PHY 242 Laboratory for 2/17/04

1. The RC Circuit

You will construct a simple RC series circuit using a battery (actually an 8 V power supply), a capacitor (C = .019 F), a resistor (R = 3 kW), and a deadman switch (Sw). (You may use the value stamped on the capacitor. However, use the ohmmeter setting of the multimeter to accurately determine the value of the resistance.) The potential across the capacitor will be monitored using the DC voltage setting of the multimeter.

For an initially uncharged capacitor, the voltage across the capacitor is given by

Procedure:

First, using a jumper wire, short the capacitor before starting to ensure the initial charge is zero. (NOTE: Never touch the capacitor terminals with your fingers!)

Construct the circuit and ask the instructor to verify that the connections are correct. Set the power supply for approximately 8 V. Depress the deadman switch and using a stopwatch, record the potential (V_c ) approximately every 10 seconds for the first 2 minutes and then every 30 seconds for an additional 3 minutes. One person should be in control of both the stopwatch and deadman switch to minimize error due to reaction time. (The process can be briefly stopped to record each value. If the potential drops during the recording process, simply tap the switch until the potential returns to the last recorded value.) When finished, short the switch and leave the capacitor to charge. After about 10 to 15 minutes the potential across the capacitor should level off to the V_m value. Record this value.

Report:

Plot V_c vs time and interpolate a smooth curve through the points. Choose four sets of values of the potential and time (about every 30 to 40 seconds) from the plot and calculate the value of t from each set. Find the average of these four values and estimate an error in the average. Compare your average with the theoretical value. Is there any trend in the calculated values of t? If so, what conclusions would you draw from this?

2. Magnetic Force on a Line Current

You will use the current flowing through a wire immersed in a magnetic field to determine the direction of the force and to verify the relationship of the magnitude of the current to the magnitude of the force.

Procedure:

1. Use a compass to determine whether the pole of the magnet, positioned next to the pivoting wire, represents a N or a S pole.
2. If not already done, set up the wire balance and adjust the magnet such that the tray on the wire balance is just across from the magnet. Connect the power supply, rheostat, ammeter and deadman switch in series with the wire balance. Be sure the rheostat is set for full resistance.
3. Turn on the power supply and press the deadman switch. The wire balance will move either up or down. Set the connections such that the wire balance moves upward. Note carefully the direction that conventional (positive) current is flowing for this case.
4. Position the laser such that the beam reflects off the mirror attached to wire balance and onto a spot that can be easily marked.
5. Add 10 mg to the tray of the wire balance. Depress the deadman switch and adjust the current until the wire balance returns to equilibrium, as indicated by the laser spot. Record the current from the ammeter.
6. Repeat using the 20 or 30 mg weight.
7. Record your values at the chalkboard.

• Indicate in a diagram the direction of the magnetic field created by the magnets, the direction of the current through the wire balance, and the resulting force. Verify that your results are consistent with the right hand rule for the equation F = I l x B.
• Using the data from all groups, compare the currents required for equilibrium with the weights placed on the tray for the two cases. You may wish to combine the values of each group in some way or look at each data set independently in your analysis. Based upon your analysis, is the data consistent with the equation F = I l x B? Explain why or why not.