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Objective To investigate the relationship between the charge on a capacitor and the p.d. across the capacitor by charging at a constant rate.

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880810.doc        p./


Ng Wah Catholic Secondary school




Tang Pak Lap        ( 19  )


AL Physics


7th May 2007



C8  Charging a capacitor at a constant rate (TAS)


To investigate the relationship between the charge on a capacitor and the p.d.

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CROConnecting leads


  1. Connect the following circuit. Set the CRO to d.c. and the sensitivity to 1 Vcm
  2. Set the time base of the CRO to a high sweep rate 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 connecting wire across it (XY). Adjust the 100 kΩ potentiometer to a suitable value for a steady current to flow (e.g. 80μA).
  4. Remove the shorting wire and the capacitor will charge up.
  5. Repeat the
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Sources of errors

  1. Error in keeping the current constant
  2. Internal resistance of the cell
  3. Non-zero value of the variable resistance when it is "minimum"


The microammeter reading is being decreased and the CRO trace is being increased when the shorting lead removed. It can be deduce that as the capacitor discharges, the p.d. across it decreases.

A-Level Practical Physics (Third Edition)© Oxford University Press 2005

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