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Sierzega: Magnetism 1
Are Electricity and Magnetism Related?
1. Recall In the last few units we have been learning about electric phenomena. Recall our study of
electric charges and forces and the observations we made with the rubbed foam tubes. Use these
observations, as well as any previous experiences with magnets, to fill in the table below:
Experiment
Observation
(Attract or Repel)
1.
2.
3.
4.
A charged object is brought near a similarly charged object.
A charged object is brought near an oppositely charged object.
A charged object is brought near an uncharged (neutral) object.
The north pole of a bar magnet is brought near the north pole of another bar
magnet.
5. The north pole of a bar magnet is brought near the south pole of another bar
magnet.
6. A charged object is brought near the north pole of a bar magnet.
7. A charged object is brought near the south pole of a bar magnet.
From these observed patterns we learned that charged objects attract and repel other charged objects and
that magnets also attract and repel other magnets. However, we found that electrically charged objects
do not exhibit magnetic properties. Are electricity and magnetism related in any way or are they totally
different phenomena?
We were able to better understand the mechanism for how electrically charged objects interact without
contact by suggesting the existence of an electric field. Similarly, objects with mass could be thought to
interact gravitationally without contact by the existence of a gravitational field. Since magnets can also
interact without contact, it’s reasonable to suggest the existence of a magnetic field as the mechanism
behind magnetic interactions. Imitating our study of electric phenomena, let’s suggest that a magnet
produces a magnetic field with which other objects with magnetic properties (another magnet, anything
made of iron, etc.) interact. The magnetic field is a magnetic disturbance produced by the magnet. The
field affects other magnetic objects.
2. Observe and find a pattern
In order to find a relationship between electricity and magnetism, we must first learn how to detect a
magnetic field. You have a bar magnet, a horseshoe magnet, and a compass. Perform the
experiments and complete the table that follows.
Experiment
Hold a bar magnet horizontally and slowly
bring it closer to the compass.
Repeat the first experiment but reverse the
direction of the bar magnet.
Hold the horseshoe magnet vertically and
slowly bring the compass between the poles
of the magnet.
Draw the relative orientations of the magnet with
marked poles and the compass with marked poles.
Sierzega: Magnetism 1
a. Describe in words a pattern between the orientation of the magnet’s poles and the orientation
of the compass.
b. Describe the same pattern representing a compass with an arrow S-N, as illustrated.
N
S
3. Representing the Magnetic Field
Using the tiny compasses in class, spread them out on a table. Notice that they all point towards Earth’s
geographical north. Now place a bar magnet in the middle of the compasses. Draw a picture of the
magnet and the orientation of each compass below.
The magnetic B-field and its representation by B-field lines
 The direction of the magnetic B-field at a point is defined as the direction of a compass north
pole when at that point.
 Magnetic field lines represent the B-field.
 The B-field vector at a point is tangent to the direction of the B-field line at that point.
 The density of lines in a region represents the magnitude of the B-field in that region—where the
B-field is stronger, the lines are closer together.
Do any other objects besides magnets create magnetic fields? We found
earlier that stationary electrically charged objects do not, but maybe moving
electric charges do…
N
A
4. Observational Experiment
In order to determine whether electric currents are produce magnetic fields,
connect a battery, a switch, some wires, and a light bulb in a circuit, as
shown at the right (the bulb indicates an electric current in the circuit).
Make sure that one of the wires in the circuit is aligned along the
geographical north-south direction. With the switch open, place compasses
under, above, and at the sides of the wires. The needles point north.
a. Perform the experiments described in the table and record your
observations.
B
Sierzega: Magnetism 1
Direction of the current
Draw an arrow representing the Draw an arrow representing the
orientation of the compass
orientation of the compass when
when placed below the wire
placed above the wire.
AB.
Current flows in the northto-south direction in the
wire, as shown in the
figure.
If you reverse the battery
poles, the current now
flows in a south-to-north
direction in the wire above
or below the compass.
b. Use the thumb of your right hand to represent the direction of the current and your four
fingers of the same hand to represent the direction of the compass. Does the orientation of
your thumb and fingers describe a pattern between the direction of the current and the
orientation of the compass for all of the above experiments?
c. Come up with a reason why electric current (moving electrically charged particles) might
affect the behavior of magnets differently than would stationary charged objects.
5. Imagine that a wire passes up through the page you are reading. Iron filings are sprinkled on the
page. We can think of the iron filings as small compasses. Draw a picture showing the filings when
no current is moving in the wire. Then draw a picture of the arrangement of the filings when there is
a significant current in the wire.
a. Is your picture consistent with the results of the experiment above? Explain your answer.
b. Repeat the same for a loop or coil of wire.
c. Draw a sketch that you think represents the orientation of magnetic field vectors produced by
the electric current in the wire at five different points. (Hint: Choose the direction of the
current out of the page.)