SP211 Lab 7
The Current Balance
We will predict the forces at play between two current carrying wires, and test our prediction by
applying a known force (with weights) and “balancing” the system back to its initial configuration.
Put your work/notes for this lab in your course notebook in some organized fashion.
1. First, some safety rules: DO NOT play with the lasers. DO NOT shine the lasers into anyone’s
eye. DO NOT exceed 15 Amps of current in the experimental appratus. Form groups at the side
lab stations, leaving the center lab stations clear.
2. Adjust the current balance apparatus with the alignment knob so that the wires are perfectly
parallel, both horizontally and vertically, and are not touching. When the system is at equilibrium
with no weights added and no current running through the wires, measure the distance between
the centers of the wires, d. Measure the lengths of wires, L.
3. For some current i moving through the wires, predict what the force between the wires would
be. Remember, we studied this in class, and saw that
µ0 Lia ib
|F~ab | = |F~ba | =
2π d
(1)
where ia = ib = i (the current is the same in both wires). Here, L is the length of the wire(s).
What direction are the forces if the currents run in the same direction? What direction are the
forces if the currents run in opposite directions?
4. Set up the laser so that it reflects off of the mirror and shines on the chalk board. Mark this
point with chalk and label it the “equilibrium” point.
5. Add a m = 100 mg mass to the weight pan on the floating wire; the wires will move closer
together due to this additional weight. Turn on the apparatus and adjust the current until the system returns to its equilibrium point. This current creates a force between the wires that balances
the force of the additional weight, mg. Record this current. Are the currents running in the same
or opposite directions?
6. Repeat step 5 with m = 200 mg.
7. Plug in your measured currents to the equation above. How does your theoretical prediction
compare to your experimental value, mg?
8. Once you’ve obtained reasonably good data, checked in with the lab instructor, and understood
all of the relevant physics, you’re welcome to leave or stay in lab to work on worksheets or WileyPlus
homework.