Physics 2054 Lab
RC circuits
Introduction
Capacitor store electric charge. If the capacitor is initially uncharged, charge from a source like a
battery accumulates on the plates and the voltage increases according to the equation
V= Vmax (1 – e-t/RC). Vmax is the voltage of that charging source. For a discharging capacitor the charge
slowly decreases so that the voltage obeys V = Vmax e-t/RC. For each situation the product RC is referred
to as the time constant, τ, which is determined by the capacitance, C, and whatever resistance, R, the
charge must move through to enter or leave the capacitor. This R is usually a circuit external to the
capacitor but can also include internal “leakage” inside the capacitor.
Once charged and disconnected from the circuit that charged it, the capacitor will discharge according
to
Materials
PASCO SE-9791 Resistor-Capacitor Circuit
Voltage measuring device: PASCO GLX
portable computer OR a computer with
PASCO DataStudio, either with its
appropriate voltage probe, OR a multimeter
in voltmeter mode.
Stopwatch if using voltmeter
Several jumper cables
A selectable, calibrated, variable resistance
(“decade box”)
Assorted extra capacitors
Procedure
Part 1
In this part you will measure τ for the circuit as set up by the manufacturer. You will do this by
measuring the voltage across the capacitor (contacts 5 and 6) in the charging/discharging circuit.
a. Connect a wire between points A and B to complete the circuit.
b. Attach your voltage measurement device across the capacitor
c. Make several trial measurements of charging and discharging to be sure that you can observe
the time dependence of the charging or discharging cycles. (i.e. you can see the “curve” of the
graph with the computer or accurately see the voltmeter display change as the capacitor’s
charge state changes.)
d. Switch the circuit to discharging mode and leave it there for 2 minutes. You may speed this part
of the procedure (to just a few seconds) by briefly connecting a wire across contacts 3 and 4.
e. Perform a charging sequence and measure the times at which the capacitor is 50% charged and
63.21% charged.
f. Repeat this for a second trial.
g. Record your data; you will use the average of the two trials to answer the questions in the
Analysis and Discussion section of your lab report.
h. Include a sketch or image of V(t) for one of your trials.
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Part 2
a. Use the resistance box to add a 10kΩ resistor in series with the built-in resistor.
b. Switch the circuit to charging mode and leave it there for 2 minutes. You may speed this part of
the procedure (to just a few seconds) by momentarily connecting a wire across contacts A (not 3
this time) and 4.
c. Perform a discharging sequence and measure the times at which the capacitor is 63.21%
discharged and 50% discharged.
d. Repeat this for a second trial.
e. Record your data; you will use the average of the two trials to answer the questions in the
Analysis and Discussion section of your lab report.
f. Include a sketch or image of V(t) for one of your trials.
Part 3
a. Remove the resistance box and return the circuit to its configuration for Part 1.
b. Now add a capacitor in parallel with the built-in capacitor. Make sure you record the value of
this capacitor in your data.
c. Switch the circuit to charging mode and leave it there for 2 minutes. You may speed this part of
the procedure (to just a few seconds) by momentarily connecting a wire across contacts 3 and 4.
d. Perform a discharging sequence and measure the times at which the capacitor is 63.21%
discharged and 50% discharged.
e. Repeat this for a second trial.
f. Record your data; you will use the average of the two trials to answer the questions in the
Analysis and Discussion section of your lab report.
g. Include a sketch or image of V(t) for one of your trials.
Analysis and Discussion Questions
Part 1
1. From your data, compute several values for τ and average these. How does this τ compare to
the expected value? Discuss any reasons for possible disagreement.
Part 2
2. From your data, compute several values for τ and average these. How does this τ compare to
the expected value? Discuss any reasons for possible disagreement.
Part 3
3. From your data, compute several values for τ and average these. How does this τ compare to
the expected value? Discuss any reasons for possible disagreement.
4. What is the significance of that strange “63.21%” in the procedure? That is, where does it come
from? Hint: it’s not your professor’s favorite number.
5. Why does shorting the resistor (placing a wire across contacts 3 and 4) help to speed up the
discharging or charging processes?
D. Boucher 2/2013
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