CAPACITANCE LAB QUALITATIVE — PHY 142/162
In this portion of the lab we will measure the length of time that a charged
capacitor block will keep a light shining, when connected as a battery. This time is
proportional to the capacitance; thus we will see how capacitance changes, when
capacitors are connected in series or in parallel. In the process, we hope to learn
how capacitors work.
CAPACITORS in SERIES
A) Single Capacitor: Connect one capacitor to the light in an open circuit. Charge
the capacitor, allowing plenty of time to reach full charge. Close the circuit
and measure how long the light shines, T1. [Note well: If T1 is longer than ½
the charge time, you did not charge it long enough. Repeat the process with
double the charge time, and redouble as needed until T1 is shorter than ½
the charge time. This applies to all parts of this lab.]
B) Two Capacitors in Series: Now connect two capacitors in series to the light in
an open circuit. Charge this capacitor block (not the individual capacitors).
What is your prediction for T2, the time that the light will shine, once the
circuit is closed? Write down your prediction, and an explanation of your
reasoning.
C) Now close the circuit, measure T2 and record it. Was it close to your prediction?
If not, what do you think was wrong with your reasoning?
D) Three Capacitors in Series: Now connect three capacitors in series to the light in
an open circuit. Charge this capacitor block (not the individual capacitors).
What is your prediction for T3, the time that the light will shine, once the
circuit is closed? Write down your prediction, and an explanation of your
reasoning.
E) Now close the circuit, measure T3 and record it. Was it close to your prediction?
If not, what do you think was wrong with your reasoning?
CAPACITORS in PARALLEL
A) Single Capacitor: You already know how long a single capacitor will make the
light shine. Again call this time T1.
B) Two Capacitors in Parallel: Now connect two capacitors in parallel to the light
in an open circuit. Charge this capacitor block. What is your prediction for
T2, the time that the light will shine, once the circuit is closed? Write down
your prediction, and an explanation of your reasoning.
C) Now close the circuit, measure T2 and record it. Was it close to your prediction?
If not, what do you think was wrong with your reasoning?
D) Three Capacitors in Parallel: Now connect three capacitors in parallel to the
light in an open circuit. Charge this capacitor block. What is your prediction
for T3, the time that the light will shine, once the circuit is closed? Write
down your prediction, and an explanation of your reasoning.
E) Now close the circuit, measure T3 and record it. Was it close to your prediction?
If not, what do you think was wrong with your reasoning?
CAPACITORS in SERIES REDUX
A) Two Capacitors in Series: Once again connect two capacitors in series to the
light in an open circuit. Charge this capacitor block. But this time, charge
each capacitor individually with the battery. What is your prediction for T 2,
the time that the light will shine, once the circuit is closed? Write down your
prediction, and an explanation of your reasoning.
B) Now close the circuit, measure T2 and record it. Was it close to your prediction?
If not, what do you think was wrong with your reasoning?
EXPLANATION of CAPACITORS
The items below contain concepts and equations related to how capacitors work.
Use them to give a comprehensive (thorough and coherent) account of what you
learned in the lab about series and parallel combinations of capacitors, explaining
their behavior in terms of your model of a capacitor.
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The definition of capacitance is C = Q/V, where C is the capacitance, Q is the
electric charge on the plates, and V is the voltage difference between the
plates.
By connecting the capacitor block to a battery, you generated a voltage
difference and charged the plates.
The light bulb shines because current, made of electric charges, flows through
it. The current is I = dQ/dt.
A standard formula for the capacitance of a parallel plate capacitor is C =0A/d,
where A is the area of the plates, d is the distance separating the plates, and
0 is a fundamental constant.