week 3 222
U of L
Popular in Fundamentals of Physics II
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This 4 page Class Notes was uploaded by Mary-elizabeth Notetaker on Thursday September 8, 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 7 views. For similar materials see Fundamentals of Physics II in Physics at University of Louisville.
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Date Created: 09/08/16
Week 3 Tuesday, September 6, 2016 11:04 AM • Same dir w Fe if q>0 Opp dir w Fe if q<0 • • The electric field vector is tangent to the electric field lines at each point • Number of electric field lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region • E vector strong when lines close(& high density) and weak when far(& low density) • Total number lines proportional to charge • If source charge pos, lines radiate outward radically in all dir ○ If neg, radial lines point inward to infinity ○ If group of charges, lines start at pos and end at neg if excess of charge, some lines will begin/end infinitely far away ○ ○ Number lines drawn leaving pos/ending neg is always same and proportional to mag of charge ○ No 2 field lines cross ○ Unlike charges, dipole(equal but opp charge) ○ 2 like charges, lines start at pos and end at infinity…electric field line is tangent to those lines Bulging out around charges indicated repulsion Low field lines btwn charges shows weak field in that region If 2+ with -, only half the lines that leave pos charge will end at neg charge…. 2x number lines leaving +2 as ending on - Phys 222 Page 1 If 2+ with -, only half the lines that leave pos charge will end at neg charge…. 2x number lines leaving +2 as ending on - • Conductors- electrons move freely in response to electric force No net motion of charge in conductor, is in electrostatic equilibrium ○ Isolated conductors: electric field is 0 everywhere inside conductor Excess charge resides only on surface □ Result of 1/r^2 repulsion..surface would be changed if charge was inside bc would repulse & cause movement disrupting equilibrium Field just outside conductor is perpendicular to conductor's surface Field accumulates on smallest(sharpest) points so stronger On flatter(wider) end charges are parallel so move apart from repulsion until equil established □ On sharper(smaller) end repulsion directed away so not as much repulsion so don’t move as far apart Larger resultant force away from surface created…may allow charges to jump from surface to air • Electric flux: ○ Field lines penetrating an area(A) perpendicular to the field ○ Product of EA is electric force ○ In general: Perp area to A is theta to the field. Theta is angle btwn the field and normal of area A Inner product of 2 vectors is: • Area vector: Area vector can have diff orientations in electric field Electric field passes thru area: Electric flux passes thru closed surface: (same eq as above) Normal of closed surface is outward Those passing out of interior of volume are pos Ex)Uniform electric field in x direction… electric flux thru each surface of cube w edges L, and net flux? 0 if no charge inside Net flux thru closed surface of arbitrary shape? Phys 222 Page 2 Net flux thru closed surface of arbitrary shape? Does total electric field pass thru closed surface depend on electric field in the space? Does electric flux thru closed surface depend on charge? ○ Gauss's Law- consider point charge Q surrounded by spherical surface centered on charge Mag of E field everywhere on surface: - E flux thru any closed surface is equal to net charge Q inside surface divided by E Technique for calculating E field on a closed surface w symm Ex) find E field inside conducting shell for r<a & outside for r>b & if add charge -2Q put in center find E field for r>b & what distribution of charge is on shape If symm, E field from any surface point always points inside E field inside shell is 0 E field outside: r>b E field outside shell is same as point charge & points out E field of charged thin sphere cell..neg charge on inner surface(Q) Phys 222 Page 3 Charge distrib: Plane symm: nonconductcing infinite sheet.. Uniform + charge.. Find E field above & below plate Field must be perp to plate Phys 222 Page 4
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