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week 4 222

by: Mary-elizabeth Notetaker

week 4 222 Phys 222

Mary-elizabeth Notetaker
U of L

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Week four notes
Fundamentals of Physics II
Dr. Ming Yu
Class Notes
phys222, Physics, fundamentalsofphysics2
25 ?




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This 6 page Class Notes was uploaded by Mary-elizabeth Notetaker on Friday September 16, 2016. The Class Notes belongs to Phys 222 at University of Louisville taught by Dr. Ming Yu in Fall 2016. Since its upload, it has received 4 views. For similar materials see Fundamentals of Physics II in Physics at University of Louisville.


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Date Created: 09/16/16
Week 4 Tuesday, September 13, 2016 10:45 AM E field of nonconducting infinite plane sheet of charge: Pos charge per unit area Symm analysis: Made up of infinite number of point charges Field must be perpendicular to sheet Field is directed towards or away from sheet For cylindrical gaussian surface: Flux thru ends is EA , no field thru curved part of surface Field is away from sheet Find field above/belowsheet with uniform negative charge per unit area (cylindrical) Same as above: sum top bottom and side. But E will be negative in bottom equation bc field is pointing towards sheet Parallel Plate Capacitor- plates of neg and pos charge w same charge density For pos plate, E field points away from plate at all positions For neg plate, E field points towards plate at all positions Total E field is sum of E+ and E- at all positions Ex) nonuniform Wab=Fconservativex d= -(Peb-Pea)= - PE Wab=Fconservativex d= -(Peb-Pea)= - PE Work done by conserv force= -change of PE Conserv force PE Where PE: electric PE diff(final-intial) Electric PE- scalar Change od electric PE is independent of path, depends on distance PE(path1)= PE(path2) To obtain electric PE diff: Ex) uniform field btwn 2 plates As charge movesfrom A to B, work is done by electric force on it PE diff in electric field: Ex) Proton released from rest in constant electric field Proton loses electric PE Ex) electron released from rest in constant electric field Does the Change in electric PE assoc w/electronbecomesmore neg w/ time? Yes it does • When electron is fired in uniform E field, find initial speed Vi if its final speed Vf has fallen by half • When electron is fired in uniform E field, find initial speed Vi if its final speed Vf has fallen by half ○ Apply conservationof E: KE+ PE=0 ○ Ex) electron fired  Change of PE of an object Work done by conservativeforce Conservativeforce generated by field Field is indep of any other object …there must be a potential associated w the field and is indep of any other object  Electric P and E due to point charges E indep of 2nd charge  E potential diff( V) btwn 2 points= change in PE of a charge q movedfrom A to B divided by size of the charge EP is characteristic of field..indep of any test charge there Positivecharge movesalong field: …. Potential dec along direction of field  EP and charge mvmts ○ When released from rest, pos charges acceleratespontaneously along field direction  Pos charge movesalong the field direction…EFdoes positive work ○ When released from rest, negative charges accelerate spontaneouslyopp to field direction  Negative charge movesopp to field dir… E field does pos work  When at rest neg charge goes to dir of lower EP, work must be done by external force against EF  Point of 0 EP is taken to be infinite distance from the charge  How to calc EP created by pos charge(+Q)? EF is vector proportionalto 1/r^2 EP is scalar proportionalto 1/4 EP of multiple point charges 1. Diagram all charges(note point of interest) 2. Calc distance from each charge to point of interest 3. Use: V=ke(q/r) a. Include sign b. If potential is pos, charge is pos…samew neg 4. Get algebraic sum Ex) If EP taken as 0 at infinity, find EP due to charges at point P How much work req'd to bring 3rd point charge q3 from infinity to p? Dipole ex: 2 charges have eq mags and opp charges EP energy of 2 charges V1 is EP due to q1 at somepoint P Bring q2 from infinity to P w/outacceleration is: PE of 2 particle system If charges have same sign, PE is positive Pos external work must be done to force 2 charges near one another If charges have opp signs, PE is neg Ex work must be done to hold back unlike charges from accelerating as they are brought close together Ex 2) suppose 3 protons on x axis, at rest relative to one another at given instant time… q3 released while others are held in place, find symbolic expression for third protons speed at infinity: Conservationof E:


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