Capacitors - A quick breakdown

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

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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:

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Since electrons are forced off one plate onto another, energy is stored by the capacitor in the form of
electric potential.
On a graph V on Q, the energy = the area underneath the graph, as E=0.5QV.
The theoretical value of energy often differs to the real value, as some energy is lost when current flows
through the resistor to the surroundings.
A thundercloud acts in a similar way to a capacitor (the cloud and the earth are effectively two parallel
plates).…read more

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