dc
the other at
dc),
reed switch box, parallel plate aluminium capacitor with insulating
sheet, 0.01µF capacitor, small plastic squares (in a box usually),
micrometer, frequency meter set on 2kHz, signal generator set
initially at 50Hz sinusoidal.
A Level Physics Notes: Experimental Physics – Using a Reed Switch to Measure Capacitance and Permittivity
Apparatus:
Lab PSU set at 8V, two digital multimeters (one set atdc
the other atdc),
reed switch box, parallel plate aluminium capacitor with insulating
sheet, 0.01µF capacitor, small plastic squares (in a box usually),
micrometer, frequency meter set on 2kHz, signal generator set
initially at 50Hz sinusoidal.
Diagram:
Note: In addition the frequency meter should be connected to the output of the frequency generator.
Theory:
The signal generator controls a reed switch. This is a switch that
at first connects the supply voltage (approx. 8V) to the capacitor.
The capacitor charges and stores a charge
given
by the equation:
A few moments later the switch then moves in such a way as to
connect the capacitor to the ammeter. The capacitor discharges over a
time
the
current
flowing
being given by:
The above cycle repeats itself at a frequency equal to twice that
of the signal generator’s frequency ,
Therefore
in a time
seconds
a chargeis
transferred via the capacitor from the power supply to the ammeter.
and so:
Procedure:
(a) Measurement of capacitance
1. Set up the apparatus as shown in the above diagram. Set the
power supply to
dc
and the signal generator to 50Hz sinusoidal. Turn up the output of
the signal generator until you hear the reed switch buzzing steadily.
At this point the voltmeter should be reading about
and
the ammeter about
2. Obtain current readings for a range of frequencies between 50Hz and 500Hz. The current should rise over this intervals and you may need to increase the output from the signal generator in order to keep the reed switch operating.
3. Draw a graph of current (I/A) against frequency (f/Hz).
4. Use the gradient of this graph to calculate the capacitance of the capacitor.
(b) Measurement of permittivity
5. Set the signal generator at 200Hz and replace the 0.01µF capacitor with the aluminium
parallel-plate capacitor as shown in the diagram below:
6. Record the current & voltage for this case and also measure
the area
of
overlap of the plates and the thickness d, of the insulating sheet.
7. Repeat stages 4 & 5 but this time with an air gap between the plates. Use the small plastic squares to separate the plates.
8. Use the current, voltage and frequency values to calculate the
capacitances in the above two cases using
9. The capacitance of a parallel plate capacitor is also given
by
where
is
the permittivity (in
)
of the insulation. Calculate the permittivity in both of the above
cases.
10. The value with the air gap should be approximately the permittivity of free space o.
Use this value to calculate the
relative permittivity
of
the plastic insulation given that
whereis
the permittivity of the insulating sheet.