Laboratory #4
Phys 2426
Dr. Joe Pizzo
The capacitance of a device is defined to be the ratio of the charge, Q, stored on the
device to the potential difference, V, across the device.
C = Q/V
(1).
Therefore, a simple method to measure the capacitance of a device is suggested. Apply a
known potential difference V to charge the capacitor, then discharge the capacitor
through a meter that has been calibrated to measure charge, Q, (a ballistic galvanometerG in the diagram below). The capacitance will be the ratio of the two measurements as
defined by equation (1).
The ballistic galvanometer on your table has already been calibrated. The constant, k, on
the label indicates the number of microCoulombs needed to deflect the meter one
division. Therefore, if the meter is deflected d divisions by a discharging capacitor, the
charge coming off the capacitor is
Q=kd
(2).
Record the value of k for your meter here and use it throughout the experiment.
k = _______________________µC/division.
The circuit used in this experiment is shown in the schematic below. When the switch, S,
is thrown toward the output, a potential difference, V, is impressed across the capacitor,
C. When the switch is thrown all the way to the right, the capacitor discharges through
the charge meter (ballistic galvanometer, G).
Output V
S
G
1
C
Part I: Individual Capacitors
The two individual capacitors to be used in this part of the experiment are on the table.
(The large capacitor is C1, and the small capacitor is C2.)
•
Check if the ballistic galvanometer is set to zero on the left side of the scale.
•
Refer to the circuit diagram. Set the switch, S, in the neutral (open) position.
•
Connect capacitor C1 at the location marked C in the circuit diagram.
•
Set and record the potential difference, V, which will be applied to the capacitor.
•
Throw the switch to the contacts with the output (the charging position).
•
(The capacitor will be fully charged to potential V in less than one second.)
•
Discharge the capacitor by flipping the switch all the way to the other side.
(Do not stop or hesitate in the open position.)
•
The charge that was on the capacitor will flow through the ballistic galvanometer.
•
Note and record the maximum deflection (in divisions, d) of the galvanometer.
•
Repeat this process three times, using different values of V.
(Try potential differences that will give a large range of deflections for the four
trials: for example, somewhere between 15 and 25 divisions; between 30 and 40
divisions; between 45 and 65 divisions; and between 70 and 80 divisions.)
•
For each trial, calculate and record the charge stored on the capacitor, Q = kd,
and the capacitance, C = Q/V.
•
Use Excel to calculate the average value of capacitance, @average, and the
uncertainty represented by @stdev.
•
Record the results in the standard format in the space provided on you lab
worksheet.
Repeat this procedure for C2.
2
DATA FOR PART I
Capacitor #1
V
(Volts)
d
Q = kd
(deflections) (microCoulombs)
C1 = Q/V
microFarads
C1 = _________________±_________________ F
Relative uncertainty ( C1/C1) = ______________
Capacitor #2
V
(Volts)
d
Q = kd
(deflections) (microCoulombs)
C2 = _________________±_________________ F
Relative uncertainty ( C2/C2) = ______________
3
C2 = Q/V
microFarads
Part II: Parallel Combination of C1 and C2
A. Theoretical Model:
Substitute the values found in part I for C1, δC1, C2 & δC2 to obtain Cp and δCp.
Cp = ____________±______________ F (Theoretical Result)
Enter this result on the number line at the bottom of the page.
Connect C1 and C2 in parallel and place the
B. Experimental Measurement:
combination at location C in the circuit diagram. Use the same procedure that you used to
find the capacitance of C1 and C2 in part I.
V
(Volts)
d
Q = kd
(deflections) (microCoulombs)
Cp = Q/V
microFarads
Cp = ____________±______________ F (Experimental Result)
Enter this result on the appropriate number line below.
Theoretical
Experimental
4
Part III: Series Combination of C1 and C2
A. Theoretical Model:
Substitute the values found in part I for C1, δC1, C2 & δC2 to obtain Cs and δCs.
Cs = ____________±______________ F (Theoretical Result)
Enter this result on the number line at the bottom of the page.
B. Experimental Measurement: Connect C1 and C2 in series and place the combination
at location C in the circuit diagram. Use the same procedure that you used to find the
capacitance of C1 and C2 in part I.
V
(Volts)
d
Q = kd
(deflections) (microCoulombs)
Cs = Q/V
microFarads
Cs = ____________±______________ F (Experimental Result)
Enter this result on the appropriate number line below
Theoretical
Experimental
5