Capacitor


A capacitor is a device that can store electrical charges.  It has many uses in electronics.  One of these is as a time delay switch that we shall look at.  One of the simplest ways to make a capacitor is to have two metal plates close together, as shown between points A and B in the picture below.  

capacitor


This pictures shows a battery on the left connected to a capacitor on the right.  In between is a resistor, shown as a rectangle at the top.  When the circuit is closed, electrical charges start following into the capactor.  The plate in the capacitor that is connected to the positive pole of the battery will start filling up with positive charges.  The other plate connected to the negative pole will fill up with negative charges.  This can happen for two reasons:  

1. the large surface areas of the plates give the charges room to stay, and

2.  the small distance between the plates means that the opposite charges can attract each other and help keep themselves on the plates.

As charges fill up in the capacitor, the voltage across the capacitor increases.  Point A gets more and more positive, while point B gets more and more negative.  This means that we can actually use A and B as if they are the positive and negative poles of a battery.  So if we connect A and B to a light bulb, the bulb will light up.

But the bulb will not light up immediately.  It will start dim and grow brighter until it reaches the full brightness.  This is because the capacitor takes time to charge up.  It is fully charged when the voltage across A and B gets as big as the voltage of the battery, because then the battery's voltage will not be enough to force more charges into the capacitor.  The time taken for the capacitor voltage to reach its maximum depends on:

1. the size of the capacitor, and

2. the value of the resistance in the circuit.

The larger these are, the longer it takes for the capacitor to be fully charged.  A light bulb connected to it will also take longer to fully light up.  In this way, the capacitor can be used as a time delay switch.