# Capacitors - A quick breakdown

Covers the basics of capacitors (what they are, how they work, time constant, energy stored and all that jazz)

- Created by: AnnaSemple
- Created on: 10-03-15 21:33

First 327 words of the document:

Capacitors

Two parallel plates which store charge.

When plugged into a circuit with a battery, electrons leave the negative terminal of the battery and flow

to one of the plates. Electrons flow from the other plate to the positive terminal of the battery.

Each plate has an equal and opposite charge.

When plugged into a circuit with a constant source of pd:

The capacitor charges as above.

The pd steadily increases, the current decreases (because the charge of the capacitor increases).

The pd tends to the pd of the source (battery) and the current tends to 0. At this point the capacitor

stops charging.

A graph of charge against time would give an inverse, decreasing exponential curve, levelling out when

Q0=CV0

The time constant:

RC the time taken for the capacitor to charge to 63% or discharge to 37% of Q0 (1/e)

Capacitance:

The charged stored per unit p.d.

If a graph is plotted with I on V, a straight line is obtained passing through the origin. The charge stored

is therefore proportional to the pd (the charge per unit for a certain capacitor is always constant).

The Farad (unit of capacitance) is essentially coulombs per volt.

When discharging through a circuit:

The discharge current steadily decreases to zero, as does the voltage (as the capacitor loses charge). The

resistor is connected directly to the capacitor and thus (since I=R/V) the current decreases.

Current, pd and charge all decrease exponentially

Measuring capacitor discharge:

Use a voltmeter or oscilloscope with an infinitely high resistance (so that current only passes through the

fixed resistor).

Time Delay circuits:

Change the resistance to alter the time delay (increasing resistance/capacitance increases the delay).

When the input pd drops below a certain value, the alarm/device goes off.

Energy stored:

## Comments

No comments have yet been made