SP212 Lab: Fiveà Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
Preliminary Measurements
Measure and record the resistance of each of the resistors on your lab bench. Now measure the equivalent
resistance of two of the resistors in series. Does it match what you expected? Now measure the
equivalent resistance of two of the resistors in parallel. Does it match what you expected?
Two-Loop Circuit with an EMF in each loop:
Build the circuit shown. Set the voltage of the power supply to be about 3 V.
Measure the current passing through the three ammeters.
After you build the circuit, take data, and then gently disconnect the D-­‐cell and power supplies. The idea
is to avoid running the D-­‐cell down, but to leave the circuit almost intact, so you can take another data set
if you need to.
Calculate what current should pass through each of the three ammeters; compare with your
measurements.
An RC Circuit
Build this circuit and collect data on V and i as a function of time for both charging and discharging. Use
the 25,000 µF (0.025 F) capacitor similar to the one shown below and the Tektronix power supply with 5
V.
Page 1 of 2 SP212 Lab: Fiveà Kirchhoff’s Rules and RC Circuits
Version: February 18, 2015
Starting with the capacitor fully discharged, start collecting data and then throw the switch to the charging
position. After the capacitor is completely charged, flip the switch to the discharge position, and
complete the data run.
Fit the appropriate exponentials to the charging and discharging curves from both the ammeter and
voltmeter. Determine the time constant from each of these, and average them together.
Calculate the theoretical time constant value for this circuit and calculate the uncertainty in your predicted
value. τ = RC
Discussion: Compare your measured value for the time constant with the theoretically-predicted value.
In addition to comparing predicted and measured values for the time constant, explain the signs and
shapes of each of the four curves in your graph.
Bulbs and Batteries in Series and Parallel
You will have two boards with small light bulbs: one board will have two light bulbs wired in parallel,
while the other will feature three light bulbs wired in parallel. Set up a circuit with two 1.5 V D-cell
batteries connected to the board with two bulbs. Observe how bright the bulbs are. Now gently unscrew
one of the bulbs. What happened to the brightness of the other bulb?
Now set up a circuit with two 1.5 V D-cell batteries connected to the board with three bulbs. Observe
how bright the bulbs are. Now gently unscrew one of the bulbs. Then, gently unscrew a second bulb.
What happened to the brightness of the other bulb?
Now connect the two batteries to both of the light bulb boards, with the two boards connected in series.
Observe the relative brightness of the bulbs. Now unscrew on of the bulbs on the board with three bulbs.
What happens to the brightness of the other bulbs? If your answer is different from your answers to parts
1 or 2, explain why the answers are different.
Discussion: What would happen if you connected your two bulbs in parallel to two batteries in parallel as
shown in the next diagram?
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