# Homework Help: Electricity and water analogy

1. Feb 6, 2008

### JasonBourneV

Hey guys:

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?

I thought the larger pipe had a greater flow because of its larger volume. Is thhat true?

2. Feb 6, 2008

### chroot

Staff Emeritus
I have moved this to the homework help forum.

Your answer is correct -- a larger pipe presents a smaller resistance. The pump, which is capable of producing some specific pressure differential, will move more current in a loop with lower resistance.

- Warren

3. Feb 6, 2008

### JasonBourneV

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

I thought the drop was the same, because the flow rate through each of these pipes is the same. So, my thinking is that the same volumes of water exit each pipe per unit time.
Thus, the drop in PE must be the same. I don't know if my answer is correct, but my explanation seems to be reasonable. What's your opnion, chroot?

4. Feb 6, 2008

### chroot

Staff Emeritus
The flow through the two pipes would not be the same; they present different resistances.

- Warren

5. Feb 6, 2008

### JasonBourneV

Okay, so flow through the larger pipe is faster. That means greatest drop in potential energy?

6. Feb 7, 2008

### chroot

Staff Emeritus
I didn't say the flow was faster in velocity; I said the current was larger in volume.

Consider a tiny drop of water moving through either pipe. Does it have any "knowledge" of which pipe it's in? Both pipes have the same pressure at the fork end, and the same pressure at the pump end. If a water drop goes from a region of high pressure to a region of low pressure, it must gain the same amount of energy, regardless of what pipe it's in.

- Warren