Phys2212_31.5+to+31.8

# Phys2212_31.5+to+31.8 - Physics 122B Electricity and...

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Physics 122B Electricity and Magnetism Lecture 15 (Knight: 31.5 to 31.8) Resistors in Circuits

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10/06/09 Physics 2212 - Lecture 15 2 Units of Power Power units watts W VA J/s = = = L 3 6 Energy units 1 kilowatt-hour (1.0 10 W)(3600 s) 3.6 10 J = × = × L At residential rates, Seattle City Light charges about 7.5¢ for a kilowatt hour of electrical energy, so one million joules ( 1 MJ) of electrical energy costs about 2¢. (Remarkably cheap!) If you operate a 1500 W hair dryer for 10 minutes, you use 0.25 kilowatt hours or 0.9x10 6 J of energy, which adds about 1.8¢ to your electric bill.
10/06/09 Physics 2212 - Lecture 15 3 Question Which resistor dissipates the most power?

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10/06/09 Physics 2212 - Lecture 15 4 Bulbs in Series Question: How does the brightness of bulb A compare with that of bulbs B and C? Answer: Bulb A is brighter than bulb B and bulb C, which are of equal brightness. Reason: The potential drop across bulb A is E , while the potential drop across B and across C is E /2.
10/06/09 Physics 2212 - Lecture 15 5 Resistors in Series ab 1 2 1 2 1 2 ( ) V V V IR IR I R R = ∆ + ∆ = + = + ab 1 2 ab 1 2 ( ) V I R R R R R I I + = = = + eq 1 2 N (series resistors) R R R R = + + + L

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10/06/09 Physics 2212 - Lecture 15 6 Example : A Series Resistor Circuit eq 15 4 8 27 R = Ω + Ω + Ω = eq (9 V) 0.333 A (27 ) I R = = = E R1 1 V (15 )(0.333 A) 5.0 V IR = - = - = - R2 2 V (4 )(0.333 A) 1.33 V IR = - = - = - R3 3 V (8 )(0.333 A) 2.67 V IR = - = - = -
Physics 2212 - Lecture 15 7 Ammeters Question: How do you measure the current in a circuit? Answer: You must break the circuit and insert an ammeter into the line of current flow. Ideal Ammeter: To have a minimum effect on the circuit being measured, the inserted ammeter must have zero resistance , so that there is zero potential difference across the ammeter. Electronic ammeters can give good approximations to this condition, but electro-mechanical ammeters may not. I = ?

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## This note was uploaded on 06/07/2009 for the course PHYSICS 2212 taught by Professor Geist during the Fall '09 term at Georgia Perimeter.

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Phys2212_31.5+to+31.8 - Physics 122B Electricity and...

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