University:
Amherst College
Course:
PHYS 117 | Introductory Physics II: Electromagnetism and Optics
Academic year:

2024
Views:

379

Pages:

11
Author:

Kimberly Kane

Electric field of single point charge  kq E P = 2 rˆq , P rq , P Another way to write k: k= 1 4p 0  0  8.85  1012 C2 /Nm 2 (called permittivity of free space) Surface area of a sphere: A = 4p r2 Electric Field Lines • Field lines start or end at charges or at infinity. – Lines are directed outwards from positive charges and inward towards negative charges. – # of lines starting or ending at a charge is proportional to the magnitude of that charge (constant of proportionality is arbitrary, i.e. up to you). The figure shows the field lines for two charges. What might be the ratio of the left charge to the right charge? 1) 2) 3) 4) 1:2 1:2 2:1 2:1 Gauss’ Law • The total electric flux (i.e. # of field lines) leaving a Gaussian (closed) surface is proportional to the charge enclosed by that surface. Gaussian Surface • A surface that encloses a volume. – It separates space into an inside and an outside. – You cannot get from the inside to the outside without going through the surface. Which of the following is not a Gaussian surface? 1) 2) 3) 4) 5) Both 3 and 4. Electric Flux • A quantitative measure of the number of field lines through a surface. – Depends on the magnitude of the electric field. – Depends on the area of the surface. Flux (FE)when Field is perpendicular to surface Flux when field is not perpendicular to surface. Figure 22.1c F E  EA cos Electric Flux F E  EA cos   E cos  A  Enˆ A n̂ - unit vector that points perpendicular (normal) to surface. - two possible directions for most surfaces; ambiguity. - for Gaussian surfaces, n points out, by convention. A cubical surface with no charge enclosed and with sides 2.0 m long is oriented with right and left faces perpendicular to a uniform electric field E of (1.6  105 N/C) in the +x direction. The net electric flux fE through this surface is approximately 1) zero 2) 6.4  105 N · m2/C 3) 13  105 N · m2/C 4) 25  105 N · m2/C 5) 38  105 N · m2/C

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