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Chapter 15

Chapter 15. Electric Forces and Electric Fields. Gauss’s Law Sections 7-9. Millikan Oil-Drop Experiment. Millikan Oil-Drop Experiment Measured the elementary charge, e Found every charge had an integral multiple of e q = n e. Active Figure: The Millikan Oil-Drop Experiment.

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Chapter 15

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  1. Chapter 15 Electric Forces and Electric Fields

  2. Gauss’s Law Sections 7-9

  3. Millikan Oil-Drop Experiment Millikan Oil-Drop Experiment Measured the elementary charge, e Found every charge had an integral multiple of e q = n e Active Figure: The Millikan Oil-Drop Experiment

  4. Van de GraaffGenerator An electrostatic generator designed and built by Robert J. Van de Graaff in 1929 Charge is transferred to the dome by means of a rotating belt Eventually an electrostatic discharge takes place MU28T33

  5. Electric Flux Field lines penetrating an area A perpendicular to the field The product of EA is the flux, Φ In general: ΦE = E A cos θ

  6. Electric Flux, cont. • ΦE = E A cos θ • The perpendicular to the area A is at an angle θ to the field • When the area is constructed such that a closed surface is formed, use the convention that flux lines passing into the interior of the volume are negative and those passing out of the interior of the volume are positive

  7. Gauss’ Law Gauss’ Law states that the electric flux through any closed surface is equal to the net charge Q inside the surface divided by εo εo is the permittivity of free space and equals 8.85 x 10-12 C2/Nm2 The area in Φ is an imaginary surface, a Gaussian surface, it does not have to coincide with the surface of a physical object Active Figure: Electric Flux Through an Arbitrary Closed Surface MU29T17

  8. Electric Field of a Charged Thin Spherical Shell The calculation of the field outside the shell is identical to that of a point charge The electric field inside the shell is E = 0 MU29T23

  9. Electric Field of a Nonconducting Plane Sheet of Charge Total charge Q is uniformly distributed over surface A Charge density σ = Q/A Use a cylindrical Gaussian surface The flux through the ends is EA, no field through the curved part of the surface Note, the field is uniform

  10. Electric Field of a Nonconducting Plane Sheet of Charge, cont. The field must be perpendicular to the sheet The field is directed either toward or away from the sheet

  11. Parallel Plate Capacitor The device consists of plates of positive and negative charge The total electric field between the plates is given by The field outside the plates is zero

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