Hydrodynamics 3/27/22

Physics with Richard Bleil

We never had a pool.  My family never had that kind of money, but as a very young boy (little more than a toddler), my parents did buy a child wading pool for us to play in.  When we were done, it was my responsibility to empty it out.  How it became the responsibility of the smallest person in the household is a mystery to me, but there it was. 

My father, for all of his faults, was actually a very smart man.  He never went to college, but he seemed to have an innate understanding of physics that was impressive.  Either that, or he used the Google of his day to figure things out, namely, he talked with people in his office instead of doing the work they were paid to do.  Anyway, he took a very small piece of hose and showed me that, if you fill it with water, and hang it over the edge of the pool, as long as the edge outside of the pool was longer than that inside the pool water, it creates a siphon and the water flowed naturally out until it was more or less empty.  No power, no electricity, no pumping, just as long as the hose outside of the pool was longer.

I actually still use this trick today, for example when I am filling my space heater with kerosene.  Once it’s started, I just sit and watch until the appropriate level is reached.  It’s all based on gravitational force.  Remember that weight is a force, so what’s happening is that, since the tube is longer out of the water than in, the weight of the water on the external side of the hose is heavier than on the inside.  This means it will flow, creating a vacuum of sufficient strength to continue pulling water out of the pool. 

This same idea is used in toilets.  No pumps, no electricity, but the water in the tank is enough to fill the bowl above the “trap” in the bowl.  The trap is not unlike the elbow under a sink.  The idea is that the trap will fill with water and prevent swamp gas from getting out and stinking up the house (and protecting us from toxins).  When you flush, the bowl will fill to a level higher than the trap causing the water (unless it’s plugged up) to flow down the tube leading to the sewer, but this line is longer than the tube leading to the trap.  So, once the water has flowed past the trap, a siphon again kicks in, which is why the water in the bowl will proceed to drain nearly completely, until the siphon is broken, and the bowl will fill with water to its normal level.  This simple concept is why you can also flush a toilet manually.  If the bowl is empty (say, for example, the water was turned off for some kind of repair), just fill a bucket with water.  When you’re ready to “flush”, dump the water quickly into the bowl (quickly enough so the excess water does not trickle past the trap) and you’ll see the flush behaves as if it had been flushed normally.

Hydrodynamics abounds.  Water towers are always built on the highest ground in the community.  By building the tower on top of that, it’s guaranteed to be above any sinks or toilets in the area.  There are, of course, pumps to push water up to those tanks, but once there, the rest is gravity.  When you turn on the tap (as I just did to fill my water bottle), flush a toilet or take a shower, the water arrives because of force of gravity.  Your sink, toilet or shower is just at a lower point than the water stored in that tank, so the water flows naturally. 

This is also why, at certain times, the water pressure will drop, or rise as the case may be.  At the University of Cincinnati, we had a flood when a tube from a reflux condenser came loose in the lab above ours.  The graduate student who hooked it up had the water flowing too rapidly (in a proper setup it should be barely a trickle).  At night, when people were sleeping, there were fewer sinks being used, showers, toilets, garden hoses, what have you.  As people shut these off for the night, the pressure built up in the lines as there was no relief for it.  The hose was already running at too high of a pressure, so the sudden increase popped the hose off (and out of the fume hood) causing it to run on the floor all night long.  The flooded water broke through the floor and into our lab creating a mess.

When water pressure drops, it’s because somebody is using it elsewhere, and it doesn’t have to be in your house.  If somebody suddenly uses a great deal of water anywhere along the line, causing a dramatic change in pressure, you’ll notice it.  But it’s all just gravity. 


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