Stream lines. front plate slightly charged induces opposite charge on back plate. Brushes pull off...

front plate slightly chargedinduces opposite charge on back plate.

Brushes pull off chargescharges collected in leyden jar (capacitor)W

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Electric Field

Definition

• Electric field is the

strength & direction of the forces in space surrounding a positive test charge

Electric field as coulomb’s law

• F = q1 * (k q2 /d2)

rewrite the force equation

F = q1 * E

q1 is the positive test charge

E is the field created by charge(s) q2

Top tips on electric fieldE=0 inside a metal (faraday) cageField lines go from + to – charges

Closer field lines are stronger

Electric Field between 2 unlike charges

+ -

A

B

C

Note that the distance betweenelectric field lines at C is shorterthan that at B.

Distance between electric fieldlines at B is shorter than at A.

Electric Field between 2 charged plates

positive

negative

Electric Field between 2 charged plates

+ + + + + + + +

- - - - - - - -

coulombs, joules, volts

• Coulomb= unit of charge

• Joule = unit of energy

• Volt= unit of energy per charge

coulomb• How many charges are in 1C? coulomb?• 1C is about 6 billion billion charges)

(1/1.6 x 10-19 = 6.25 x 1018)• The man, the law

Charles Coulomb~1750

Joules of energy

• Charges have a type of energy called

electron potential energy (PEe or U)

High energy: + is close to +: ++

or + is far from –: + -

It takes work to move charges against (opposite) an electric field High energy work needed to move + charge

(W = F*d)

E

F

+

low energy

POSITIVE charge moving in an E field.

(a) When a positive charge moves in the direction of an electric field, the field does positive work

and the potential energy decreases. Work = qo

E d

Volts is an energy density

• Voltage is also called potential• 1 volt = 1 joule / 1 coulomb• Example: 12 V battery: every coulomb of

charge has 12 joules of energy

Examples

• Static balloon 9 V battery (1hr,1A)

1 joule of energy 9x104 joule energy

0.001 C of charge 1x104 C of charge

1/.0001 = 1000 volts 9/1 = 9 Volts

Another c,j,v example

• Van de graaf static generator has..

1,000,00 volts (high) 1 joule of energy (low) .00001 coulombs of charge

1,000,000 v = 1 j/ .00001c

Uniform electric field 2 situations Point Charges

Force: F=q*E Force= k q1*q2 / d2

Electric potential energy Electric potential energy

PE= F*d = q*E*d PE = F*d = k q1*q2 / d

Voltage (electric potential) Voltage (electric potential)

V= PE/q1 = q*E*d /q = E*d V= PE/q1 = k q2 / d2

Voltage difference: Voltage difference:

DVe = D PE/q1 = E* Dd DVe = D PE/q1 = k q2 / d2 - kq2/ d1

It takes work to move + charge against static forces like it takes work to lift against gravity

• PEfinal

• +++++

• +

• Peiniitial work = gain in PE = F*d

work

Electrostatics gravitational

Force: F= kq1 q2 / d2 F= Gm1m2/d2

Field E= F/q1 = kq2/d2 g = F/m1 = Gm1/d2

potential energy PE= F*d = kq1 q2 / d PE = F*d= Gm1m2/d

(using field) =qE*d = mg*h

Potential V= PE/q1 = kq2 / d U= PE/m1 = G m2 / d

(Using field) = E*d = g*h