second midterm review

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

This preview shows pages 1–2. Sign up to view the full content.

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.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

## 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.

### Page1 / 4

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

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online