Chapter 21Electric Fields

An electric force of4.5 x 10-5 N is measured

between two particles. One particle has a charge of

2.0 x 10-6 C & the other has a charge of 3.0 x 10-8 C. Calculate the distance

between them.

Electric force like gravitational force is

inversely proportioned to the square of the

distance between the two points of concern

Electric Field (E)•A vector quantity that relates the force exerted

on a charge to the amount of the charge

Electric Field (E)

E =Fon q’

q’

Electric Field (E)

Fon q’ = q’E

Calculate the electric field strength when a 25 N force at 37o NoE is exerted on a charge

of + 5.0 x 10-6 C

Typical Field Strengths

Field Value (N/C)TV tube 1 x 105

Spark req 3 x 106

H orbital 5 x 1011

Electric Field Lines

•Lines representing the force vectors in

an electric field

Electric Field Lines

+

Electric Field Lines

-

+ -

Electric Field Lines

Electric Field Lines

•Always point from positive to

negative

Electric Field Lines

•Do not exist , but provide a model

of a field

The electric field between two

parallel plates is uniform

+ -

Electric Potential•The electric

potential difference of charges

measured in volts

Electric Potential•As with heat, we can only measure

potential difference (V)

Electric PotentialDifference (V)•The change in potential energy per unit charge

Electric PotentialDifference (V)•The work done moving a charge

thru a field charge

Electric PotentialDifference (V)

•Measured in J/C

•J/C = volt (V)

Electric PotentialDifference (V)

W on q’

q’V =

Electric PotentialDifference (V)

U = W

Electric PotentialDifference (V)

Uq’

q’V =

Electric PotentialDifference (V)

W on q’

q’V =

Electric PotentialDifference (V)

W = Fd

Electric PotentialDifference (V)

Fd on q’

q’V =

Electric PotentialDifference (V)

F

q’V = x d

Electric PotentialDifference (V)

F

q’E =

Electric PotentialDifference (V)

V = Ed

Basic Equations•V = Ed•W = qV•F = qE

Equipotential

•When the electric potential

difference is 0

Equipotential

•Charge rearranges itself to reach equipotential

Equipotential•When two spheres have

the same charge, the larger one has lower

electric potential

Equipotential•When two spheres have

the same electric potential, the larger one has the greater charge

Equipotential•When a charged object comes in contact with a

neutral one, the charge in equally distributed

Equipotential•Because of the size of

Earth, when objects touch Earth, their charge

is passed to the Earth

Grounding•When a charged object

touches Earth, all its charge flows to Earth creating equipotential

Electric Fields

•All charges are on the outside of a conductor

Electric Fields

•In pointed object, the field strength is

greatest at the point

Capacitor

•A device designed to store a charge

Capacitance

•The ratio of charge to electric potential

difference

Capacitance (C)

C =

qV

Farad (F)

•Unit for capacitance measured in coulombs

per volt: F = C/V

Basic Equations•V = Ed•W = qV•F = qE•q = CV

A charge of 1.6 x 10-6 C is stored to create a

capacitance of 4.0 x 10-3 F acting over

2.0 m. Calculate: V, E, F, & W

A charge of 1.5 x 10-6 C is stored to create a

capacitance of 4.0 x 10-3 F acting over

2.0 mm. Calculate: V, E, F, & W

A charge of 3.2 x 10-4 C is stored to create a

capacitance of 8.0 mF acting over 4.0

m. Calculate: V, E, F, & W

Charge =1.6 x 10-6 CForce = 3.2 x 10-3 NDistance = 64 nm.

Calculate: V, E, C, & W

Calculate: 3.2 x 10-144

x 1.5 x 10162

8.0 x 10-256 7.5 x 10175

x 4.0 x 10122 =

Calculate: 3.2 x 10144

x 1.5 x 10162

8.0 x 10-254 7.5 x 10-175

x 2.0 x 10125 =