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Eðlisfræði 2, vor 2007
25. Current, Resistance, and Electromagnetic Force
Assignment is due at 2:00am on Wednesday, February 14, 2007
Credit for problems submitted late will decrease to 0% after the deadline has passed. The wrong answer penalty is 2% per part. Multiple choice questions are penalized as described in the online help. The unopened hint bonus is 2% per part. You are allowed 4 attempts per answer.
Current as Charge Flow
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: Answer not displayed 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 _____
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MasteringPhysics: Assignment Print View
Hint B.1 Water conservation Hint not displayed ANSWER: Answer not displayed Part C Part not displayed Part D Consider a new circuit: water is pumped to high pressure and fed into only one pipe. The pipe has two distinct segments of different diameters; the second half of the pipe has a smaller diameter than the first half: Which of the following statements about the flow and change in pressure through each segment is true?
ANSWER: Answer not displayed
Capacitor Supplies Current to Bulb
A large capacitor is charged with on one electrode and on the other. At time , the capacitor is connected in series to two ammeters and a bulb. The ammeter connected to the positive side of the capacitor reads , and the ammeter connected to the negative side of the capacitor reads . Both ammeters will read positive if current flows in a clockwise sense through the circuit (from the + to the - terminal of the meter).
Part A Immediately after time , what happens to the charge on the capacitor plates?
a. Electrons flow through the circuit from the positive to the negative side of the capacitor. b. Electrons flow through the circuit from the negative to the positive side of the capacitor. c. The positive and negative charges attract each other, so they stay in the capacitor. d. Current flows...