Electric Energy and Current Chapter 18

Electrical Potential Energy- the potential energy between charges at a distance, or between a charge and an electric field.

PE = -qEd

As a charge moves, it gains KE, and loses PE.

• Potential Difference- the work that must be done to move a charge.

• Many different names- Potential, Potential Difference, Voltage, Emf. Symbol is V, units are V = J/C.

V = PE/q V = -Ed V = kq/r

In order to have energy stored, you must keep the terminals of the battery at different potential (ie different levels of charge)

• Capacitance- the abilityof a conductor to store energy in the form of electrically separated charges.

• Symbol is C, unit is the Farad, F.

• C = Q/V 1 Farad = 1 Coulomb/Volt

• Parallel plate Capacitor (most common)

C = 0A/d A=area, d= distance between plates

= 8.85 E-12

• Capacitance can be changed by putting something between the plates of a capacitor. We call this a dielectric.

• A dielectric is an insulating material- examples are glass, rubber, wood, waxed paper, etc. Molecules in dielectric become polarized, line up with electric field. This allows for a weaker electric field between the plates, so the plates can store more charge.

• Capacitance increases with a dielectric.

• An example of a capacitor is the flash in a camera.

• Energy stored in a charged capacitor

• PE = ½ QV

• Since Q = CV, PE = ½ CV2

• Usually capacitance and charge are small units

• pF = 10-12 F

• nC = 10-9 CC = 10-6 C