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PHYS 241 Recitation

PHYS 241 Recitation. Kevin Ralphs Week 3. Overview. HW Questions Flux/Gauss’s Law Conductors vs Insulators. Flux/Gauss’s Law. History The 18 th century was very productive for the development of fluid mechanics

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PHYS 241 Recitation

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  1. PHYS 241 Recitation Kevin RalphsWeek 3

  2. Overview • HW Questions • Flux/Gauss’s Law • Conductors vs Insulators

  3. Flux/Gauss’s Law • History • The 18th century was very productive for the development of fluid mechanics • This lead physicists to use the language of fluid mechanics to describe other physical phenomena • Mixed Results • Caloric theory of heat failed • Electrodynamics wildly successful

  4. Gauss’s Law • What does it tell me? • The electric flux (flow) through a closed surface is proportional to the enclosed charge • Why do I care? • You can use this to determine the magnitude of the electric field in highly symmetric instances • Flux through a closed surface and enclosed charge are easily exchanged

  5. 3 Considerations for Gaussian Surfaces Gauss’s law is true for any imaginary, closed surface and any charge distribution no matter how bizarre. It may not be useful, however. • The point you are evaluating the electric field at needs to be on your surface • Choose a surface that cuts perpendicularly to the electric field (i.e. an equipotential surface) • Choose a surface where the field is constant on the surface *Note this requires an idea of what the field should look like

  6. Common Gauss’s Law Pitfalls • Your surface must be closed • The charge you use in the formula is the charge enclosed by your surface • The Gaussian surface need not be a physical surface • Start from the definition of flux and simplify only if your surface/field allows it

  7. Conductors vs Insulators • Conductors • All charge resides on the surface, spread out to reduce the energy of the configuration • The electric field inside is zero • The potential on a conductor is constant (i.e. the conductor is an equipotential) • The electric field near the surface is perpendicular to the surface Note: These are all logically equivalent statements

  8. Conductors vs Insulators • Insulators • Charge may reside anywhere within the volume or on the surface and it will not move • Electric fields are often non-zero inside so the potential is changing throughout • Electric fields can make any angle with the surface

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