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Measurement of capacitance by reed switch

Extracts from this document...

Introduction

881798.doc        p./

School

Class (No.)

                        (      )

Name

Subject

AL Practical Physics

Date

Mark

C10   Measurement of capacitance by reed switch

Procedure

1.        Connect the circuit. Set the resistance of the variable resistor at its maximum value. Record the reading of the voltmeter.

                Voltage = ______ 5.9 __________V

4.        Decrease the resistance of the variable resistor slowly until the current shown on the microammeter does not increase any more. Record the reading of the microammeter.

5.

...read more.

Middle

200

250

300

350

Current (I) / μA

1200

2000

2500

3200

3600

4300

        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).

image00.png

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

...read more.

Conclusion

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                © Manhattan Press 2005

...read more.

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