Mastering Physics: Electricity and Water Analogy

Mastering Physics: Electricity and Water Analogy

Figure 1

Part A

Question: (Figure 1) 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: Large pipe

Part B

Figure 2

Question: 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: (Figure 2). The overall flow of water that enters the system before the fork is equal to _____

Answer: the sum of the flows through L and S.

Part C

Question: What can you say about the drop in potential energy (per unit mass or volume) of water traveling through either pipe?

Answer: The drop is the same for both pipes.

Part D

Figure 3

Question: 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: (Figure 3). Which of the following statements about the flow and change in pressure through each segment is true?

Answer: The flow through each segment is the same as the overall flow; the sum of the changes in pressure through each segment equals the overall change.