Capacitance

A capacitor is a device used to store energy using charges

and electric field.

They are a number of uses for capacitors in our modern world: Pulsed Lasers/Weapons Help engines run Touch screens (like this SMARTboard) Filters for electrical circuits

Capacitors work using the physics of capacitance.

Capacitors

Capacitance – the ratio of charges on a

conductive plate to the voltage between the conductive plates.

Capacitance is like Capacity The Capacity of a milk carton is the volume it

holds Heat Capacity is the amount of energy an

object can store without an increase in temp.

Capacitance

Basic capacitance equation:

Capacitance of different objects: Isolated sphere

Parallel Plates

Capacitance

Capacitance 𝐶=𝑄∆𝑉

Charge on Plate

Potential Difference (Voltage)

8.854 x 10-12 C2/Nm2

Radius

Area

Separation

Unit: Farad Scalar Quantity

Since charges are being held in place there is

also potential energy that is being stored (which can then be turned into kinetic energy of the moving electrons!)

Capacitance

Parallel Plate Capacitor

We can change the capacitance by affecting the area of the plates, the distance between the plates, or the voltage of the battery.

How a Capacitor Works

Battery+ -

Electric Field in Wire

Electric Field in Wire

+++++

-----

Electrons move from the plate leaving it positively charged Electrons build up on the

plate leaving it negatively charged

The separation between the two charged

plates creates potential energy.

The potential energy comes from the chemically stored potential energy in the battery.

Parallel Plate Capacitors

+++++

-----

One way to increase the capacitance of a

parallel plate capacitor is to insert an insulator (like glass) between the two conductive plates.

This insulator is called a dielectric.

Dielectrics

This slightly changes our equation for a

parallel plate capacitor to look like this:

Dielectric cont.

Dielectric Constant

Capacitor with dielectric

Capacitor without dielectric

Basic Circuit Symbols

Wire

Capacitor

Battery

Switch

Combinations of Capacitors

Parallel Circuits are ‘stacked like pancakes.’

Capacitors in Parallel

C1

C2

C3

Battery

Parallel Circuits have two important

characteristics:

The Current varies over each component in the circuit:

The Voltage stays constant over each component in the circuit:

Parallel Circuits

To find the equivalent capacitance (a single

capacitor to replace all of the capacitors), we use the following equation:

Capacitors in Parallel

C1

C2

C3

Battery

Battery

CEQUIV

Series Circuits are placed next to each other.

Capacitors in Series

C2 C3C1

Battery

Series Circuits have two important

characteristics:

The Current stays constant over each component in the circuit:

The Voltage varies over each component in the circuit:

Series Circuits

To find the equivalent capacitance (a single

capacitor to replace all of the capacitors), we use the following equation:

Capacitors in Series

C2 C3C1

Battery

CEQUIV

What is the potential difference between the

plates of a 3.3 F capacitor that stores sufficient energy to operate a 75 W light bulb for 30 seconds?

Remember: Power =

Practice Problem