Charging and discharging of a capacitor

 

 

 

A comprehensive curriculum

  • electricity

  • electrical circuits

  • electrical components

  • capacitance

Purpose of the experiment

Determine the capacitance of the capacitor, if the voltage across the plates of the capacitor is monitored while it charges or discharges and a curve is fitted to the resulting U vs. t plot.

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Background notes

It is better to select a capacitor which capacitance is bigger then 1 mF. A capacitor like this charges and discharges within a few seconds with the chosen 1 kW resistor.

 

Experimental data

Example e-ProLab file of the capacitor charging

Example e-ProLab file of the capacitor discharging

Example curve of the capacitor charging

Example curve of the capacitor discharging

The data table of charging the capacitor

The data table of discharging the capacitor

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Instructions for data analysis

Students can determine the capacitance of the capacitor C from the curve of discharging a capacitor e.g.

As we can see from the chart of voltage vs. time, the fitted exponential function is

 

From theory

where R = 2200 W.

The capacitance is approximately

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Conclusions

When the capacitor is discharged through a resistor, the voltage across the capacitor and the charge on the capacitor decreases exponentially. When the capacitor is charged through a resistor, the voltage exponentially rises.

 

Student activities

Investigate to see how the U vs. t plot changes when the values for R and C are changed. Explain the results obtained.

 

Additional activities

To express the units of resistance, ohms, and the units of capacitance, farads, in fundamental units.

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