Group:___ Names: __________Rubric/Key_________________
Magnetic Field Exploration Group Worksheet
Introduction
Magnets have two poles: north and south. The N (north-seeking) pole is defined to be the end of a
bar magnet that tends to point towards the Earth’s north geographic pole. The direction of the
magnetic field at a point is defined to be the direction in which the north end of a compass needle
points when the compass is placed at that location. In most locations on the surface of the Earth,
the geomagnetic field is ~0.5 gauss.
Investigation – answer these questions as a group to the best of your ability based on your observations *in*
the classroom. If you ever find that the results seem contradictory to what you expected, spend some extra
time on that particular question. You do not have to answer the questions in order so don’t get hung up
waiting (for example) on a Magnaprobe or magnetic viewing film.
1. Place a compass on the table away from any magnets.
a. Which end points north? _Towards Franklin Street (possibly 180 degrees as well)______
b. Check with your classmates to see if all the compasses at your table point in the same direction? If
not, why? There is a good chance that some of the compasses have had their polarity changed and
do not all point in the same direction (this is a pretty open ended question though, because it
depends on the random distribution of compasses in the room).
c. Is it possible for a compass needle to change polarity? Yes, it is possible!
d. Is it possible that the Earth’s magnetic field can change? Some students will over think this and
discuss on geological era timescales – yes, the magnetic field can change, but over the course of
this class, probably not.
2. Tie a piece of thread around the middle of a bar magnet so that it can be suspended in the air and freely
rotate.
a. How does the behavior of this magnet compare with the compass? Will spin a bit but orients itself
with labeled N pole (if the magnet in question is strong enough) pointing towards Franklin Street.
b. What are the similarities and differences? Pretty much a large compass – difference is the stability
(hindered in part by our own ability to hold the string still/constant)
3. Find a Magnaprobe (small red/blue magnet that can pivot within a black plastic handle) and observe how it
behaves compared with the compass and suspended magnet.
a. Which of the three devices is most sensitive to the Earth’s magnetic field? Why? Magnaprobe
seems to be more sensitive – has three independent degrees of motion and so small changes are
easier to see.
b. What limits the sensitivity of each? Open answer here – depends on which compass and the
strength of the magnet they are comparing to – but degrees of freedom and strength of magnet per
mass (how well/quickly it can align)
4. Place a bar magnet on the table and carefully observe what happens as you move a compass around the
magnet. If you have multiple compasses available, place them around the magnetic to get a better sense of
the field around the magnet. Sketch your observations below. Use an arrow to show the direction of the
magnetic field, remembering to follow the convention that the magnetic field points in the direction that the
north end of a compass points at that location. On your diagram, label the poles of the bar magnet and use
the compass to “map” the magnetic field around the magnet.
Students should be able to produce a good quality schematic of magnetic field lines for a bar magnet.
5. Place magnetic viewing film on top of the bar magnet to get a finer view of the magnetic field.
a. Where is the magnetic field strongest? How can you tell? By the color of the film, near the poles
of the magnet.
b. How is this similar to the electric field around a dipole? Lots of ways to describe here – but
essentially that at a distance, the field drops off more quickly than with a point charge.
6. Discuss and summarize how the strength of the magnetic field depends on the distance from the
bar magnet and the location around the bar magnet. How can you determine the magnetic field
strength with the materials provided? Discuss with your group members about how a compass can
be used to measure magnetic field strength. Use a ruler and compass to find the distance from one
end of the magnet where the field is the same strength as the Earth’s (~0.5 gauss).
a. Describe your experimental procedure. Use the reference strength of the Earth’s magnetic
field. Start with an isolated compass and see the position a magnet needs to be brought
with respect to the compass to make the needle point in the other direction. That
maximum distance should be when the field is equal to the Earth’s field.
b. Based on your observations, do magnetic fields obey the principle of superposition? Yes, it
should obey the principle of superposition.
7. Find a magnet that has already been broken in half. Separate the two halves and investigate using
a compass. How many north and south poles result? With two pieces, there are now 2 North
poles and 2 South poles.
8. From your observations, what can you infer about the direction of the magnetic field inside a bar
magnet? Sketch the magnetic field vectors for a few points inside the magnet. Magnetic field lines
are continuous and it is impossible to have a monopole.
9. Find a rare-earth neodymium (Nd) magnet, and compare the strength of the field with that of the Alnico bar
magnet. Be sure to handle the magnets carefully since they break easily! These magnets are MUCH
stronger. Any number of different comments are appropriate here.
10. Bring a charged rod near the compass and observe the result. How are magnetic poles and charges similar?
How are they different? Many people won’t have time to get to this question, but if they do, they should
find that while like charges repel like charges and like poles repel like poles, a charged surface or object
doesn’t attract or repel a magnet.
11. Experiment to see what else you can discover about these magnets and how they behave. Open ended
question – depends on the interest and creativity of the group.
GRADING – points for completion and attempting all the questions. This should be turned in at the end of the
class, not additional homework. If groups are focused, there should be enough time to make it through the
worksheet. Grade on a 0-5 scale, if all questions have a reasonable answer or attempt at an answer, the group earns
a 5. The grading for this assignment should be super quick!!