# Circuit Theory/Capacitors

**Capacitors** are passive circuit elements
that can be used to store energy in the form of an electric field. In
the simplest case, a capacitor is a set of parallel metal plates
separated by a *dielectric* substance. Dielectric used can be air or any
other substance.

![](Capacitor.svg "Capacitor.svg"){width="200"}

Electric charges build up on the opposite plates as a voltage is applied
to the capacitor.

With a constant voltage across the capacitor there will be no change in
electric charge and thus the steady-state current becomes zero. Stored
energy can be discharged from a capacitor by connecting its terminals
with a wire. Big capacitors can create lightening bolts, so tape the
wire to the end of a long wooden stick.

### Capacitance

**Capacitance** is defined as the
capability of a capacitor to store the charge of a voltage. Capacitance
is measured in units called \"Farads,\" abbreviated by an \"F\".

The ratio of charge over voltage gives a value of capacitance

$$C = \frac{q}{v}$$

### Capacitor Terminal Relation

The relationship between the current and the voltage of a capacitor is:

$$i = C\frac{dv}{dt}$$

The real world disturbs a capacitor by changing the voltage across it
rapidly. This instantly turns the capacitor into a short. The capacitor
eventually catches up to the changed condition, the voltage stops
changing, the current drops to zero and the capacitor acts like an open.

### Capacitor Safety

The bigger a capacitor is, the more dangerous it is.

A capacitor can continue storing its energy after the circuit is turned
off. A capacitor the size of a soda bottle can throw a person across the
room and/or damage nerves. Touching a capacitor the size of a kitchen
trash destroyed a persons arms to the point they were cut off at the
shoulders. Touching a capacitor the size of an oil drum has killed
people. Super
capacitors are packing
more power into smaller, lighter objects.

If capacitor is connected to a circuit wrong, it can explode youtube
video.

A leyden jar is a capacitor. Aluminum foil
of about 200 mm by 450 mm placed on either side of glass can store
charge. If charged with a static electricity generator such as a
Wimshurst machine can cause the
glass, at the edges of the aluminum foil to shatter into little shards
like a grenade exploding.

### Example

Problem: Given a current of 10 amps flowing through a 1uF capacitor for
2 us, what is the voltage across the capacitor?

Solution: The problem appears to be a capacitor hit by a pulse of
current: $i(t) = \begin{cases}
 0, & \text{for } t {\leq} 0\\
 10, & \text{for } 0 < t {\leq} .000002\\
 0, & \text{for } .000002 < t
\end{cases}$

The initial voltage across the capacitor is not given. Assume it is zero
for time less than 0. While current is flowing into the capacitor it is
going to charge. When the current stops, the capacitor will hold it\'s
charge and the voltage will remain constant.

The charging period requires transforming\
$i = C\frac{dv}{dt}$\
into the integral\
$v(t) = \frac{\int\limits_{0}^{t}i({\tau})\,d{\tau}}{C}$

The solution involves an integral: $v(t) = \begin{cases}
 0, & \text{for } t {\leq} 0\\
 \frac{\int\limits_{0}^{t}10\,d{\tau}}{.000001}, & \text{for } 0 < t {\leq} .000002\\
 v(.000002), & \text{for } .000002 < t
\end{cases}$

To calculate the answer, the integral needs to be evaluated twice (links
are to wolfram alpha):

-   once with the limits
    0,t
-   second with the limits
    0,0.000002

This results in the solution$$v(t) = \begin{cases}
 0  \text{  volts}, & \text{for } t {\leq} 0 \text{  seconds}\\
 10^7t \text{  volts}, & \text{for } 0 < t {\leq} .000002 \text{  seconds}\\
 20 \text{  volts}, & \text{for } .000002 < t \text{  seconds}
\end{cases}$$

