TBR Physics Chapter 7, PV, Q28 - Toilet Fluid Dynamics


Full Member
Nov 15, 2011
  1. Pre-Medical
    Does anyone want to give a shot at explaining to me exactly how a toilet works?

    So I get that the idea is to create a pressure difference between the bowl and the output end of the tube, and that water flowing from the tank into the bowl increases the hydrostatic pressure at the input end of the tube and reduces pressure in the Erkel tube via an aspirator. But I don't really get how the aspirator works or how it reduces pressure in the Erkel tube. I'm picturing the aspirator as a thin tube full of air that's above the water level of the tank, is that right? If so, then wouldn't the pressure in the aspirator always be equal to atmospheric pressure? Can somebody explain how the water level in the tank can change the pressure in the aspirator tube if it's open to air?

    Also for Q28, why is II true:

    II. Water flows because the hydrostatic pressure is greater at the output end of the tube than the input end.

    Shouldn't this be the opposite? Fluids flow from greatest pressure (input) to lowest pressure (output), no?
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    New Member
    7+ Year Member
    Aug 20, 2012
    1. Pre-Medical
      Since no one has replied to you yet I'll see if I can help. You seem like you have a good understanding of what should happen in general. They key to understanding the aspirator tube is to realize that the tank where the water is held (at the top of the aspirator) is completely, or nearly completely, sealed from the atmosphere for the purposes of this question. This does cause more questions about how the tank cycles through its range of pressures, but that isn't involved in the question so I won't address that (they also don't talk about refilling the tank with water).

      Regarding question 28, I think their wording is poor. To me, combining the question and explanation they give, it seems they are talking about the difference in potential energy between the input end of the siphon and the output. Instead of talking about potential energies they decided to focus on hydrostatic pressure (thus the pressure of a column of water above each end of the tube) and the way it was worded makes the question terrible. Don't get hung up on this question. Regardless, III had to be correct and I was incorrect, so there was no other answer choice.
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