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

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Introduction

881799.doc        p./

 School Class (No.) (      ) Name Subject AL Practical Physics Date Mark

C9   Investigation of factors affecting the capacitance of a parallel plate capacitor using electrometer

### Procedure

A.        Relationship between charge and p.d. in capacitor

7.        Touch the upper plate with the screened wire of the electrometer to transfer the charge from the parallel plate capacitor to the electrometer. Record the reading of the milliammeter , which represents the charge stored (Q) in the internal capacitor.

9.        Repeat steps 6 to 8 with the other values of voltage outputs (V) of the HT supply in steps of 5 V. Tabulate the results.

 Voltage (V) / V 5 10 15 20 25 30 35 Charge (Q) / × 10−8 C 0.19 0.36 0.5 0.7 0.93 1.05 1.24

Middle

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).

 No. of spacers (n) 1 2 3 4 5 6 7 1/n 1 0.5 0.33 0.25 0.2 0.17 0.14 Charge (Q) / × 10−8 C 1 0.5 0.29 0.25 0.12 0.06 0.04

13.        Repeat steps 6 to 8 with different overlapping areas (A) between the

Conclusion

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.

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