Optional Practical: Charging and discharging a capacitor

Introduction

In this practical you are going to use the PhET AC circuit construction kit to experiment with the time taken to charge and discharge a capacitor through a resistance. First set up the following circuit.

To change the value of a component right click it.

When complete, try charging the capacitor by closing switch A. You can see how the PD across the capacitor plates changes by connecting the “Voltage chart” across it.

Now discharge by closing switch B. Oops, I forgot to mention opening switch A again, What happens if you don’t do this (try it)? Good job this is a simulation :-)

Measuring the time constant

  • Stop the animation by clicking the pause button at the bottom of the screen
  • Connect a voltmeter across the capacitor plates of the capacitor
  • Add a stop clock and click the start button. It won’t start since the animation is paused but will start when you press play.
  • Discharge the capacitor by right clicking it and choosing “discharge”.
  • Close switch A (make sure B is open)
  • Click play and stop the animation when the charge reaches 2/3 of the final value (20 V).
  • Record the time and compare it with the value of RC

Combinations of capacitor

  • Using the same method as before measure the time period for parallel and series combinations of 0.1 F capacitor. What can you deduce about the affect of adding capacitors in series and in parallel?
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