ism_ch21

# ism_ch21 - Chapter 21 Electric Current and Direct-Current...

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Chapter 21 Electric Current and Direct-Current Circuits Answers to Even-numbered Conceptual Questions 2. No. An electric current is produced when a net charge moves. If your body is electrically neutral, no current is produced when you walk. 4. Yes. There is a net charge on the comb, and by moving it from one place to another you have created an electric current. 6. Car headlights are wired in parallel, as we can tell by the fact that some cars have only one working headlight. 8. The equivalent resistance decreases because there are now more paths through which the electric current can flow. 10. Material A satisfies Ohm’s law because the relationship between current and voltage is linear. 12. Yes. Just connect two of these resistors in parallel and you will have an equivalent resistance of R /2. 14. Assuming the lights are connected to the same potential difference, V , the resistance of the lights can be compared using P = V 2 / R . Since light A has twice the power of light B, it must also have half the resistance of light B. 16. Resistors connected in parallel have the same potential difference across their terminals. 18. When the switch is closed, light 2 is shorted out and the equivalent resistance of the circuit drops from 3 R to 2 R . It follows that the current in the circuit increases. 20. Originally, light 3 is dark; after the switch is closed, light 3 is illuminated. The intensity of lights 1 and 2 are unaffected by closing the switch; after all, they still have the same potential difference and the same resistance. Therefore, it is clear from P = V 2 / R that they will continue to dissipate the same amount of power. 22. The total power dissipated in the circuit increases when the switch is closed. Before the switch was closed light 1 dissipated the power P = ε 2 / R , as did light 2. Now all three lights dissipate the power P = 2 / R . 24. Resistivity is an intrinsic property of a particular substance. In this sense it is similar to density, which has a particular value for each particular substance. Resistance, however, is a property associated with a given resistor. For example, the resistance of a given wire can be large because its resistivity is large, or because it is long. Similarly, the weight of a ball can be large because its density is large, or because it has a large radius. 26. If two heaters, each with resistance R , are connected in series, the equivalent resistance is now 2 R . The heaters are connected to the same potential, however, and therefore they draw half the original current. From P = I 2 R we can see that each heater now dissipates one quarter the power of the single heater, for a total power consumption of one half the original value. 56

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Physics: An Introduction Chapter 21: Electric Current and Direct-Current Circuits 28. A number of factors come into play here. First, the bottom of a bird’s foot is tough, and definitely not a good conductor of electricity. Second, and more important, is the fact that a potential difference is required for there to be a flow of current. Just being in contact
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## This note was uploaded on 04/08/2008 for the course PHYS 104 taught by Professor Pengyi during the Fall '08 term at UNC.

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ism_ch21 - Chapter 21 Electric Current and Direct-Current...

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