# Capacitors

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Capacitors
A capacitor is a device that can store charge and release all of it very quickly into a circuit. It
is used in devices like a flash in a camera to release a large charge for a bright flash when only
needed for a short time, a battery simply couldn't deliver this type of charge in one go. A
capacitor is made of two conducting plates with an insulator sandwiched between them. The
charge can be stored on the conductors. The symbol for a conductor is shown below;
It looks very similar to a battery except for the fact that each
line is the same in length.
Capacitors are defined on their `capacitance' which is defined as the charge stored per unit
potential difference. This is shown in the equation below;
Q
C= V
Where;
`C' = Capacitance
`Q' = Charge
`V' = Potential Difference
The unit for capacitance is the Farad (F). However this unit is so large that most capacitors
are measured in micro-farads (F)
Charging and Discharging a Capacitor Experiment
This circuit show the general setup for charging
and discharging a capacitor. When connected to
terminal 1, a current flows through the circuit
from the battery and charges the capacitor. The
resistor is there to slow the flow of current such
that a charging graph can be obtained. Without a
resistor the capacitor would charge almost
instantly. After the readings indicate the
capacitor is fully charged, the switch is then
flipped to terminal 2 where the discharging
begins. Here the resistor plays a similar role as before, lengthening the time in which the
capacitor discharges. It is also helpful however as discharging a capacitor without one by
touching both wires can produce a large snap sound and ruin the wires. After both of these
experiments are complete and values for the voltage and current have been recorded with
time, graphs can be drawn to show the process.

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### Page 2

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When a capacitor is charging;
Charge flows from the battery to the capacitor
o Voltage Increases
o There is a current
Until the capacitor voltage is equal to the battery voltage
As more charge is stored it becomes harder to push more charge on so the current
decreases
o Current falls
o Rate of Voltage decreases
Because of these each graph follows an exponential growth or decay curve.…read more

### Page 3

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When a capacitor discharges the voltage, current and stored charge all fall; following an
exponential decay. The time constant () is the time taken for the charge, current or voltage
to fall to 0.368 of its original value. Why this number? It's e-1 .…read more