Electric Energy and Current Chapter 17

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.

DV = DPE/q DV = -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)

The gap between electrodes in a spark plug is .060 cm. Producing an electric spark in a gasoline-air engine requires an electric field of 3x106V/m. What minimum potential difference must be supplied?

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 = e0A/d A=area, d= distance between plates

e0= 8.85 E-12

A capacitor connected to a 12V battery source hold 36μC of charge on each plate. What is the capacitance?

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 C

C = 10-6 C

Find the electrical potential energy stored in the capacitor in the previous problem