Phys2212_31.5+to+31.10

Phys2212_31.5+to+31.10 - Physics 2212 Electricity and...

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Unformatted text preview: Physics 2212 Electricity and Magnetism Lecture 15 (Knight: 31.5 to 31.10) Resistors in Circuits 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!) I f 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? 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 10/06/09 Physics 2212 - Lecture 15 6 Example : A Series Resistor Circuit eq 15 4 8 27 R = + + = 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 = - = - = - 10/06/09 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. I deal 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 = ? x x Note: Clip on ammeters that measure AC current without breaking the circuit are commercially available. They use magnetic induction (see L20). 10/06/09 Physics 2212 - Lecture 15 8 Real Batteries (1) An ideal battery provides a potential difference that is a constant, independent of current flow or duration of use. But real batteries sag under load and become weak or dead as their chemical energy is used up. How can we include such effects? A reasonable approximation is to include an internal resistance r i nt . The internal resistance may increase as the battery ages and supplies energy. The rule is that the larger and more expensive the battery, the lower is r i nt ....
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Phys2212_31.5+to+31.10 - Physics 2212 Electricity and...

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