T2+-+K - PHY 121 Test 2 Key Explanations are provided only...

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PHY 121 – Test 2 – Key Fall 2011 Explanations are provided only for questions requiring quantitative answers. 1. Different types of light bulbs are connected to a battery. The bulb with the least electrical resistance will drive the a) least amount of current. b) greatest amount of current c) least amount of energy d) least amount of electric power e) least amount of potential difference 2. Which of the statements below is false about the electric circuit described in the previous problem? a) The resistance of the bulbs makes necessary the presence of the battery in order to circulate the current. Otherwise, if the resistance were zero, the current would flow without a battery. b) In order to keep the current constant, the supplied energy (and so the voltage) must stay constant c) The battery uses its internal chemical energy to do work per transported charge, given by its difference of potential The bulb glows due to the energy released as the electrons make their way through the filament e) The current decays in time (and the bulb gets dimmer and dimmer) since all the electrons in the battery are gradually depleted 3. A bulb is connected across a battery and the current around the circuit is measured to be 3 amperes. Knowing that the difference of potential delivered by the battery is 12 Volts, what is the resistance of the bulb? a) 4 ohms b) 36 ohms c) 0.25 ohms 180 ohms e) It's impossible to know, since the difference of potential will be substantially modified by the fact that the bulb carries a current depleting the battery of electrons 4. A 100 watt bulb is connected across a 10 volt car battery. Recall that, while the voltage of the battery describes its ability to supply energy, the power of the device connected across the battery describes its ability to use that energy. That is, the power of the device should be both proportional to voltage (energy available) and current (how fast the energy is used). So, what is the current through the circuit? a) 3.6 kilowatt-hour b) 3.6 amperes c) 10 amperes 1000 amperes e) 0.1 amperes 5. When electric power is transmitted from the generating site to a community, the company tries to keep the electric current small along the power lines by using transformers. Why? a) To make the electrical resistance of the wires small. b) To minimize the energy lost due to heating of the transmission wires. c) To speed up the transmission of the electric power. To protect the birds that might perch on the transmission wires, and preserve the environment. e) To minimize magnetic energy losses. Explanation : According to Ohm s law, the resistance of the bulb is given by 12 V 4 3 A V R I Explanation : Using a suitable expression for the power dissipated by the bulb, we get 100 W 10 A 10 V P P IV I V  
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PHY 121 – Test 2 – Key Fall 2011 6. Electric utilities, in an effort to reduce energy loss in transmission wires, employ ____ to step-up the ____ at the power plant to a very high level. (Similar devices are employed at the home to reverse this process.) a) transformers, voltage b)
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T2+-+K - PHY 121 Test 2 Key Explanations are provided only...

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