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Charging a capacitor at a constant rate

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Introduction

Experiment P: Charging a capacitor at a constant rate

Date:

1/11/07

Marks : ______________

Aim: To investigate how the charge on a capacitor is related to the p.d. applied across it by charging the capacitor at a constant rate.

Apparatus:

  • Capacitor (electrolytic type) 500 μF
  • Microammeter 100 μA
  • Clip component holder
  • Stop-watch
  • CRO
  • Connecting leads

Theory:

From definition, the capacitor 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

image00.png

If a capacitor is charged up at a constant rate, i.e. image01.png

...read more.

Middle

image03.png

  1. The time base was set to any high value so that a steady horizontal trace is displayed. The trace was shifted to the bottom of the screen.
  2. The capacitor was shorted out by connecting a lead across it and adjust the 100 k Ω potentiometer for a suitable current, say 80 μA.
  3. Shorting lead was removed and the capacitor will charge up. Note what happens to the microammeter reading and the CRO trace.
  4. The procedure was repeated but this time start the stop-watch and continuously adjust the potentiometer to keep the current constant as the capacitor charges up.
  5. The times was measured for the CRO trace to move up by 1 cm, 2 cm, 3 cm, etc. These are the times for the p.d.
...read more.

Conclusion

Therefore if V across the capacitor is directly proportional to t, Q is directly proportional to time as current was constant.

image04.png

Conclusion

We can find out that the p.d. across the capacitor is directly proportional to the time needed. Given that the charging current is constant.

Sharing

The experiment is much easier than the last one , but we encountered some obstacles in connecting wires , as usual , we messed up positive and negative terminals and couldn’t conduct it smoothly. At last, we had to call for help.

Suggestion

and there may be some personal error , for example counting the time taken for the capacitor be charged to extent value was rather inaccurate. Perhaps, we could conduct the experiment several times and compute out the average value.

Reference list

http://en.wikipedia.org/wiki/Capacitor

http://www.elecsound.cn/Ceramic-Capacitor.htm

...read more.

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