PHY 242 Laboratory for 9/16/99

The RC Circuit and the Kirchoff Circuit Challenge

The RC Circuit (5pts)

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). The potential across the capacitor will be monitored using a high impedance digital voltmeter (V).

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

Vc = Vm (1 - e -t/t )

Vm is the maximum voltage across the capacitor which occurs as t goes to infinity.
t = RC is the time constant of the circuit.

Procedure:

Construct the circuit and ask the instructor to verify that the connections are correct. Short the capacitor before starting to ensure the initial charge is zero. Set the power supply for approximately 8 V. Depress the deadman switch and using a stopwatch, record the potential (Vc ) 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 Vm value. Record this value.

Plot Vc 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?

The Kirchoff Circuit Challenge (3-5 pts)

When you enter the lab, you will be given a diagram of three circuits consisting of resistors and a battery. The circuits will be assigned point values of 3, 4, or 5, consistent with their difficulty.

  1. Choose one of the circuits and determine what the voltage will be across the specified resistor. Although the approximate values of the resistors are specified by the color coded bands, use the ohmeter to obtain more exact values.
  2. Carefully construct the circuit. Do not turn on the power supply that will serve as the battery.
  3. When you are ready, the instructor will collect your calculations, turn on the power supply and check the voltage. If the voltage reading matches your calculation (within a reasonable error), you will receive the points for the circuit you have chosen. There will be no further work required for this section.

    If the values do not match, you must either redo the calculations and submit them with the lab report OR you will be required to reconstruct the circuit before leaving the lab, depending upon the error you have committed. Good Luck!