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QUICK QUIZ 27.1 You are driving along a dark country road at night when you start experiencing car trouble. You pull over, open the hood and try to figure out what is wrong with the car. You dont have a flashlight in the car but you do find a regular household light bulb in the trunk. The bulb is labeled 100 Watts. You manage to find some wires and connect the light bulb to the 12-volt car battery. The power delivered to the light bulb will be approximately: a) 1 Watt, b) 10 Watts, c) 100 Watts, d) 1000 Watts, or e) Watts.

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QUICK QUIZ 27.1 ANSWER (a). A 100 Watt household light bulb is designed to operate at 100 Watts with 120 volts normal house voltage. If you reduce the voltage by a factor of 10, the power is reduced by a factor of 100 according to the equation P = V 2 /R, since the resistance of the light bulb remains roughly constant.

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QUICK QUIZ 27.2 You have the opportunity of using one of two power supplies for a certain application. Power supply A has a maximum voltage capacity of 1000 volts and a maximum current capacity of 0.1 amps. Power supply B has a maximum voltage capacity of 10 volts and a maximum current capacity of 10 amps. Either power supply would be suitable for your application but you would like to choose the power supply that is the safest. Your lab partner tells you that it is the current that kills you, not the voltage and suggests you pick the one with the lower current capacity. In the interest of safety you should, a) pick power supply A, b) pick power supply B, or c) pick either power supply - they are equally safe.

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QUICK QUIZ 27.2 ANSWER (b). It is true that it is the current that kills you, but a large current cannot travel through your body if the voltage across your body is small. You may think of yourself as a giant resistor. Typical human resistances can be in the neighborhood of to ohms, depending on the situation. A current of 0.1 A is generally lethal. For power supply B, with a maximum voltage across you of 10 volts and a moderate resistance of ohms, the current through you would be: I = V/R = 10/ = A, quite safe. For power supply A, with 1000 volts across you and a resistance of ohms, the current would be 0.1 A – generally lethal.

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QUICK QUIZ 27.3 Three identical resistors (A, B, and C) are shown with the voltage on each end of each resistor indicated. The relationship between the magnitude of the currents through the three resistors is a) I A > I B > I C, b) I C > I B > I A, c) I A > I C > I B, d) I B > I A > I C, or e) I B > I C > I A.

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QUICK QUIZ 27.3 ANSWER (e). Recall that V in Ohms law, I = V/R, refers to the voltage difference between the two ends of the resistor. This difference is greatest for resistor B and least for resistor A.

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QUICK QUIZ 27.4 Of the materials listed in Table 27.1, the material that would make the most sensitive thermometer would be a) silver, b) quartz, c) nichrome, or d) silicon.

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QUICK QUIZ 27.4 ANSWER (d). Sensitivity refers to the ability of a device or instrument to detect small changes in some physical parameter. The most sensitive thermometers are made from materials that have some physical property that changes dramatically with small changes in temperature. The temperature coefficient of resistivity determines how dramatically the resistivity changes with temperature. From the materials in the chart, this coefficient has the greatest magnitude for silicon.

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You perform a simple experiment on a light bulb. You steadily increase the voltage across the light bulb as you measure the current through the bulb. In addition, as you increase voltage, you note that the temperature of the bulb increases. Which curve below would best represent your actual data? QUICK QUIZ 27.5

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QUICK QUIZ 27.5 ANSWER (c). As you increase voltage, the temperature of the bulb increases and the resistance of the bulb increases. Since the slope of the graph is I/ V = 1/R, the slope of the graph will decrease as the resistance increases. Graph c best illustrates this dependence.

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