Turn the crank of the Genecon and record what
happens to the compass needle.
10) Turn the crank in the opposite direction and
record what happens to the needle in this case.
11) Remembering that the compass needle is a
magnetic dipole and will align with the direction of the magnetic field, what do questions 7) - 10) tell you about the
direction that the magnetic field of a solenoid points. Specify the direction in terms of the
orientation of the coils.
12) Does the magnetic field depend on the
direction that the current flows in the solenoid? If so, how?
13) Summarize your observations about the
strength and direction of the magnetic field of a solenoid.
Electromagnets
Wrap the patch cord tightly around the
iron rod with as many turns as you can fit.
Have one person turn the crank while another holds the
electromagnet.
14) Use the compass needle to demonstrate that
the wrapped iron rod behaves like a permanent magnet when current flows through
the wire. Record your observations
demonstrating this result.
Remove
the iron rod from the coil but leave the coil the same. Repeat your procedure with the compass to
verify that the coil without the iron rod acts like a permanent magnet.
15) Record your observations demonstrating that
the coil without the rod behaves like a permanent magnet.
16) Does the magnet seem to be stronger with the
iron rod or without? Explain.
17) Remembering that the iron rod is a temporary
magnet, explain why the magnetic field of the electromagnet is stronger when
the iron rod is present.
A Very Simple Motor
Set the U-shaped magnet on the table with
one pole on the table and one pole in the air.
Use the ring stand and the clamp to hold the wood base with two
hooks. Suspend the U-shaped wire from
the hooks, adjusting the height so that bottom of the wire is halfway between
the poles of the magnet. Make sure that the wire is free to swing on the hooks.
Turn the crank of the Genecon back and
forth about a quarter turn each way.
18) Record what happens to the wire.
19) Explain this behavior in terms of the
magnetic fields of the U-shaped magnet and of the field due to the current in
the wire. Draw a suitable picture.
Stop
the wire and turn the Genecon continuously in one direction.
20) Record what happens in this case.
21) Why is the result different when you turn the
crank continuously in one direction than when you turn the crank back and
forth?
The Equivalence of a Generator and a Motor
Connect the alligator clips of two
Genecons together. Place one on the
table and turn the crank of the other.
22) Describe what happens?
Turn
the crank in the opposite direction.
23) Describe what happens. How is it different than previously?
Note
that when you turn the crank of the Genecon, it acts like an electric
generator. The generator converts
mechanical work into electrical energy.
24) What process is happening in the second
Genecon?
25) How do a generator and a motor seem to be
related?
We
will explore this connection more in ch. 22.
The Magnetic Field of Earth
In this brief activity we will determine
the magnetic field of Earth. We will use
an apparatus called a tangent galvanometer.
The tangent galvanometer is essentially a coil with a compass at its middle.
1. Set-up
Make
sure that the DC power supply is turned off, the knob on the right is set to
the 0 – 28 V setting and the knob on the left is fully counter clockwise. Connect the coil to the DC output of the
power supply between the middle and an outer terminal. This means that the power supply is connected
across 5 turns in the coil. Rotate the
apparatus so that compass needle is oriented in the plane of the coil. Identify which side of the compass needle
points north, and note it for later reference.
Rotate the compass case so that the needle aligns with N on the case.
2. Turn on the power supply and very slowly turn up the current. Be careful to monitor the current meter on
the power supply and not the voltage. Do
not exceed 5 A, and probably you will need considerably less.
26) Do you observe any effect? Describe it.
3. Turn up the current a little more.
27) What change is there?
4. Turn down the current and reverse the leads
to the coil. Slowly turn up the current
again.
28) Does the effect change in way?
5. Turn down the current and connect the power
supply across the outer terminals on the tangent galvanometer. The power supply is now connected across 10
turns in the coil..
Very slowly turn up the current.
29) Was there any change in the effect?
30) When current flows in the coil, what is
produced?
31) In the center of the coil, what direction
does the magnetic field produced by the coil point compared to the coil?
32) How
does the strength of the produced magnetic field depend on the current in the
coil?
33) How does the strength of the magnetic field
produced by the coil depend on the number of turns with the coil?
34) Draw a vector diagram showing the sum of the
magnetic field due to the coil and the magnetic field due to Earth.
35) Use your diagram to find an expression for
the angle, q, through which
the needle deflects in terms of Earth
magnetic field and the magnetic field of the coil.
An expression for the magnitude of the
magnetic field, in units of teslas, at the center of the coil is given by , where B is the magnetic field strength, N is the number of
turns in the coil, r is the radius of the coil, I is the current flowing in the
coil, and m0 = 4p x 10-7 Tm/A is a fundamental constant called the
permeability of free space.
36) Describe a procedure to measure Earth's
magnetic field using this apparatus.
37) Carry out your procedure and record your
results and calculations below.