General Physics 2Current & Resistance1 E5 - Current So far we’ve looked at stationary charges – electrostatics Now we’re ready for moving charges!!!

General Physics 2Electric Field2 Motion of q in a uniform E field The figure shows an electron entering a uniform electric field of N/C. What is the acceleration of the e - once inside the field? m e = 9.11 x kg q e = x C

General Physics 2Electric Field3 Motion of q in a uniform E field If the electron is initially moving with a horizontal velocity of 3.0 x 10 6 m/s, and l = 20 cm, what is the magnitude and direction of the electron’s velocity as it leaves the parallel plates? m e = 9.11 x kg q e = x C

General Physics 2Current & Resistance4 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

General Physics 2Current & Resistance5 Drift Velocity The average drift speed is proportional to E In between collisions, the acceleration is proportional to the electric field

General Physics 2Current & Resistance6 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 σ c is the conductivity Note that σ ≠ σ c

General Physics 2Current & Resistance7 Current Flow Current is the total rate at which charge crosses a boundary Unit of current is the ampere See Table E5.1

General Physics 2Current & Resistance8 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?

General Physics 2Current & Resistance9 Electric Current conventional current - direction a positive charge would flow

General Physics 2Current & Resistance10 Surface Charges Direct Flow Surface charges in a conductor move so that current is constant

General Physics 2Current & Resistance11 Resistance Resistance of a conducting object Where Δ ϕ is the potential difference between the ends Unit of resistance is the ohm

General Physics 2Current & Resistance12 Ohm’s Law: Resistors For a metal conductor Ohm (  ) = unit of resistance metal nonohmic device ohm’s law resistance

General Physics 2Current & Resistance13 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 m 3 /s 3

General Physics 2Current & Resistance14 Plug & Chug (a) Determine the resistance of, and current through, a 75-W lightbulb connected to its proper source voltage of 120 V.

General Physics 2Current & Resistance15 Think-Pair-Share You buy a 75-W lightbulb in Europe, where electricity is delivered to homes at 240 V. If you use the lightbulb in the US at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V bulbs? [Hint: assume that brightness is proportional to power consumed.]

General Physics 2Current & Resistance16 Think-Pair-Share How many kWh of energy does a 550-W toaster use in the morning if it is in operation for a total of 15 min? At a cost of 9.0 cents/kWh, estimate how much this would add to your monthly electric bill if you made toast four mornings per week.

General Physics 2Current & Resistance17 Discharging a Capacitor The potential difference V C at any time t Where R is the resistance and C is the capacitance

General Physics 2Current & Resistance18 Discharging a Capacitor

General Physics 2Current & Resistance19 Group Problems Exploration of Physics E&M Conduction Answer questions in hints tab Capacitor activity sheet E5B.2 E5B.11 E5S.5