Electricity and Magnetism
DEM09.1 - Storage of Charge (Capacitor)
- To illustrate the storage of charge or energy by a capacitor.
Comment: The use of a capacitor discharge to pulse a light has a familiar application
in flash photography.
- a metered, 0- 20 V or greater DC power supply
- DEM 9.1 discharge apparatus
- two banana-banana leads, length at least 0.5 m
- spare #40 (6.3 V) light bulbs (as used in lab EM10)
Wire the circuit taking care to observe the correct polarity between the power supply and the capacitor board. Turn on the power supply, and set the supply to about 20 V.
- A blocking diode has been included in the charging circuit to protect
the electrolytic capacitor from damage should the demonstrator fail
to observe the correct +/- polarity when connecting the power supply.
Reverse polarity will result in zero charging current to the capacitor.
The diode also allows an AC supply to be used in place of the specified
DC supply should one need/wish to do so.
- The capacitor supplied has a maximum voltage rating of 30 V. DO NOT EXCEED THIS VOLTAGE.
Press the pushbutton switch labelled "charge" to store energy
on the capacitor. An optional step at this point is to disconnect the
power supply to convince the student that the power supply is no longer
delivering energy to the circuit.
Press the pushbutton switch labelled
"discharge." A bright flash of light is produced by the light
bulb. The flash is bright because the circuit delivers a peak voltage
of 20 V to the nominally 6.3 V bulb. Since the pulse of current is of
short duration the filament does not burn out. Of course, the filament
may "crisp" the first time you try the demo, hence a spare
bulb is supposed to be supplied.