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Unformatted text preview: 1/19/2012 1 Lecture 4-1 Gausss Law: Qualitative Statement (Review) s Form any closed surface around charges s Count the number of electric field lines coming through the surface, those outward as positive and inward as negative. s Then the net number of lines is proportional to the net charges enclosed in the surface . Lecture 4-2 Electric flux (Review) # of field lines N = density of field lines x area To state Gausss Law in a quantitative form, we first need to define Electric Flux. Surface can be of any shape Surface can be open or closed Must specify which way N E A E An = ch ch i E E An ca area=A = A cos if A is tilted A Lecture 4-3 Electric flux through Arbitrary Surface General definition of electric flux: E S E n dA = ch i (must specify sense , i.e., which way ) E E An = ch i Divide the surface into many very small, nearly flat plaquettes and sum over the contributions from all of them. Lecture 4-4 Electric Flux through Closed Surface E n a i E S E ndA = ch i I This integral is over a CLOSED surface. Since is a scalar product, the electric flux is a SCALAR quantity The integration element is a vector normal to the surface and points OUTWARD from the surface. Out is +, In is - n E Net # field lines going outward So, deforming the surface (such as shown) makes no difference!shown) makes no difference!...
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This note was uploaded on 02/05/2012 for the course PHYS 241 taught by Professor Wei during the Spring '08 term at Purdue University-West Lafayette.
- Spring '08