Memorial University of Newfoundland
Department of Physics and Physical Oceanography
Physics 2055 Laboratory
Charge and Discharge of a Capacitor
Introduction
In a series circuit consisting of a resistor, a capacitor and a battery, the rate at which the capacitor
charges depends on its capacitance, C and the value of R. [See Serway/Jewett, Chapter 28 for
further details]. For the circuit shown on page 789 (7th Edn.) the voltage across the capacitor
plates as a function of time is given by
−t/RC
v(t) = V◦ 1 − e
where V◦ is the voltage across the battery and the parameter RC is called the time constant, sometimes denoted by the Greek letter τ.
Procedure
• Large capacitances (>∼ 1µF) are generally electrolytic, and are constructed in such a
way which allows for a relatively high capacitance in a small volume. Most electrolytic
capacitors are polarized and require one of the electrodes to be positive relative to the
other. Polarity is indicated with the leads labeled + or −; the negative terminal lead may
be shorter than the positive lead. They must be connected correctly in a circuit.
• Safety note:- It is good practice to make sure that the capacitors are fully discharged
before using to avoid the possibility of electric shock. Short circuit the capacitor by
touching the terminals with a piece of insulated wire.
1. Construct a series circuit using a 5 volt d.c. power supply, R ∼ 100 kΩ and C ∼ 1 mF. Sketch
the circuit in your lab notebook.
2. Turn on the power supply and record the voltage across C as a function of time until it is fully
charged. Plot your data and use a suitable curve fitting routine to determine the best value of
V◦ and the time constant τ. What is the physical significance of each of these parameters?
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3. Starting with the capacitor fully charged, discharge it by replacing the power supply with a
short circuit. Similarly record the voltage across the capacitor as a function of time and plot
your data. In this case the voltage is described by the equation
v(t) = V◦ e−t/RC .
Determine the time constant and try to account for any difference in its value during charge
and discharge.
4. Replace the 100 kΩ resistor with ∼ 50 kΩ and repeat the charge and discharge experiments,
determining the time constant in each case. Plot these curves on the same axes as above so
that you can compare them easily.
5. Explain how changing the resistance (and hence τ) affects the charge and discharge characteristics of the capacitor.
6. The behaviour of the current during capacitor charge and discharge may be illustrated using
a light bulb in place of the load resistor. Use a 1 F capacitor to describe and explain what
happens to the light as the capacitor is charged and discharged. Explain also what happens
to the current in the circuit during charge and discharge.
7. It has been suggested that the capacitor acts as a gate or a filter in the circuit. From your
observations, which do you think is the more appropriate description?
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