charging a capacitor at a constant rate(C08)

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School

True Light Girls' College

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Name

Tam Teresa (???)

Class (Class No.)

6S (22)

/10

Group No.

3

Date (dd-mm-yy)

20-05-2009

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AL Physics TAS - Experimental Worksheet

C08 - Charging a capacitor at a constant rate

Time Allowed

2 periods

Objective

To investigate how the charge on a capacitor is related to the potential difference applied across it by charging the capacitor at a constant rate.

Apparatus

* Capacitor (electrolytic type) 500µF

* Microammeter 100µA

* Potentiometer 100k?

* Clip component holder

* Stop-watch

* CRO (Cathode Ray Oscillator)

* Connecting leads

Theory

From definition, the capacitance C of a capacitor is found from

C=Q/V

where Q is the charge stored on the capacitor and V is the potential difference across it.

==> Q = CV

==> = C

If a capacitor is charged up at a constant rate, i.e. = I , where I is a constant , then is also constant.

Hence the potential difference across the capacitor increases linearly with time.

Procedure

. Connect up the circuit as shown in Fig. (a). Set the CRO to d.c. and the sensitivity to 1V/cm.

2. Set the time base to any high value so that a steady horizontal trace is displayed. Shift the trace to the bottom of the screen.

3. Short out the capacitor by connecting a lead across it and adjust the 100k? potentiometer for a suitable current, 80µA.
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4. Remove the shorting lead and the capacitor will charge up. Note what happens to the microammeter reading and the CRO trace.

5. Repeat the procedure but this time start the stop-watch and continuously adjust the potentiometer to keep the current, 70µA, constant as the capacitor charges up.

6. Measure the times for the CRO trace to move up by 1cm, 2 cm, 3 cm, 4 cm, 5cm and 6cm.These are the times for the pd across the capacitor to reach 1V, 2V, 3V, 4V, 5V and 6V.

7. Tabulate the results.

Results

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