week 7 notes 222
week 7 notes 222 Phys 222
U of L
Popular in Fundamentals of Physics II
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This 4 page Class Notes was uploaded by Mary-elizabeth Notetaker on Thursday October 6, 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 6 views. For similar materials see Fundamentals of Physics II in Physics at University of Louisville.
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Date Created: 10/06/16
Week 7 Thursday, October 6, 2016 11:07 AM • Nichrome wire from last week melted and recast w 2x original length. Find Rn as multiple of old resistance Ro Suppose electrical wire replaced w one having every linear dimension doubled With higher T, atoms vibrate leading to thermal scattering of electrons Electrons find it more diff to pass thru atoms bc of more collisionsw atoms Increased e- scattering w inc T= inc R For most metals, R inc approx linearly w T over limited T range Ex) platinum resistance thermometer-measures T by measuring change in resistance of conductor ○ Principle of thermometer measured by resistance formula above ○ When device is immersed in vessel containing melting indium, its resistance inc to 76.8 ohms(Rmp). Find the mp(Tmp) of indium (RoTo= 50 ohms at 20C) ○ Indium heated further until reaches 235C, what is ratio of new current I in platinum to current Imp at mp? Back to resistance formula…. Measure T coeff of resistivity Electrical E in a circuit… power Electrical E in a circuit… power As charge moved thru battery from A to B, pot E of system inc by: … Chem E of battery dec by same amt As charge moved thru resistor, from C to D, it loses E in collisions w the atoms in the resistor When charge returns to A, net result is some chem E of battery delivered to resistor and caused T inc …rate of E lost is power: From ohms law(delta V=IR), alternate power forms: Power unit: Watts ….I in Amperes, R in ohms, and voltage in V Kilowatt-hour: amt of time power is supplied 1kWh=3600s x 1000W=3.6x10 J 6 Ex) V applied across ends of nichrome heater wire w A and L… same V applied across ends of second nichrome heater wire w A and 2L. Which wire gets hotter? Shorter wire does bc R inc so P dec Ex) rank currents: Current proportional to power since pot diff same Current btwn pos/neg higher than thru other resistors in circuit bc power splits btwn other connected resistors but all goes back thru pos/neg part Ex) diameter of A > diameter B… lengths and resistivities are equal. For given V diff across ends, relationship btwn Pa and Pb? Pa>Pb bc diameter a larger so power dec Ex) light A 25W 120V and light B is 100W 120V…. Diff power when connected to constant 120V source Resistance of each? During what time interval does a 1C charge goes into bulb A? In what time interval does 1J pass into light A? Cost of running light A contn=inuously for 30 days assuming company sells at .11 per kWh? Superconductors- direct drop in resistance w/out slowly going down… dec to 0 ○ Resistance os same as normal metal above certain T Tc ○ Suddenly drops to 0 at Tc
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