MasteringPhysics: Assignment Print View
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i 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
To understand the analogy between water pressure, water flow, voltage, and current
As suggested by the fact that we call both
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
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 not displayed
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
The overall flow of water that enters the system before the fork is equal to