Chapter 19 - Electric Potential Energy and Electric Potential
• Recap: Work done = change in PE.
• WAB = EP EA − EP EB , is the work done by the electrical field in moving a
unit of positive charge from A to B. EP E is the electrical potential energy.
• Unit of electrical potential energy is Joules.
• Electric potential: EPE per unit positive charge
WAB
EP AA EP EB
=
−
.
q0
q0
q0
The quantity, EP E/q0 is the electric potential. Unit is Joules/Coulomb or
Volts.
•
V =
EP E
.
q0
• Ex 1, p. 567
• A positive charge accelerates from a region of higher electric potential to a
region of lower electric potential.
• A negative charge accel;erates from a region of lower electric potential to a
region of higher electric potential.
• Ex 3, p. 568
• Electron-Volt (eV): One electron volt is the magnitude of the amount by
which the potential energy of an electron changes through a potential difference of one volt,
1eV = 1.6 × 10−19 J.
• Potential of a point charge:
V =
kq
.
r
• Ex. 5,6,7 p. 573
• When two or more charges are present, the potential due to all the charges
is obtained by adding together the indvidual potentials.
• Equipotential surface - connects all points at teh same potential.
• The net electric force does no work as a charge moves on an equipotential
surface.
1
• The electric field created by any charge or group of charges is everywhere
perpendicular to the associated equipotential surfaces and points in the
direction of decreasing potential.
• Field is potential gradient,
E=−
∆V
,
∆s
with units of volts/meter.
• Ex. 9, p. 578.
• Capacitors are two conductors that are placed near each other without
touching each other. Sometimes the space between the two plates is filled
with an insulating material called a dielectric.
q = CV,
where C is the capacitance of the capacitor, q is the charge on the capacitor
and V is the potential difference across the capacitor. C is measured in
Coulombs/V olt or Farads.
• The capacitance of a capacitor of area A, with the plates a distance d apart
is
κǫ0 A
,
C=
d
where κ is the dielctric constant of the material between the plates.
• Ex. 10, p. 561.
• Energy stored in a capacitor is
E = 0.5qV,
E = 0.5CV 2 ,
E=
• Ex. 13, 14, p. 567
2
q2
.
2C