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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
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
e0= 8.85 E-12
A=area, d= distance between
A capacitor connected
to a 12V battery source
hold 36μC of charge on
each plate. What is the
Capacitance can be changed by putting
something between the plates of a capacitor.
We call this a dielectric.
A dielectric is an insulating materialexamples 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
 mC = 10-6 C
 Find the electrical potential energy stored in the
capacitor in the previous problem