db_lec03_post

db_lec03_post - PHYS 221 Lecture 3 Last lecture: E field...

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PHYS 221 Lecture 3 • Last lecture: E field • For a point like charge • For any charge distributions • Today we look at another way to determine E 1/17/12 1 E = F q 0 F = k q 0 q 1 r 2 ˆ r D. Bortoletto PHYS221 E = k q 1 r 2 ˆ r E = k dq r 2 ˆ r
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1/17/12 2 Gauss Law *relates the electric fields at points on a closed Gaussian surface and the net charge enclosed by that surface. What are in the boxes? D. Bortoletto PHYS221
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1/17/12 3 Concept of Flux θ v Air stream with uniform velocity, v , flows through a loop of area A Φ = volume time can define a flux: D. Bortoletto PHYS221
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1/17/12 4 Flux of an Electric Field area lines of number ~ E EA ~ lines of number area Works only when lines are perpendicular to surface! Need to quantify the pattern. More lines comes from the box – more charge inside! Q: What is the number of lines that cross the surface? Electric field is proportional to the number of lines per unit cross-sectional area: D. Bortoletto PHYS221
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1/17/12 5 Flux of an Electric Field Analogy : think of rain which falls into a bucket. Less rainwater enters the bucket if it is tilted A E EA EA = = θ cos ~ lines of number The number of lines crossing a surface is proportional to the perpendicular component of the field. Definition of electric field flux: cos EA A E EA E = = = Φ D. Bortoletto PHYS221
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1/17/12 6 Electric field flux Field lines can either enter the closed surface area, or exit it.
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db_lec03_post - PHYS 221 Lecture 3 Last lecture: E field...

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