5. Repeat steps 3 and 4 with the other frequencies of the signal generator from 400 Hz to 10 Hz. Tabulate the results.
Note that low switching frequency is advised since for high one, the reed switch may not be sensitive enough to respond, and the time may be too short for complete discharge.
6. Plot a graph of frequency (f) against current (I).
Results and Discussion
1. Why the charging and discharging of the capacitor must be complete?
If the charging of the capacitor is not complete, the voltage of the capacitor will not reach
that measured by the voltmeter, and the formula I = fVC will be invalid. The same
argument can be applied for discharging process.
2. Why we need to set the resistance of the variable resistor at its maximum value at first?
This avoids excess current and protects the meter from damage when the reed switch
contacts close.
3. From the f-I graph in step 6, what is the relationship between the frequency of the reed switch and the current flowing in the capacitor?
The frequency of the reed switch is directly proportional to the current flowing in the
capacitor.
4. Determine the capacitance of the capacitor by using Equation (1).
The slope is found to be 2.08 μF.
5. Compare the measured capacitance of the capacitor with the value marked on it. Calculate the percentage error.
Percentage error =
6. State the sources of error and suggest improvements for this experiment.
(a) Estimate the order of magnitude of the reed switch first so the appropriate ammeter is
used.
(b) From I = fCV, since the capacitance of the capacitor is so small, high switching
frequency and high voltage have to be provided to give measurable current. However, the
switching frequency of the reed switch is not too high, since for high one, the reed switch
may not be sensitive enough to respond, and the time may be too short for complete
discharge. Also the voltage is not too high, otherwise, the heating effect at the contact
points of the reed switch may be too large.
New Way Physics for Advanced Level
