Electric Potential Electric forces are conservative. Work done by an electric force is W=-q o Ed DU=-W
Electric Potential Electric forces are conservative. Work done by an electric force is W=-q o Ed U=-W.
Dec 15, 2015
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Electric Potential
Electric forces are conservative.Work done by an electric force is W=-qoEdDU=-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 20-1Change in Electric Potential Energy
Electric Potential
,
o o
U WV
q q
J jouleSI unit volt V
C coulomb
Definition of Electric Potential V
Electric Potential
When working with electric potential we measure only changes in electric potential.The zero point can be set arbitrarily.
Figure 20-2Electric Field and Electric Potential
Electric Potential
Electric field and the rate of change of electric potential.
o
o o
q E sWV E s
q q
VE
s
Electric Potential
Electric potential of point charges.
electric potential energy for a point charge
oo
kqV
r
kqqU q V
r
Electric potential
Equipotentials are surfaces where the electric potential is the same.
Figure 20-6Equipotentials for a Point Charge
Figure 20-7Equipotential Surfaces for a Uniform Electrical Field
Figure 20-8Equipotential 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 19-18Charge 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 19-19Electric Field Near a Conducting Surface
Electric fields around conductors
Electric field lines enter conductor surfaces at right angles.
Figure 19-21Shielding Works in Only One Direction
Figure 19-20Intense Electric Field Near a Sharp Point
Electric fields around conductors
Electric field lines are more intense near a sharp point.
Charging by induction
Figure 19-22abCharging by Induction
Figure 19-22cdCharging by Induction
Capacitors
A capacitor is a device that has the ability “capacity” to store electric charge and energy.
Capacitors
Capacitance
charge
: :
is defined as
QC Q the on the plates
VV electric potential across the plates
coulombUnit farad F
volt
Figure 20-13A Parallel-Plate Capacitor
Capacitors
2
122
1 C =8.85 10 Coulomb's constant
4
o
o
Permitivity of free space
kk N m
Capacitors
Capacitance of a parallel plate capacitor
d= distance between plates
A= area of the plates
oACd
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 20-15The Effect of a Dielectric on the Electric Field of a Capacitor
Capacitors
Capacitance of a parallel plate capacitor filled with
a dielectric
d= distance between plates
A= area of the plates
dielectric constant
oACd
Capacitors
22
Energy stored in a capacitor
1 1
2 2 2
QU QV CV
C
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