http://www.ck12.org Chapter 16. Electric Potential
16.3 Capacitance
Objectives
The student will:
- Understand capacitance.
- Understand how to solve problems involving capacitance.
Vocabulary
- capacitance:The constant ratio of charge to voltage, which is a property of all capacitors.
- capacitor:Common name for a parallel plate conductor.
- farad:The unit of capacitance, which equals coulombs per volts.
We begin our study ofcapacitorswith the study ofcapacitance, a property of all capacitors. In order to do this,
we must consider the physical dimensions of a capacitor. What is the area of the capacitor plates? What is their
separation distance? It will be shown that the physical dimensions of a capacitor are very important in determining
capacitance.
Capacitance
As was mentioned earlier, a parallel plate conductor is often referred to as a capacitor. The charge on either plate of
the capacitor is referred to asQ. It should be kept in mind that the net charge on the capacitor is zero.
Consider a capacitor charged by a voltage source, in this case a 12-Volt battery as shown inFigure16.8.
Experiments show that the chargeQacquired by a capacitor is directly proportional to the voltageVof the source.
If the voltage is increased, each plate acquires more charge, but the ratio of charge to voltage remains constant
Q
V
→constant(C)→Q=CV.
The constant of proportionalityCis called the capacitance.
The unit of capacitance is coulombs per volts, which is given the name farad(F)in honor of Michael Faraday, the
great 19th-century British physicist and chemist.
1 C
V =^1 F