Force Fields Gravitational fields -act on masses - Strength ~ Mass / d 2 -attractive -cannot be shielded -can store energy -overall very weak force Electrical fields -acts on charges -Strength ~ Charge & 1 / d 2 -attractive or repulsive -can be shielded -can store energy -generally stronger than gravity

Electric Field

Definition Electric field is the strength & direction of the forces in space surrounding a positive test charge

Top tips on electric field Field lines go from + to – charges Closer field lines are stronger field lines can’t cross another!

Question 1 Suppose an electric field points south Which way would a proton move: a. south B. north C. west D. east.

Question 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 2 Suppose an electric field points south Which way would an electron move: a. south B. north C. west D. east.

Question 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 law F = q 1 * (k q 2 /d 2 ) rewrite the force equation F = q 1 * E q 1 is the positive test charge E is the field created by charge(s) q 2

Question 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 Answer F=q*E F= (40 N/*C)(-10 C) F = N (attractive force since negative answer, +*- = - )

Question 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 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- Faraday 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 cage Don’t try this at home!

Electric 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 Cages Purpose: 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

coulomb How many charges are in 1C? coulomb? 1c is about 1 million charges! The man, the law Charles Coulomb~1750

Joulesof energy Charges have a type of energy called electron potential energy High energy: + is close to +: ++ or + is far from –: + -

Question 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 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. increases since field points from high to low energy + E

Question 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 6 Imagine moving a positive charge against an electric field If it takes 2000 Joules of work to move the charge..? a J----positive work needed to add energy? B J--- negative work done by the field, (field loses energy while charge gains) + E

Electric Potential (Voltage) Analogies: voltage is like water pressure energy= height water= charge  V=  PE/Q voltage= energy/Charge Volts= joules/coulombs

Question 7:coulombs, joules, volt match these! Coulomb unit of energy/charge Joule unit of charge Volt unit of energy

Question 7:coulombs, joules, volt match these! Coulomb unit of charge Joule unit of energy Volt unit of energy per charge

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

Example 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 8 What’s the voltage on a Van de Graaf static generator 1 joule of energy (medium) coulombs of charge (v.low) v = ?

Question 8 What’s the voltage on a Van de Graaf static generator 1 joule of energy (medium) coulombs of charge (v.low) v = 1 j/.00001c = 100,000 Volts ouch!