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Physics 122B Electricity and Magnetism

Physics 122B Electricity and Magnetism. Lecture 15 (Knight: 31.5 to 31.8) Resistors in Circuits April 30, 2007 . Martin Savage. Lecture 15 Announcements. Lecture HW Assignment #5 has been posted on the Tycho system and is due at 10 PM on Wednesday.

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Physics 122B Electricity and Magnetism

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  1. Physics 122B Electricity and Magnetism Lecture 15 (Knight: 31.5 to 31.8) Resistors in Circuits April 30, 2007 Martin Savage

  2. Lecture 15 Announcements • Lecture HW Assignment #5 has been posted on the Tycho system and is due at 10 PM on Wednesday. • The second midterm exam is this Friday. It will cover everything up to the end of Wednesdays lecture, emphasizing the most recent material, but assumes understanding of all material inclusive. Physics 122B - Lecture 15

  3. Units of Power 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 0.9x106 J of energy, which adds about 1.8¢ to your electric bill. Physics 122B - Lecture 15

  4. Question Which resistor dissipates the most power? Physics 122B - Lecture 15

  5. Bulbs in Series Question: How does the brightness of bulb A compare withthat 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 dropacross B and across C is E/2. Physics 122B - Lecture 15

  6. Resistors in Series Physics 122B - Lecture 15

  7. Example:A Series Resistor Circuit Physics 122B - Lecture 15

  8. Ammeters x x I = ? Answer: You mustbreak the circuit and insert an ammeter into the line of current flow. Question: How do you measure the current in a circuit? Ideal Ammeter: To have a minimumeffect 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. Note:“Clip on” ammeters that measure AC current without breaking the circuit are commercially available. They use magnetic induction (see L20). Physics 122B - Lecture 15

  9. 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 rint. 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 rint. A regulated electronic power supply provides a very good approximation to a zero-resistance constant-potential ideal battery. Physics 122B - Lecture 15

  10. Real Batteries (2) DVbat Question: How can you measure rint? Answer: One (rather brutal) way is to vary an external load resistance R until the potential drop across R is ½E. Then R=rint because each drops ½E. Physics 122B - Lecture 15

  11. Example:Lighting Up a Flashlight A 6 W flashlight bulb is powered by a 3 V battery having an internal resistance of 1 W. (Assume ideal wires.) What is the power dissipation of the bulb? What is the terminal voltage of the battery? 1/ 2/ Note that in this situation, 14% of the available energy goes into heating the battery rather than providing light from the bulb. Physics 122B - Lecture 15

  12. A Short Circuit What happens if you “short out” a battery, i.e., connect an ideal resistanceless wire across its terminals, so that the potential difference across the terminals becomes zero? Then, Ishort=E /r. In other words, all of the battery’s potential is dropped across its internal resistance. Ishort is the maximum possible current that a battery can supply, and is a measure of the internal resistance r of the battery (r = E / Ishort). Physics 122B - Lecture 15

  13. Example:A Short-Circuited Car Battery What is the short-circuit current of a 12 V car battery with an internal resistance of 0.020 W? What happens to the power supplied by the battery when it is shorted? 0.02 W 12 V All of this power would be dissipated internally in the battery, making it likely to explode. Therefore, do not short out your car’s battery! Physics 122B - Lecture 15

  14. Bulbs in Parallel • Initially, bulbs A and B have the same brightness and C is out. What happens to the brightness of the bulbs when the switch is closed? • Bulb A gets brighter (because the overall current increases); • Bulb B gets dimmer (because current is diverted to Bulb C); • Bulb C glows with the same brightness as B (because they split the current equally). Physics 122B - Lecture 15

  15. Resistors in Parallel Physics 122B - Lecture 15

  16. R1 I I R2 Series vs. Parallel DV R1 R2 Resistors in series divide the overall potential difference between them in proportion to their resistances. Resistors in parallel divide the overall current through them in proportion to their inverse resistances. The current splits and tends to favor “the path of least resistance”. Physics 122B - Lecture 15

  17. Example:A Parallel Resistor Circuit Three resistors are connected across a 9 V battery. Find the current through the battery. Find the potential differences across and currents through each resistor. I I2 I1 I3 DVbat Physics 122B - Lecture 15

  18. Example:A Combination of Resistors Physics 122B - Lecture 15

  19. Voltmeters Question: How do you measure the potential difference between two points in a circuit? Answer: Youcan connect one lead of a voltmeter to each point. Ideal Voltmeter: To have a minimum effect of the circuit being measured, the connected voltmeter must have infinite resistance, so that no current is diverted through the voltmeter. Electronic voltmeters can give good approximations to this condition, but electro-mechanical voltmeters may not. Physics 122B - Lecture 15

  20. V A I I Voltmeters vs. Ammeters An ideal voltmeter has infinite internal resistance. It must be connected between circuit elements to measure the potential difference between two points in the circuit. An ideal ammeter has zero internal resistance. It must be inserted by breaking a circuit connection to measure the current flowing through that connection in the circuit. X Physics 122B - Lecture 15

  21. Question What is the correct order of bulb brightness (from brightest to dimmest)? (The bulbs are identical) (a) A=B=C=D; (b) A>B=C=D; (c) A=B>C=D; (d) A>B>C=D; (e) A>B>C>D Physics 122B - Lecture 15

  22. Example:Analyzing a Complex Circuit (1) Four resistors are connected to a 12 V battery as shown. Find the current through the battery. Find the potential differences across and currents through each resistor. Physics 122B - Lecture 15

  23. Example:Analyzing a Complex Circuit (2) Physics 122B - Lecture 15

  24. End of Lecture 15 • Before the next lecture, read Knight, sections 31.9 through 31.10. • Lecture HW Assignment #5 has been posted on the Tycho system and is due at 10 PM Wednesday. • Midterm 2 is Friday Physics 122B - Lecture 15

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