Conduction and Ohm's LawProf. Stephen Sekula9/29/2010Supplementary Material for PHY1308 (General Physics – Electricity and Magnetism)Announcements●Homework 6:●Due next Monday by 5pm●In-class quiz●Friday, covering homework 5 (capacitors)●Exam grading●Ongoing, expect results later todayExtra Credit #2●Attend the Physics Research Fair●Friday, 2:30-5:00 p.m. in Fondren Science 127●Undergraduate Research●Talk to the presenters●format: poster presentations with speaker-driven narratives●No speaker for the poster you want to use? Contact the author and interview them outside of the fair. CITE THE INTERVIEW.●Write a 1-2 page paper●http://www.physics.smu.edu/sekula/phy1308/PHY1308_PaperGuidelines.pdfGeorg Simon Ohm1789-1854German physicist who began his studies using the newly developed “electrochemical cell” (battery) invented by Italian Count Allesandro Volta.Published in 1827 his discovery of a relationship between current and voltage.Conductor Economics●Commodity Prices●Gold: ~$1300/troy ounce●Copper: ~$4/troy ounce–1 troy ounce = ~31 gramsExample: copper wire●How much potential (voltage) is needed to drive 15A of current through a 1.8mm diameter copper wire?●copper resistivity: 1.68 10⨯-8 Ωm●what do you think? Big voltage or little voltage?Example: copper wire●How much potential (voltage) is needed to drive 15A of current through a 1.8mm diameter copper wire?●copper resistivity: 1.68 10⨯-8 Ωm●each meter of wire needs only 99mV of potential to drive 15AE =J =I A=99 mV/mEXAMPLE: Copper wire●Flip the problem around: how much current can a 120V potential drive through 1.0 of such wire?●Answer: about 18,000A●Yikes!●Might it be bad to remove the appliance from the end of its electrical cable and then short the cable?EXAMPLE: Copper wire●How much power is being dissipated in the copper wire when it's shorted?●Answer:●What does this equal? House fire!●NEVER SHORT CIRCUIT YOUR HOUSE WIRING!!!P =IV =1.8×104A120 V =2.2×106J/s=2.2 Million WattsEXAMPLE: HUMAN●You accidentally stick one finger from each hand in the slots of an electrical outlet (120V). How much current goes through your body under perfectly dry conditions?●Answer:● Will you feel this? Is it enough to kill?I =V / R= 120 V/105=1.2 mAUseful numbers●Current and human biology●0.5-2.0 mA–threshold of sensation●10-15 mA–involuntary muscle contractions; can't let go●15-100 mA–Severe shock; muscle control lost; breathing difficult●100-200mA–Fibrillation of heart; death within minutes●>200 mA–Cardiac arrest; breathing stops; severe burnsDon't TaSe me, bro!●What are the properties of a Taser (stun gun)?●Voltages in the range 50-1000 kV●Delivered to the body? More like 1500V●What current goes through your body?●ANSWER: –That's about 15mA – in the range to
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