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PHYS 219 Recitation

PHYS 219 Recitation. Kevin Ralphs Week 3. Overview. HW Questions Dielectrics Conductors vs Insulators Current Resistance. HW Questions. Ask Away…. Dielectrics.

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PHYS 219 Recitation

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  1. PHYS 219 Recitation Kevin Ralphs Week 3

  2. Overview • HW Questions • Dielectrics • Conductors vs Insulators • Current • Resistance

  3. HW Questions Ask Away….

  4. Dielectrics • Put simply, a dielectric is a material (an insulator) that in all, but very rare cases weakens the electric field around it • This allows more charge to be placed on the plates for the same voltage (i.e. capacitance is increased) • The permittivity of a dielectric tells you how it affects the capacitance • The ratio of the permittivity of a dielectric and the permittivity of free space is the dielectric constant

  5. 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 and valid ONLY when the charges are in equilibrium

  6. 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

  7. Current • What does it tell me? • The amount of charge flowing through a boundary • The unit of measure is the ampere: • The word “flow” implies there should be an equation similar to flux that describes this • is the drift velocity. It is the average velocity of the charge carriers

  8. Resistance • What does it tell me? • The ratio of the potential drop in the “direction” of the current and the current in a segment and is measured in ohms () • Essentially it is telling you how tough it is to push charge through an object • Why do I care? • All things have resistance so it is critical to understand how it affects electric current • The resistor is another one of our linear electronic components

  9. Resistivity • The resistivity () tells us how easy it is to push charge through a material, regardless of its dimensions

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