Force FieldsForce Fields• Gravitational fields

-act on masses-Strength ~ Mass / d2

-attractive-cannot be shielded-can store energy-overall very weak force• Electrical fields

-acts on charges-Strength ~ Charge & 1 / d2

-attractive or repulsive-can be shielded-can store energy-generally stronger than gravity

Electric FieldElectric Field

DefinitionDefinition

• Electric field is the

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

Top tips on electric fieldTop tips on electric field

Field lines go from + to – chargesField lines go from + to – chargesCloser field lines are strongerCloser field lines are strongerfield lines can’t cross another!field lines can’t cross another!

Question 1Question 1

• Suppose an electric field points south• Which way would a proton move:

a. south• B. north• C. west

• D. east.

Question 1 answerQuestion 1 answer

• Suppose an electric field points south

• Which way would a proton move:A. south

Since electric field shows direction of a positive charge from high to low

energy

Question 2Question 2

• Suppose an electric field points south• Which way would an electron move:

a. south• B. north• C. west

• D. east.

Question 2 answerQuestion 2 answer

• Suppose an electric field points south

• Which way would an electron move:B. north

Since electron is a negative charge it moves opposite direction of field

Electric field as coulomb’s lawElectric 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

Question 3Question 3

• The electric field in a certain region of space is 40 N/C.

A. What is the force on a -10 C charge placed in the region?

B. Is it attractive or repulsive?

Question 3 AnswerQuestion 3 Answer

• F=q*E

• F= (40 N/*C)(-10 C)

• F = - 400 N

(attractive force since negative answer, +*- = - )

Question 4Question 4

• Imagine a metal ball placed in an electric field

• IF an electric field moves charges and conductors are full of electrons which can move, where will all the electrons go?

a. stay on left side• B. spread out on outer surface

• C. stay in middle• D. don’t move

Question 4 answerQuestion 4 answer

• Imagine a metal ball placed in an electric field

• IF an electric field moves charges and conductors are full of electrons which can move, where will all the electrons go?

B. spread out on outer surface since the charges move away from another to lower their energy

• the charges on opposite sides cancel out forces so E=0 inside

Electric Shielding- Electric Shielding- Faraday CageFaraday Cage

• Inside a Conductor, the Electric Field MUST be ZERO!

• Any net charge on the inside of a conductor will be pushed to the surface & distributed to equilibrium!

• Electric field E is perpendicular to the outer surface,and zero inside any conductor (metal)

Faraday cageFaraday cage

• Don’t try this at home!

Electric field demosElectric field demos

Metal cage blocks out the electric field from the radio station or cell phone tower-so no music!

Pith balls inside the metal screen don’t move since there is no electric field inside a faraday cage

Common Faraday CagesCommon Faraday CagesPurpose: • Keep in or out electro-magnetic fields used in

electronics/telecommunications

Examples:• Metal foil around wires inside phones

(so less pick-up of unwanted noise/calls)• Rebar in concrete or bridges- bad reception• Metal boxes around electronic trigger switches

coulombcoulomb

• How many charges are in 1C? coulomb?• 1c is about 1 million charges!• The man, the law

Charles Coulomb~1750

JoulesJoules of energyof energy

• Charges have a type of energy called

electron potential energy

High energy: + is close to +: ++

or + is far from –: + -

Question 5Question 5

• Imagine moving a positive charge in an electric field

• If you move the + charge to the left against the electric field, what happens to the energy of the + test charge?

a. no change• B. increases• C. decreases

• D. not enough info

+E

Question 5Question 5

• Imagine moving a positive charge in an electric field

• If you move the + charge to the left against the electric field, what happens to the energy of the + test charge?

B. increasessince field points from high to low energy

+E

Question 6Question 6

• Imagine moving a positive charge against an electric field

• If it takes 2000 Joules of work to move the charge..?

a. how much energy does the charge gain?• B. how much work does the field do to move the charge

back to the right?

+E

Question 6Question 6

• Imagine moving a positive charge against an electric field

• If it takes 2000 Joules of work to move the charge..?

a. 2000 J----positive work needed to add energy?• B. -2000 J--- negative work done by the field,

(field loses energy while charge gains)

+E

Electric Potential Electric Potential (Voltage)(Voltage)

•Analogies:voltage is like water pressureenergy= heightwater= charge

V= PE/Q

•voltage= energy/Charge

•Volts= joules/coulombs

Question 7:coulombs, joules, voltQuestion 7:coulombs, joules, voltmatch these!match these!

• Coulomb unit of energy/charge

• Joule unit of charge

• Volt unit of energy

Question 7:coulombs, joules, voltQuestion 7:coulombs, joules, voltmatch these!match these!

• Coulomb unit of charge

• Joule unit of energy

• Volt unit of energy per charge

Volts is an energy densityVolts 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

Example of high voltage, low chargeExample of high voltage, low charge

• What’s the voltage on a balloon if it has…

1 joule of energy (medium) .001 coulombs of charge (low)

voltage= energy/charge v = 1 J / .001C = 1000 Volts !

Question 8Question 8

• What’s the voltage on a Van de Graaf static generator

1 joule of energy (medium) .00001 coulombs of charge (v.low)

v = ?

Question 8Question 8

• What’s the voltage on a Van de Graaf static generator

1 joule of energy (medium) .00001 coulombs of charge (v.low)

v = 1 j/ .00001c

• = 100,000 Volts ouch!