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online assignment 5-4-1

# online assignment 5-4-1 - Assignment 15 Due 11:00pm on...

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Assignment 15 Due: 11:00pm on Friday, December 4, 2009 Note: To understand how points are awarded, read your instructor's Grading Policy . [Return to Standard Assignment View ] Electricity and Water Analogy Learning Goal: To understand the analogy between water pressure, water flow, voltage, and current As suggested by the fact that we call both currents, the flow of charged particles through an electrical circuit is analogous in some ways to the flow of water through a pipe. When water flows from a small pipe to a large pipe, the flow (measured, for instance, in gallons per minute) is the same in both pipes, because the amount of water entering one pipe must equal the amount leaving the other. If not, water would accumulate in the pipes. For the same reason, the total electric current is constant for circuit elements in series. Water pressure is analogous to total electric potential (voltage), and a pump is analogous to a battery. Water flowing through pipes loses pressure, just as current flowing through a resistor falls to lower voltage. A pump uses mechanical work to raise the water's pressure and thus its potential energy; in a battery, chemical reactions cause charges to flow against the average local electric field, from low to high voltage, increasing their potential energy. Part A Consider the following water circuit: water is continually pumped to high pressure by a pump, and then funnelled into a pipe that has lower pressure at its far end (else the water would not flow through the pipe) and back to the pump. Two such circuits are identical, except for one difference: the pipes in one circuit have a larger diameter than the pipes in the other circuit. Through which circuit is the flow of water greater? ANSWER: Small pipe Large pipe Correct The cross sectional area of the pipe is analogous to the area of a wire: the smaller the area the higher the resistance and the more the pipe/wire impedes flow. If the change in pressure (proportional to the potential energy per unit mass) of water traveling through two pipes is the same, the flow will be less through the pipe with smaller cross sectional area. The electrical analog is Ohm's law , where resistance is inversely proportional to the area of a wire/resistor. Part B Now consider a variant on the circuit. The water is pumped to high pressure, but the water then faces a fork in the pipe. Two pipes lead back to the pump: large pipe L and small pipe S. Since the water can flow through either pipe, the pipes are said to be in parallel : The overall flow of water that enters the system before the fork is equal to _____ Hint B.1 Water conservation Hint not displayed ANSWER: the flow through pipe L. the sum of the flows through L and S. the average of the flows through L and S. Correct Part C What can you say about the drop in potential energy (per unit mass or volume) of water traveling through either pipe?

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