Physics Dept. Demonstration Manual

DEM 09.1 - Storage of Charge (Capacitor)

Objective:

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.

 
Apparatus:  
  • 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 EM 10)



Method:

See figure 1 for the setup.

Figure 1: Circuit diagram. Note that the DEM 9.1 board supplied has been designed so that as viewed by the students, the L/R order of components is power supply/charging circuit/discharge circuit.

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.

Notes:

  1. 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.

  2. 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 by the demonstration technologist.

 

Additional Work: Energy Analysis

One may wish to show that the energy stored by the capacitor is a function of the charging voltage. The brightness of the light flash can be used as an energy indicator. Please see the following table for guidance.

Charging Voltage E = 1/2 C V2 Observation (volts) (joules)

8.0 0.13 dull
12 0.29 easily seen flash
16 0.51 quite bright
20 0.80 got your shades on buddy?

The above energy calculations are based on C = 4000 microfarads.