Investigation of factors affecting the capacitance of a parallel plate capacitor using electrometer

10.        Plot a graph of the charge stored (Q) in the parallel plate capacitor against the potential difference (V) across it.

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B.        Factors affecting the capacitance

12.        Repeat steps 6 to 8 with different numbers of spacers (n) between the metal plates. Tabulate the results. Find the reciprocal of the number of spacers (1/n).

13.        Repeat steps 6 to 8 with different overlapping areas (A) between the metal plates by changing the protruding length x. Tabulate the results.

14.        Replace the spacers with a polythene sheet between the parallel plates. Repeat steps 4 to 8 with the 30 V voltage output of the HT supply. Record the reading of the milliammeter.

                Charge (Q) = _____ 2.52_______× 10−8 C

Results and Discussion

1.        From the Q-V graph in step 10, find the slope of the graph. What does the slope represent?

The slope is found to be 0.035 × 10−8 F, and it represents the capacitance of the parallel

plate capacitor.

2.        From the results in steps 12 and 13, describe how the separation between the parallel plates and their overlapping area affect the capacitance of the parallel plate capacitor.

The capacitance of the capacitor is inversely proportional to the plate separation but is

directly proportional to the overlapping area between the plates.

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3.        From step 14, compare the charge stored in the capacitor separated by polythene with that in the capacitor separated by air of the same plate separation. Hence, find the relative permittivity of polythene.

Relative permittivity of polythene

4.        State the sources of error and suggest improvements for this experiment.

(a) Avoid touching the metal plates with an earth hand.

(b) Use screened cable to prevent charge leakage.

(c) The sensitivity of the electrometer may drift after using for a long time, so it is

necessary to work through the whole experiment quickly.

(d) Clean the metal plates with methylated spirit, in order to remove the stray resistance

due to grease and dirt on the plates.

(e) Stray capacitance between the metal plates and the bench may also give experimental

errors.