Storing charge - the capacitor
To understand how another component in electronic circuits works, imagine the following:
Imagine that we have two 'boxes' to put charged particles separated by a
piece of plastic. We fill the top one with positive charges and the
bottom one with negative charges, only the positive charges are allowed
to move. We know that they will try to come together because of the
forces generated by opposite charges, but since they cannot get out of
the box, the are just stored there not doing any work.
Now imagine that we connect the two boxes with a pipe through where the
charges can move. The positive charges will move to meet with the
negatives and be in equilibrium. Now that they are moving, there's work
and energy being expended that can be put to use.
The device that accomplishes this is called a capacitor. Basically they
are two conductor plates separated by an insulator layer, in effect
creating the two boxes mentioned above.
When we connect a voltage source to a capacitor, the capacitor is
'empty', with no charges, then charges from the source will start
filling it up. As more and more charges reach the capacitor, they will
start exerting a force on the charges trying to come in from the source,
so it will start filling slower and slower.
Once the capacitor is filled, no more charges flow from the source to
the capacitor. If the voltage source is removed, the charges the
capacitor has remain there, waiting for something to allow them to meet
with opposite charges to reach equilibrium.
If we connect a resistor across, the potential difference created by the
separated charges in the capacitor allow it to function as a voltage
source, so these charges start flowing through the resistor. As more
charges flow, the capacitor starts emptying, causing less potential
difference over time, until it can no longer provide charges and the
current flow stops.
One way to picture the charging and discharging of a capacitor is to
think of a balloon with two mouths, one connected to an air pump and the
other left open simulating the resistor through which charges escape.
The pump will inflate the balloon to a certain pressure that will be
kept constant by the air that escapes the balloon through the other
mouth.
If a sudden increase in pressure from the pump occurs, the balloon will
inflate more but the air coming out will remain at about the same level,
increasing until the air that comes in is the same air that comes out.
If the increase in pressure is short, the balloon will inflate and
deflate quickly, and the air coming out would remain almost the same
throughout.
Same happens with capacitors, when a sudden spike in voltage occurs, the
capacitor stores the charges and the voltage in it rises slowly,
outputting about the same current throughout the process. This property
gives capacitors most of its uses with direct current circuits.
