Learning objectives By the end of this chapter, you should be able to:
state the relationship of charge, voltage and capacitance state the factors affecting a parallel plate capacitor, with reference to a demonstration
illustrate how to charge and discharge a capacitor describe the applications of capacitors used in various equipment.
Introduction
Most modern electrical appliances have capacitors built into them. A capacitor can hold an electrical charge until it is required. Capacitors are responsible for such varied roles as volatile memory storage when power supplies are interrupted or pulsed weapon systems requiring high-voltage discharge. They can even be used for information transfer in binary form.
The ratio of charge to voltage on a device is called its capacitance. Capacitance is based on ‘capacity’. The more charge a conductor is capable of holding (its capacity), the bigger the capacitor it is.
As you add more and more charge (coulombs) to an insulated conductor, you increase its potential. This means that it takes more and more work to add or bring more charge to it. As a result, voltage increases.
Therefore charge Q is proportional to voltage V (Q ∝ V ). From this we get:
Charge Q = CV
Q = charge (C), C = capacitance (F), V = voltage (V) C is the constant of the insulated conductor. Changing capacitance
1. Try to bring a positive charge near a positive insulated conductor. The conductor will repel the charge. This means that more work is required to bring the charge near the conductor, which increases pd or voltage.
2. Try to bring a positive charge near a positive insulated conductor with a negative insulated conductor adjacent. The pd has been reduced by
LEAVING CERTIFICATE PHYSICS 247
FIG. 22.1 Capacitors hold an electric charge until it is required
A capacitor is a device capable of storing an electrical charge.
