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Charging a capacitor at a constant rate

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Lam Cheuk Ying Regina

   F.6C (12)

Date: 25th March, 2010

Title: Charging a capacitor at a constant rate

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


  • capacitor (electrolytic type) 500 µF
  • microammeter 100kΩ
  • potentiometer 100kΩ
  • clip component holder
  • stop-watch
  • CRO
  • Connecting leads


  1. The circuit was connected up as shown above. The CRO was set to d.c. and the sensitivity was set to 1 V/cm.
  2. The time base was set to any high value so that a steady horizontal trace is displaced. The trace was set to the bottom of the screen.
  3. The capacitor was shorted out by connecting a lead across it and the 100kΩ
...read more.




If a capacitor is charged up at a constant rate, i.e.image02.png, where I is a constant,

Then image03.png is also constant.

Hence the potential difference across the capacitor increases linearly with time. image04.png


...read more.


image06.pngV.From the graph, the slope of the straight line = (5-1)V/(40-8)s= 0.125Vs-1

 since image03.png

    0.125 = 80µA/C

            C=6.4 X 10-4CV-1

  • Sources of errors:

- There is leakage of charge from the capacitor.

- Error in keeping the constant current.

- There is internal resistance of the cell.

- Non-zero value of variable resistance when it is set "minimum".


By charging the capacitor at a constant rate, the charge on a capacitor is directly proportional to p.d. applied across it

...read more.

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