Current Balance
In this lab you will quantify the magnetic field produced by a current in a wire. CAUTION: the currents and voltages you may use in this experiment are strong enough to injure you. Be mindful of your own and your colleagues’ safety, follow the setup instructions below, and check with your instructor or lab TA if you’re not certain about how to connect something. Don’t touch metal wires or components that have current flowing through them, and disconnect power when you are not using it. The metal rods through which power flows in the current balance apparatus may become hot after long use, so be careful when touching them even if power is disconnected.
Objective
- Determine the force between two current-carrying wires, and show that it is in agreement with that predicted by the Lorentz force and the Biot-Savart Law.
Resources
- Current balance apparatus
- Step-down transformer (Vin/Vout ≈ 20)
- Variable autotransformer (variac)
- Connector wires
- Tiny weights (~1-10 grams)
- Laser mounted on a stand with a clamp
- Digital caliper
Background
The Lorentz Force Law states that the force exerted on a current I of length L by a uniform external magnetic field B is \vec{F}_B = I\vec{L}\times\vec{B}
The Biot-Savart Law describes the magnetic field produced by a current. At a distance r from a long, straight wire carrying a current I this can be shown to be |\vec{B}|=\frac{\mu_0 I}{2\pi r}
Guideposts/Hints
show/hide- The tools you have will produce alternating current (AC) in the wires. This won’t affect the force, but if you try to measure DC you will get zero!
- You can calculate the magnetic force between the wires using your values of current, etc. but you need to compare it with something. How can you measure the effective magnetic force? Remember this is a current balance – what force is balancing the magnetic force?
- Estimate uncertainties in your measurements, so you can compare your data to the predicted results. You should perform multiple measurements to reduce your overall uncertainty.