Sierzega: Magnetism 3
Magnetic Force Exerted on a Current Carrying Wire
Magnetic force exerted on a current: The magnitude of the magnetic force FB on W that a
magnetic field B exerts on a current I passing through a wire of length L is
F B on W = ILBsinθ
where θ is the angle between the directions of the B-field and the current I. The direction of this
magnetic force is given by the right hand rule for the magnetic force.
3.1 You wonder if instead of supporting your clothesline with two poles you could replace it
with a wire and then support it magnetically by running an electric current through it and using
Earth’s B-field, which near the surface has magnitude 5 x 10-5 T and points north. Assume that
your house is located on the island of Dominica near the equator where the B-field produced by
Earth is approximately parallel to the earth’s surface. The clothesline is 10 m long and with the
hanging clothes has a 2.0 kg mass. What direction should you orient the clothesline and what
electric current is needed to support it? Finally, decide if this seems like a promising way to
support the clothesline—no poles needed! (The wires in homes will only carry currents around
20 A before circuit breakers start to trigger for safety reasons.)
3.2 A 2.0-m long wire has a 10-A current through it. The wire is oriented south to north and
located near the equator. Earth’s B-field has a 4.0 x 10-5 T magnitude in the vicinity of the wire.
What is the magnetic force exerted on the wire?
3.3 An east-west power line at the equator carries 100 A of current toward the east. At this
location Earth's B field has a magnitude of 3.5 x10-5 T and points north. (a) Determine the
magnitude and direction of the magnetic force exerted by Earth’s magnetic field on the 230-m
long 160-kg wire. (b) Compare this force to the gravitational force exerted on the line by Earth.
3.4 Summary: There are significant differences between the force caused by a magnetic field
and the forces caused by gravitational and electric fields. After writing each difference, answer
the question, “How do I know this?”
Sierzega: Magnetism 3
3.5 TESTING EXPERIMENT: Force Exerted By The Magnetic Field On A Current-Carrying Wire (Right Hand Rule For The Magnetic Force)
Available equipment: A horseshoe magnet whose poles are known (Red: North, White: South), a
scale, an assortment of rigid wires (mounted on plastic backings. They look a bit like slides.), a
power source, multimeter, connecting wires, ring stand, meter stick, PASCO current balance
(WARNING: Do not let the current in the circuit exceed 5A since this could damage the
current balance or possibly the power supply).
Think of how you can use this equipment to test the right hand rule for the direction of the force
exerted by the magnetic field on a current carrying wire. Hint: What physical quantities can you
determine using the scale?
Warning: Do not leave the power source on after you finish the measurements. There will be a
large current flowing in the circuit so shut the power source off when it’s not needed.
a) First, recall the right hand rule for the magnetic force exerted by the magnetic field on a
current carrying wire. Write what quantities it relates and describe the rule with a picture
and using words. Consider the available equipment and how you could use it to achieve
the goal of the experiment. Brainstorm and write down your potential experiments. Think
ahead about what you will measure and how you will measure it.
b) Describe the experimental procedure you have chosen. The description should contain a
labeled sketch of your experimental setup, an outline of what you plan to do, what you
will measure, and how you will measure it. Explain in detail how you will experimentally
measure the direction of the force exerted by the magnetic field on the wire. Hint: The
reasoning here is more complicated than it seems at first. Use force diagram(s) and
Newton’s second and third laws to help.
c) Use the hypothesis you are testing to make a qualitative prediction for the reading of the
scale (more than some value, less than some value) for your particular experiment. Show
the reasoning used to make the prediction with force diagrams. Call Mr.Sierzega over
once you have done this but before you turn on the current.
d) Perform the experiment and record the outcome. (WARNING: Do not let the current in
the circuit exceed 5A since this will damage the current balance or possibly the
power supply).
e) Did the outcome match your prediction? If not, list possible reasons.
f) Based on your prediction and the experimental outcome, make a judgment about the right
hand rule.
Sierzega: Magnetism 3
Magnetic Forces Lab Rubric
Scientific
Ability
Force Diagram
Name:
Not Proficient (0)
Partially Proficient (2)
Proficient (3)
Advanced Proficient (4)
No representation
is constructed.
Force diagram is
constructed but contains
major errors such as
incorrect mislabeled or
not labeled force vectors,
length of vectors, wrong
direction, extra incorrect
vectors are added, or
vectors are missing.
Force diagram
contains no errors
in vectors but lacks
a key feature such
as labels of forces
with two subscripts
or vectors are not
drawn from single
point or axes are
missing.
The diagram contains no
errors and each force is
labeled so that it is
clearly understood what
each force represents.
Is able to
distinguish
between a
hypothesis and
a prediction and
make a
reasonable
prediction
based on the
hypothesis/relat
ion.
Is able to decide
whether the
prediction and
the outcome
agree/disagree
No prediction is
made. The
experiment is not
treated as a testing
experiment.
A prediction is made but
does not follow from the
hypothesis/relation.
A reasonable
prediction is made
that follows from
the
hypothesis/relation.
A prediction is made,
follows from the
hypothesis/relation, has
an if-and-then structure,
and describes the
outcome of the designed
experiment.
No mention of
whether the
prediction and
outcome
agree/disagree.
A decision about the
agreement/disagreement
is made but is not
consistent with the
outcome of the
experiment OR
experimental uncertainty
is not taken into account.
A reasonable
decision about the
agreement/disagree
ment is made and
experimental
uncertainty is taken
into account.
A reasonable decision
about the
agreement/disagreement
is made and experimental
uncertainty is correctly
taken into account.
Is able to make
a reasonable
judgment about
the hypothesis
No judgment is
made about the
hypothesis.
A judgment is made but
is not consistent with the
outcome of the
experiment OR
assumptions are not
taken into account.
A judgment is made
and is consistent
with the outcome of
the experiment and
assumptions taken
into account.
A reasonable judgment is
made and assumptions
(including effects of
assumptions) are taken
into account.
Rubric Turned in With Thoughtful Self-Assessment: (required or no grade)
Total: ____________/ 16 points
Typed, stapled lab report due: __________________________