Capacitor Demos

Equipment:

2 90 microfarad electrolytic capacitors
PASCO 36 Direct Current supply
red, black, green and yellow cables
grounding ring with insulated handles

The positive pole of the capacitors is the center three leads (all soldered together). The negative pole is the outer can, which also has four posts protruding on the top.

Safety precautions: These capacitors deliver shocks that can be much worse than the Van da Graff. You should really avoid getting shocked:

always ground the capacitors with the grounding ring before handling (they might still have a charge from the previous demo)
always handle the cables by the insulated part
when possible, work with one hand in your pocket, to avoid a discharge though your chest

1. Energy stored in a capacitor:

Connect the red cable from the supply + (red) to the center post of one capacitor. Connect the black cable from the - (black) to one of the posts on the can. Turn on the supply and let it charge up to 200 V. The supply delivers very little current, so this might take a few minutes. Be prepared to talk about something in the mean time. When it is fully charged:

disconnect the red cable from the supply. Be careful to not let it touch anything grounded.
turn off the supply
disconnect the black cable fro the supply and let it rest on the table.
point out to the student that the supply is completely disconnected and turned off.
touch the red banana plug to the black banana plug: Pow.

Concepts: The spark came from the capacitor, and not the supply. The capacitor is a passive element, but it can store charge, voltage, energy. The energy stored is (0.5)(90e-6F)(200V)^2=1.8 J. Also, a capacitor is capable of delivering large currents. You can show the difference by connecting the leads to the supply (and not the capacitor), setting it to 200 V, and touching the leads together (with the power on). There will not be any noticeable spark, because the supply cannot provide enough current to spark. Related applications: flash bulb circuits, auto ignitions, PC power backups.

2. Series and parallel capacitance.

Repeat demo 1 with two capacitors connected in series.

red: supply + to cap 1 +
yellow: cap 1 - to cap 2 +
black: cap 2 - to supply -

Charge, disconnect, and discharges as in demo 1 (yellow cable remains in place). pow. Only 0.9 J.

Repeat with two capacitors in parallel:

red: supply + to cap 1 +
yellow: cap 1 + to cap 2 +
green: cap 2 - to cap 2 -
black: cap 1 - to supply -

Charge, disconnect, and discharges as in demo 1 (yellow cable remains in place). POW. 3.6 J. (Note: this might weld the plugs together. A little force will get them apart again.)

Concepts: Capacitances are larger when hooked in parallel, smaller when hooked in series. For a fixed voltage, more energy is stored in a larger capacitor. Note also that the two capacitors in series charge much more quickly than the single capacitors, and the parallel ones charge much more slowly. You can talk about RC circuits, or you can just point out that the supply can only deliver a limited amount of charge per unit time, and a larger capacitance needs a larger charge to get to the same voltage.