Week 2 notes Physics 222
Week 2 notes Physics 222 PHYS 222
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This 5 page Class Notes was uploaded by Marimann on Saturday September 5, 2015. The Class Notes belongs to PHYS 222 at Iowa State University taught by Lajoie in Fall 2015. Since its upload, it has received 56 views. For similar materials see Intro to classic physics 2 in Physics 2 at Iowa State University.
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Date Created: 09/05/15
Physics 222 Week 2 Electric field lines and Electric Dipole 4 0 Review of last week 0 electric field of positive and negative charges 0 EgtkQ1r2rgtr 0 electric field lines I instead of a field of arrows physicists prefer to run them together in electric field lines I E field vectors are tangent to the lines lines leave charges and return to charges infinitely no closed system c noloops I two infinite planes intersecting with opposite charges 0 Electric dipole 0 an electric dipole is a pair of point charges having equal but opposite sign and separated by a distance I pgtQdgt 0 direction from to o 7 393 I the net force on a dipole 0 there is only torque I on rotating dipole 0 work done by electric field 0 dWth o 1 p gt x E O UpgtExt unstable O o E versus distance 0 point charge E1r2 o infinite line of charge E1r o infinite plane of charge Econstant o Dipole E1r3 0 electric field of an electric dipole o EandorEke Qr2 Electric fluX 5 o a concept of flux 0 ex I a wire rectangle in a fluid I Q how much fluid flows through a rectangle in a given time the amount will change as we change the angle of the wire rectangle I o the most moves through when perpendicular o A 0 less flux when the rectangle is tilted o AcosG o flux0 when the wire is parallel o AO flu F JIF quot39 3 r i W Jr ill innu x ir p is 5Iirr 0 Area vector Agt I is a vector with o Magnitude the area of the surface Direction perpendicular to the surface I dot product between the area vector and the vector of the fluidelectric field 0 electric flux I DEA EACOSG o EElectric field o Aarea of shape 0 eangle between electric field lines and vector J to shape I units 0 WC m2 orVm o flux as the flow out of an imagined box I if we put a box around a charge we can think of flow as the electric charge coming out of the box 0 a box containing a charge 0 doubling the charge doubles the flux 0 doubling the box dimensions DOES NOT change the flux 0 sign matters I if you reverse the direction of A you reverse the sign of the flux 0 for an open surface choose any direction as long as you make it clear 0 for a closed surface the convention is to take the area vector pointing outwards o Fluxgt0 lines going out charge inside 0 FluxltO lines going in charge inside 0 Flux0 no lines or cancelling lines neutral charge inside l o Q how does flux change with concentric spheres of differing sizes I the area changes but the flux is the same unless it envelops another charged object o Gauss s law o I E 935E gt dA gt Q encs 0integral form El surface Qenclosed charge Aarea of surface Eelectric field I s O885103912 o centered of a sphere I I Eqls 0 o Fluxchargeepsilon not 0 does not depend on radius Applications of Gauss s law and conductors 6 0 practical applications conductor 0 in a conductor charges can only be on the surface Gauss s theorem tells us that in such case electric field inside a conductor is always zero 0 charges on conductors 0 solid conductor with charge qC I EgtO within the conductor I qC is only on the surface of the conductor 0 same conductor with an internal cavity I Same as before 0 isolated charge q placed in a cavity I for Egt to be zero at all point on the Gaussian surface the surface of the cavity must have a total charge q a O Copy ght d iann Wesley Lungman Inca quencloseaSOZQQm80 QQin0 QmQ oinQinl41TR2mltO Net charge on the surface must be zero 0 Qout39QinQ o if the enclosed charge is off center it still does not make a difference OOOO field at the surface of a conductor o E 58 0
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