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Stream lines . front plate slightly charged induces opposite charge on back plate. Brushes pull off charges charges collected in leyden jar (capacitor). Wimshurst Machine. Electric Field. Definition . Electric field is the

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Presentation Transcript
slide2

front plate slightly chargedinduces opposite charge on back plate.Brushes pull off chargescharges collected in leyden jar (capacitor)

Wimshurst Machine

definition
Definition
  • Electric field is the

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

electric field as coulomb s law
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

slide6
Top tips on electric fieldE=0 inside a metal (faraday) cageField lines go from + to – chargesCloser field lines are stronger
slide7

A

B

-

C

+

Electric Fields

Electric Field between 2 unlike charges

Note that the distance between

electric field lines at C is shorter

than that at B.

Distance between electric field

lines at B is shorter than at A.

Electrostatics

slide8

Electric Fields

Electric Field between 2 charged plates

positive

negative

Electrostatics

slide9

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

Electric Fields

Electric Field between 2 charged plates

Electrostatics

coulombs joules volts
coulombs, joules, volts
  • Coulomb= unit of charge
  • Joule = unit of energy
  • Volt= unit of energy per charge
coulomb
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
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
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

slide14

(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

POSITIVE charge moving in an E field.

volts is an energy density
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
Examples
  • Static balloon 9 V battery (1hr,1A)

1 joule of energy 9x104 joule energy

0.001 C of charge 1x104C of charge

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

another c j v example
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

slide18
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
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

slide20
Electrostaticsgravitational

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

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