second midterm review

second midterm review - Charge is conserved: Current...

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1 1 Midterm II Review Exam Tomorrow For Thursday’s lecture: HRW 30.1-30.5 We’ll be starting: Induction and Faraday’s Law 2 Current •C h a r g e i s c o n s e r v e d : I 1 =I 2 +I 3 I 1 I 3 I 2 r r I= J dA I J d A = JA, for a uniform current The current density is the current per unit area. It gives the local direction of current flow. d J=env Microscopically, the current density results from the slow drift of many free electrons dq I = , q = Idt dt Current is the positive charge passing through a wire cross section per unit time. 3 Resistivity and Resistance Resistance is due to electrons colliding with vibrating nuclei r r ρ = E J Resistivity: a property of a material Resisitance: a property of macroscopic objects = R V I I V L A Ohm’s Law ρ L A == EL JA For a wire of constant cross section 0 U=Vdq The energy lost by a bit of charge passing through the resistor is 2 2 V P=IR or P= or P=IV R the voltage drop across R ρ T 0 Conductors ⎛⎞ Δρ αΔ = ⎜⎟ ρ ⎝⎠ 0 T 4 EMF Devices I V ab int -IR -Ir + ε =0 int ε I= R+r ab ba V= - I R R ε r int a b Non-ideal EMF device : ε depends on the circuit, because there is internal resistance to the movement of charge. charge gains energy in a battery Add + ε going from – to +. Add - ε going from + to -. charge loses energy in a resistor When passing through a resistor, add IR in the direction of current, + I R opposite the direction of current.
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This note was uploaded on 04/05/2008 for the course PHYS 212L taught by Professor Huang,tai-yin during the Spring '07 term at Penn State.

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second midterm review - Charge is conserved: Current...

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