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E5 - Current

E5 - Current. So far we’ve looked at stationary charges – electrostatics Now we’re ready for moving charges!!!. Drude Model. When an electric field is present, the electron’s collisions with atoms results in a drift velocity opposite to the direction of the electric field.

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E5 - Current

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  1. E5 - Current • So far we’ve looked at stationary charges – electrostatics • Now we’re ready for moving charges!!! Current & Resistance

  2. Drude Model • When an electric field is present, the electron’s collisions with atoms results in a drift velocity opposite to the direction of the electric field Current & Resistance

  3. Current Density and conductivity • Current density indicates how rapidly charge is flowing near a certain point in a conductor • Where ρ is the density of charge carriers, and n is the number density of charge carriers • Current density is proportional to the electric field • σc is the conductivity • Note that σ ≠ σc Current & Resistance

  4. Current Flow • Current is the total rate at which charge crosses a boundary • Unit of current is the ampere • See Table E5.1 Current & Resistance

  5. Microscopic View of Current • Current is the amount of charge in coulombs passing a given point per second. • How fast are individual electrons moving? • Electrons move due to electric field inside wire, but collisions with other particles slow them down. • e- are moving to the left. Which way is E? Which way is I? Current & Resistance

  6. Electric Current • conventional current - direction a positive charge would flow Current & Resistance

  7. Surface Charges Direct Flow • Surface charges in a conductor move so that current is constant Current & Resistance

  8. Resistance • Resistance of a conducting object • Where Δϕ is the potential difference between the ends • Unit of resistance is the ohm Current & Resistance

  9. For a metal conductor Ohm () = unit of resistance ohm’s law Ohm’s Law: Resistors metal nonohmic device resistance Current & Resistance

  10. Power Dissipated in a Conductor • Power is the rate at which electrostatic potential is converted to thermal energy • Unit of power is the watt = J/s = N m/s = kg m3/s3 Current & Resistance

  11. Discharging a Capacitor • The potential difference VC at any time t • Where R is the resistance and C is the capacitance Current & Resistance

  12. Discharging a Capacitor Current & Resistance

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