Chapter 15 & 16

Electrostatics: Forces & Energy

Capacitance

 

Capacitance is a measure of the ability of a device to store charge.

 

Definition of Capacitance, C

Units: coulomb/volt = farad, F

 

Example 16-9

A capacitor of 0.75 µF is charged to a voltage of 16 V. What is the magnitude of the charge on each plate of the capacitor?

 

Solution:

Using , solve for Q:

 

Parallel-Plate Capacitor

Capacitance of a Parallel-Plate Capacitor

is the permittivity of the substance between the parallel-plates. The chart 15-3 on page 550 in the book has the values needed. For air .

Factors Affecting Capacitance

Example 16-10

A parallel-plate capacitor is constructed with plates of area and separation 0.550 mm. Find the magnitude of the charge on each plate of this capacitor when the potential difference between the plates is 20.1 V.

 

Solution:

 

Example 16-11

A parallel-plate capacitor is connected to a battery that maintains a constant potential difference V between the plates. If the plates are pulled away from each other, increasing thier separation, does the magnitude on the plates (a) increase, (b) decrease, or (c) remain the same?

 

Solution:

(b) The charge on the plates decreases.

 

Capacitance of a Parallel-Plate Capacitor Filled with a Dielectric:

where is the dielectric constant found also in table 15-3.

 

 

Example 16-12

A parallel-plate capacitor is constructed with plates of area and separation 0.550 mm. The space between the plates is filled with a dielectric with dielectric constant . When the capacitor is connected to a 12 V battery, each of the plates has a charge of magnitude . What is the value of the dielectric constant ?

 

Solution:

Find the capacitance:

Solve for :

Substitute the numerical values:

 

Capacitors in Combination

Capacitors in Parallel

 

The voltage across the combination of capacitors is equal to the voltage across the battery.

The net charge stored is the sum of the individual amounts stored on each capacitor.

The capacitance is the sum of the individual capacitors.

 

Capacitors in Series

 

The amount of charge on each capacitor is equal.

The voltage is the sum of each voltage across the individual capacitors.

The total capacitance:

 

On to example of Circuits with Capacitors