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Agenda

This agenda provides information about today's physics class, including completing Chapter 24, homework due date, and the return of quizzes. It also highlights the topics covered in Chapter 25, such as scalar and vector fields, potential, and capacitance.

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Agenda

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  1. Agenda • Today • Chapter 24 (Finish Fri & Start 25) • HMWK Due Fri • Return Quiz Friday • Reading • Finish ch. 24 by Friday • Skim 25 (at least last page) Friday • Read 25 by Monday

  2. Field vs. Potential Turns Scalar into Vector

  3. Field vs. Potential Gradient: Think Slope of PE plot 3D Fun Potential surrounding + & - charge pair Slope at any given point gives Electric Field

  4. Simple ExampleGravity of course

  5. Simple ExampleGravity of course Force from PE Field is same (w.o. the “m”)

  6. Spherical Case • Normally quite complicated • Wait till calc 3 • Radial component only:

  7. Fields in a material? • Electric Field changes in a material • Example: Metal field = 0 • Insulators change as well • Interaction of neutral object with a field? • Polarization • Polarization takes energy (lining up atoms) • Some of the energy in the field goes to polarizing material • Reduces the effective strength of a field

  8. Mathematically • Electric Field of a point charge • Equivalent Equations • Epsilon (e) is permittivity • How easily field permeates a material • is a material dependent constant • in vacuum is one • in air is one • in materials (insulators) is > 1

  9. Dielectrics • Materials used specifically for their permittivities (e or ke0) • Talked about when dielectric properties are important

  10. Dielectric Breakdown • When electric field too large • Strips electrons from material components • Arcing – shorting – lightning • Damaging to components (& people!)

  11. How many volts in static electricity? • Dielectric Breakdown in air ~ 3x106 V/m • Units for field V/m or N/C (same thing) • Most shocks between mm & cm • Assume field constant • Big || plates = finger & cat for example Field = Breakdown field V ranges from 1 to 10 kV Deadly?

  12. Dielectric Usage • Stores Field energy in form of polarization • Can be used as electrical energy storage • Capacitors

  13. Simplest Form: || Plate Apply Voltage Across Parallel Plates V = Ed E (plate) = s/(2ke0) E (Between) = s/(ke0) s = Q/A Infinite just approximation E = Q/(Ake0) V = Ed V = Qd / (Ake0)

  14. Simplest Form: || Plate Apply Voltage Across Parallel Plates V = Qd / (Ake0) Capacitance = Ability to store Charge from V Q = V C [C is capacitance] V = Q / C || plate: C = Ake0/d Examine parameters Geometric & material only, no field dep.

  15. Energy Storage: Capacitor • Consider how much energy it takes to move charges from one plate to another • “Assemble” Charges • Work done in moving charges stored as Potential Energy

  16. Energy Storage: Capacitor Can Apply Top Equation to Bottom Mix & Match, Q,V,C As usual, ½  Integration

  17. Other Geometries • Spherical, other all have different formulas for capacitance • Universal: • C depends only on dielectric & geometry • Q = VC • PE = Q2/2C

  18. Agenda • Today • Chapter 24 (Finish Fri & Start 25) • HMWK Due Fri • Return Quiz Friday • Reading • Finish ch. 24 by Friday • Skim 25 (at least last page) Friday • Read 25 by Monday

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