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Electric Potential
Electric forces are conservative.
Work done by an electric force is
W=-qoEd
DU=-W
Electric Potential
• The convention in our text is
that the work done on a
system is has a negative sign.
• Unfortunately the college
board help sheets use the
opposite convention.
Figure 20-1
Change in Electric Potential Energy
Electric Potential
Definition of Electric Potential V
DU W
DV 

qo
qo
J
joule
SI unit  
 volt ,V
C coulomb
Electric Potential
When working with electric
potential we measure only
changes in electric potential.
The zero point can be set
arbitrarily.
Figure 20-2
Electric Field and Electric Potential
Electric Potential
Electric field and the rate of
change of electric potential.
W   qo E Ds 
DV 

 E Ds
qo
qo
DV
E
Ds
Electric Potential
Electric potential of point
charges.
kq
V

r
electric potential energy for a point charge
kqqo
U  qoV 
r
Electric potential
Equipotentials are surfaces
where the electric potential
is the same.
Figure 20-6
Equipotentials for a Point Charge
Figure 20-7
Equipotential Surfaces for a Uniform Electrical Field
Figure 20-8
Equipotential Surfaces for Two Point Charges
Solve problems 1-7,
16-19, and 21-24 on
pages 672 and 673.
Electric fields around conductors
Excess electric charges move to
the surface of a conductor
because that is where they are
least affected by the internal
charges of the material.
Figure 19-18
Charge Distribution
on a Conducting Sphere
Electric fields around conductors
When electric charges are
at rest, the electric field
within a conductor is zero.
This works even if the
conductor has an internal
cavity.
Figure 19-19
Electric Field Near a Conducting Surface
Electric fields around conductors
Electric field lines enter
conductor surfaces at right
angles.
Figure 19-21
Shielding Works in Only One Direction
Figure 19-20
Intense Electric Field Near a Sharp Point
Electric fields around conductors
Electric field lines are more
intense near a sharp point.
Charging by induction
Figure 19-22ab
Charging by Induction
Figure 19-22cd
Charging by Induction
Capacitors
A capacitor is a device that
has the ability “capacity” to
store electric charge and
energy.
Capacitors
Capacitance is defined as
Q
C
Q  the charge on the plates
V
V  electric potential across the plates
coulomb
Unit :
 farad : F
volt
Figure 20-13
A Parallel-Plate Capacitor
Capacitors
Permitivity of free space  o
o 
1
4 k
=8.85 1012
C2
k  Coulomb's constant
2
N m
Capacitors
Capacitance of a parallel plate capacitor
o A
C
d
d= distance between plates
A= area of the plates
Capacitors
A dielectric material is an
insulator that increases the
capacitance of a capacitor when
placed between the plates.
Each material has a dielectric
constant k not to be confused
with k.(p 665)
Figure 20-15
The Effect of a Dielectric on the Electric Field of a Capacitor
Capacitors
Capacitance of a parallel plate capacitor filled with
a dielectric
k o A
C
d
d= distance between plates
A= area of the plates
k  dielectric constant
Capacitors
Energy stored in a capacitor
2
1
1
Q
2
U  QV  CV 
2
2
2C