|MadSci Network: Physics|
A good site that explains toilets is at howstuffworks. The starting of the siphon is best explained with water momentum and not hydrostatic pressures, just like truing to explain water hammer in pipelines is not a simple hydraulic analysis. In order for the siphon pipe in the toilet to work, all of the air must be forced down the drain. This is why when a cup of water is dropped in the bowl, nothing will happen because it will raise the level in the siphon pipe only slightly and spill over into the drain, air will remain at the crest of the pipe and the siphon will not start. When a large volume of water is dumped in the bowl, there is the momentary raise in the level of the bowl that will start the momentum or pressure wave to expel all of the air in the siphon; and as discussed at the howstuffworks, there is also an extra water line directly into the siphon pipe that aids in expelling all of the air. Once the siphon starts, it is a simple hydraulic analysis that shows that the water in the bowl will be sucked out until air gets back in the siphon pipe and breaks the siphon.
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