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Announcements

Announcements. Homework for tomorrow… (Ch . 30, CQ3, Probs. 3, 6, & 7) 29.24: 2.0 x 10 -10 F 29.52: 25 μ F 29.53: Q 1 =Q 2 = Q 3 = 6.0 x 10 -5 C, Δ V 1 = 5.0 V , Δ V 2 = 15 V , Δ V 3 = 10 V Office hours… MW 12:30-1:30 pm TR 9-10 am F 10-11 am

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Announcements

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  1. Announcements • Homework for tomorrow… (Ch. 30, CQ3, Probs. 3, 6, & 7) 29.24: 2.0 x 10-10 F 29.52: 25 μF 29.53: Q1=Q2=Q3 = 6.0 x 10-5 C, ΔV1 = 5.0 V, ΔV2 = 15 V, ΔV3 = 10 V • Office hours… MW 12:30-1:30 pm TR 9-10 am F 10-11 am • Tutorial Learning Center (TLC) hours: M 8-1 pm, 3-6 pm, W 8-1 pm, 2-6 pm TR 8-6 pm, F 8-2 pm

  2. Chapter 30 Current & Resistance (Creating a Current)

  3. Review… • Energy stored in a capacitor… • Energy density of a capacitor… • Electron current… Drift velocity e-number density cross-sectional area

  4. i.e. 30.1:The size of the electron current What is the electron current in a 2.0 mm diameter copper wire if the electron drift speed is 1.0 x 10-4 m/s? Given: ne=8.5 x 1028 m-3 for copper.

  5. Discharging a capacitor… How long does it take to discharge the capacitor?

  6. 30.2:Creating a Current • Q: What creates a current?

  7. 30.2:Creating a Current • Q: What creates a current? • Q: But a conductor in electrostatic equilibrium has an E-field of ZERO inside a conductor?

  8. 30.2:Creating a Current • Q: What creates a current? • Q: But a conductor in electrostatic equilibrium has an E-field of ZERO inside a conductor? • Q: If there is a non-zero E-field, then there is a non-zero F, so shouldn’t my electrons accelerate? • instead of move at a constant drift velocity, vd?

  9. Establishing an E-field in a Wire Notice: • conductors are in electrostatic equilibrium. • E = 0 inside the wire, all excess charge resides on the surface. • Surface charge density is uniform.

  10. Establishing an E-field in a Wire Notice: • Within Δt ~ 1 ns, sea of electrons shift slightly. • conductors are NOT in electrostatic equilibrium. • Surface charge density is no longer uniform. • Non-zero E-field inside the wire. • E-field creates a current.

  11. Establishing an E-field in a Wire Notice: • Surface charges are NOT the moving charges. • ie (electron current) is inside the wire, NOT on the surface.

  12. i.e. 30.2: The surface charge on a current-carrying wire Consider a typical E-field strength of 0.01 V/m. Two 2.0 mm diameter rings are 2.0 mm apart. They are charged to ±Q. What is Q?

  13. A Model of Conduction Q: If there is a non-zero E-field, then there is a non-zero F, so shouldn’t my electrons accelerate? • instead of move at a constant drift velocity, vd?

  14. A Model of Conduction Q: If there is a non-zero E-field, then there is a non-zero F, so shouldn’t my electrons accelerate? • instead of move at a constant drift velocity, vd?

  15. A Model of Conduction Q: If there is a non-zero E-field, then there is a non-zero F, so shouldn’t my electrons accelerate? • instead of move at a constant drift velocity, vd? And so the electron current is..

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