Capacitor

  This type of device is used to store electrical charges and prevent voltage spikes. The insulated plates area provide a form of storage into which electricity can flow and this prevent any surge of voltage. The unit for this is Farad (F) capacitance. The key thing that makes the capacitor unique is that the storing of charges takes time, hence it can be uses to dampen voltage spikes, or for event timing.
I. RC time Delay or "charging time":
   1.Theory:
   What really happens here? This can only be explained using electron flow theory. So the plate connected to the Pos is the positive plate, and the negative plate connected to the negative of the battery. Eventually, positive charges are attracted to electrons and vice versa, and hence the effect is similar with the diode. But because between 2 plates is the insulator, positive and negative charge don't meet but still get attracted hence keep storing up on the plates storage, this is called charging. At first there is plenty of room of storing so the charging is really really fast, then it is getting tighter so the charging speed drop exponentially, until the capacitor reach it maximum capacity, there is no more storing.
   When the capacitor is disconnected from the battery, there is no force to keep the charges together, hence they are more attracted to the rest of the circuit( conductor), or yet trying to flow through an easiest way to get to each other( positive and negative), this is called discharging. Discharging can be dangerous, because if charging is exponential-like, so is discharging. The speed of discharging at initial is the biggest because the maximum number of charges bunching up wanting to get out! And that is the magnitude of the current flow. Remember physic 101! Current is essentially Coulomb/sec it means how much charge flowing through a second!!! So with a big capacitor releasing its energy, DON'T BOTHER TOUCHING IT!!!  
   2.Calculate how fast the capacitor should charge
               T=RxCx5
R: resistance Ohm ; C: capacitance Farad; T is time in second.

II. Build the circuit

This circuit has a resistor in series with a bridged capacitor, and a voltage source of 12V.
The resistor is, as always to control the current in the circuit. Excessive current can damage the capacitor, overheating the plate, damaging the insulator. The capacitor is there for our benefit of understanding how it works in a circuit and how it charges and discharges.
The capacitor once hooked up with a source will autoly charge up. Therefore we make a bridge across it so electricity will flow through the bridge, as the easiest way to get to battery negative, completing the circuit, plus most importantly it prevents the capacitor from charging.

Once the bridge is disconnected, the capacitor starts charging up. By correctly hooking up th Voltmeter, we can read the charging value as voltage drop across the C keeps increasing,as it is charging. So once we disconnect the bridge, we start recording the VD every 10 seconds for 180 seconds, hence draw a graph for voltage change over time. And what we've got is a logarithism-like graph. The very steep slope from the start indicating the fastest charging speed as the capacitor is initially empty. As the storage is getting fuller, the slope becomes more gradual over time, indicating that it is actually harder to get charges stored fast, hence the charging speed gradually decreases. The last few seconds, the slope is getting very close to zero, showing that the capacitor is full.