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