Investigating the Capacitance of a Parallel-Plate Capacitor Using a Reed Switch

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Date: 25th March, 2010
Title: Investigating the capacitance of a parallel-plate capacitor using a reed switch Objective: To investigate the factors which affect the capacitance of a parallel-plate capacitor using a reed switch. Apparatus:

- reed switch
- signal generator
- capacitor plates of area about 0.24m X 0.24 m 1 pair - polythene spacers ( 10 X 10 X 1 mm )
- polythene sheet, same area as capacitor plate 1 mm thick
- battery box with 4 cells 2 - voltmeter
- light-beam galvanometer
- standard mass, e.g. 100g
- resistance substation box
- Connecting leads
The reed switch
1) The reed switch was examined. The plastic box was opened up and looked inside. 2) The yellow terminals of the reed switch module were connected to low impedance input of a signal generator. The frequency was gradually increased. If necessary, the voltage output was increased. 3) Note that a diode was connected in series to the coil inside the module. Setting up the apparatus

4) The apparatus was set up as above. The capacitor plates were separated with 4 polythene spacers placed at the corners. The frequency and voltage of the signal generator were adjusted such that a sound was heard and the spot in the light-beam galvanometer was deflected. 5) When a signal of frequency f was applied, X vibrated between Y and Z at the same frequency. When X made contact with Y, the capacitor was charged by the d.c. supply: when X made contact with Z, it discharged through the light-beam galvanometer. 6) An essential condition for which I=Qf was stated.

7) A variable resistor was connected in series with the galvanometer to protect it from damage. Damage may result from an accidental connection of the d.c. supply to the galvanometer. 8) If the resistance was too low, it had little effect. If it was too high, the discharge of the capacitor may be incomplete. A CRO was connected across the variable resistance to monitor the discharging current pulse. The viable resistor was adjusted to a maximum value such that each pulse fell to zero before the next one. Charge and applied p.d.

9) The signal generator was set to the maximum allowed frequency marked on module. 1 cell in the battery was connected up. The readings on the galvanometer were taken. 10) The voltage was increased in steps by connecting up more cells in the battery box. In each case, the readings on the galvanometer were taken. 11) The sensitivity of the galvanometer was note and it was used to convert the deflection to current. The results were tabulated. Effect of plate separation

12) Without changing the voltage, the number of spacers n was increased in steps and the reading [pic] of the galvanometer were taken. The results were tabulated. Effect of area of overlap

13) Without changing the charging voltage and the number of spacers used, the area of overlap of the metal plate was charged as shown above. Relative permittivity
14) With a one-spacer separation between the plates, the reading [pic]of the galvanometer deflection was taken. 15) The spacers were replaced with a sheet of polythene of the same thickness as a dielectric. The galvanometer reading[pic]0was taken. The ratio of[pic]was taken. Precautions:

• The resistance of the variable resistor should neither be very high and very low. If the resistance is too high, the charges stored in the parallel-plate capacitor may not be fully discharged, so that the measured current is not accurate, which is definitely smaller. If the resistance is too low, it may have a chance to damage the light-beam galvanometer because of large current. • We should not assume the electromotive force provided by one battery is exactly the value which is claimed to be. As there is current flow, the voltmeter, variable resistor, light-beam galvanometer and the reed switch may draw a little current, which results in lower potential difference. Moreover,...
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