# Capacitors Flashcards

Define capacitance.

The amount of charge stored per volt.

Define the farad.

1 farad is the capacitance when 1C is stored over a potential difference of 1V.

State the equation for the total capacitance of two or more capacitors in series.

total capacitance = (1/C1 + 1/C2…)^(-1)

State the equation for the total capacitance of two or more capacitors in parallel.

total capacitance = C1 + C2…

What does the area under a potential difference against charge graph represent?

The energy stored by a capacitor.

Describe the graph that occurs for potential difference/charge/current against time for a discharging capacitor.

These graphs are all the same, and show exponential decay.

Define time constant.

The time constant is the time taken for current, charged stored, or potential difference across to fall to about 1/e (37%) of it’s initial value.

Is we measure an amount that shows exponential decay at regular intervals, how can we show that this is exponential decay?

The measurements next to each other will show a constant ratio. eg. A2/A1 = A1/A0

Describe how a capacitor is used in flash photography.

Charge from a capacitor flows through xenon gas, emitting a bright light. The capacitor discharges really quickly to give a short pulse of high current that produces a brief flash of light.

Give an example of a use of a capacitor.

In flash photography, nuclear fusion, and as a back-up power supply.

How are capacitors used in nuclear fusion?

The capacitors control the lasers that give the reaction its energy. They release a huge charge in a fraction of a second to maximise the amount of energy in the laser-light.

How are capacitors used in back-up power supplies for computers?

A large capacitor will release charge slowly if the power supply fails. This gives you time to save a retrieve data if needed.