week 6 222
U of L
Popular in Fundamentals of Physics II
Popular in Physics
This 4 page Class Notes was uploaded by Mary-elizabeth Notetaker on Thursday September 29, 2016. The Class Notes belongs to Phys 222 at University of Louisville taught by Dr. Ming Yu in Fall 2016. Since its upload, it has received 4 views. For similar materials see Fundamentals of Physics II in Physics at University of Louisville.
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Date Created: 09/29/16
Week 6 Thursday, September 29, 2016 11:13 AM If there is no EF inside conducting material, e-'s move randomly If field is present, e- force F=-eE imposes small drift on e-'s motion Zig-zag line represent motion of charge carrier in conductor ○ Avg speed: ○ Drift speed: (much smaller) ○ Current (I) is opp dir of net displacement of e-'s Macroscopic currents can be related to motion of microscopic charge carriers making up the current ○ Current depends on: avg drift speed of charge carriers in the dir of the current # of charge carriers per unit volume Size of charge carried by each ○ Consider identically charged particles moving thru conductor of cross-sectional area A x is displacement of charge carriers moving in t N is # of charge carriers per unit V N=n x is the total # charge carriers Total charge is # carriers x charge per carrier(q) Ex)Wire w cross sectional area that gradually dec along wire so wire has shape of long cone, if I constant: ○ How does Vd vary along wire? Speeds up as cross sectional area becomes smaller Bc I constant, if A dec, Vd must inc to make up for it Ex) copper wire w A=3x10^-6 carrier I=10A.. Each copper atom contributes 1 free e- to metal, find Vd of e-'s in the wire(density of copper is 8.92g/cm^3 and atomic mass is 63.6u Circuits ○ Switched on, bulb bright.. When switched on, current flows Current across bulb when bulb is connected to battery in closed path Circuit diagram ○ Battery pumps charge thru bulb and around loop ○ Charge flows from pos terminal into one side of bulb and out other side, thru conducting wires back to neg terminal of battery ○ No charge would flow w/out complete conducting path ○ .. Must characterize how bulb works in diff situations Need current in bulb.. Ammeter measures current Need current in bulb.. Ammeter measures current □ Must be connected in series □ All charge passing thru bulb must also pass thru ammeter Pot diff across bulb □ Voltmeter measures voltage (pot diff) □ Must be in parallel… pot diff across bulb is same as pot diff across voltmeter Resistance Pot inc means strong field, so F strong, so Vd inc, which is proportional to current … relation btwn V and I is proportional Constant of proportionality is resistance of conductor R Ohm's law: Emperical relationship is valid only for certain materials… Materials that obey Ohm's law are ohmic Materials w resistance that changes w V or I are nonohmic ..for ohmic device, resistance is constant over wide range of Vs Relationship btwn current and V is linear Slope is related to resistance Ohmic: For nonohmic: Diode- common nonohmic ex V-I relationship nonlinear Resistance is characteristic of mat and indep of current and pot diff Origin of resistance: e- don’t move in straight line path thru conductor ..undergo repeated collisions w metal atoms ..collisions randomize e- velocity, reducing it in dir opp of Vd Internal friction- results from force of repeated collisions (origin of mat's resistance) So: resistance in circuit arises from collisionsbtwn e-'s carrying current w the fixed atoms in the conductor e- undergo more collisions in a longer conductor Smaller cross sectional area slows electric flow Resistance R of ohmic conductor is proportional to its length along current(L) and inverselt proportional to its cross sectional area(A) is the constant of proportionality and is called resistivity of the material (in Ex) calc resistance per unit lenth of nichrome wire of radius .32 nm If pot diff is 10V is maintained across 1m of wire, what is current?
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