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 eProLab file of the capacitor charging
Example eProLab 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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