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

Title:   Investigating The Capacitance of a Parallel-plate Capacitor Using a Reed Switch

Objective : To investigate the factors affecting the capacitance of a parallel-plate capacitor
using a reed switch.
Apparatus : Reed switch 1
Signal generator 1
Capacitor plate 2
Polythene spacers 16
Polythene sheet 1
Bateery box with 6V 1
Voltmeter 0 – 5 V 1
Resistance substitution box 1
Light-beam galvanometer 1
Standard mass 0.1 kg 1
Plastic ruler 1
CRO and connecting lead 1
Adapt 1
Connecting wires 10

Theory : The reed switch used in this experiment is a two-way switch controlled by an a.c.
source. It consists of a magnetic reed X and two contacts Y and Z as shown in
Figure 1.

Figure 1
The magnetic field is created by a current that flows in the coil. The diode is used to
allow the alternating current to flow only in alternate half cycle. As a result X will
be in contact with Y and Z alternately with a frequency equal to that of the a.c.
source.
If a capacitor is connected with the reed switch as shown in Figure 2, it will be
charged by the battery and discharge through the resistor R alternately. Assume that
the capacitor is fully charged up and discharged every time, the total quantity of
charge passes through the resistor in one second is
Q/t = CVf     where C is the capacitance of the capacitor,
V is the voltage across the capacitor and
f is the frequency of the a.c. source.
This gives the current [pic] passing through the resistor.
Therefore the capacitance of the capacitor can be estimated by[pic]

Procedure :

(A)Setting up the necessary conditions:

1. Construct a parallel-plate capacitor with two metal plates separated by four
polythene spacers placed at the corners.
2. Connect the circuit as shown below. Set the variable resistor to high
resistance before switching on the signal generator.

3. Connect the coil of the reed switch to the low impedance (4 Ω) input of the...