The Physics of Toilets

You may be dealing with a plumbing issue. You may get sent back in time and realize you don't have the strength to go on without indoor plumbing. You may get asked this by a small child who, up until that moment, was the one person who believed you had the answer to everything. Trust me. Sooner or later, you are going to want to know this.

Why does the water rush out of the bowl of the toilet so fast when the lever is in the tank? Why does a slow rush of water fail to clear the bowl completely? Why do toilets always seem to block up on Friday night before a long weekend? We can answer two of those questions with physics. The last will just have to depend on, oh, let's say karma.

Toilets have three major components. The first is a bowl. We all know what that's for. The second is a tank. We can all see that. The last is behind the bowl, in that little tube from whose bourn - hopefully - no traveler returns. That part is a siphon. As simple machines go, siphons get less respect than wheels, levers, or inclined planes; but to be fair, not that many people know that siphons make flush toilets possible. If they did, siphons would be celebrated.

A siphon works because it allows water to move like a chain instead of like discrete particles. Grab a pitcher and fill it with water. Stick a length of flexible tube deep in the water and let the tube droop down over the side of the pitcher. Then suck one the end of the tube until the water comes up over the edge of the pitcher and down the tube a ways. The water in the tube will splash on the floor. (Oops. Did I not tell you to put a container there to catch the water? My bad.) But the water in the length of tube climbing up the side of the pitcher will not fall back down into the pitcher. It'll keep going, drawing more and more water over the side until the pitcher empties onto the floor. (Really my bad. I mean. Did I have to tell you a whole pitcher? Couldn't I have just said a glass?) The water will be drawn over the side the same way a length of beads will be drawn over the side of a container if the beginning of the strand is pulled over the side.

It doesn't seem to make sense that water will act like a chain when it breaks apart so easily under other circumstances. Water molecules, though, have their own cohesive forces. That's why they even stick together in the first place. The cohesion, under the constraints of the walls of the siphon tube, will allow the water to act like string. There are even self-siphoning fluids, where the molecules are so tangled and long that even a bit over the lip of glass will siphon off the whole thing. (Some sticklers will argue that atmospheric pressure, which pushes on the water in the pitcher, plays a part. If the water filling the tube "breaks apart," there will be a vacuum, which will suck the water back together again. Most, though, say it's just plain cohesion that makes it work.)

How does this help a toilet flush? Behind the bowl, there's a little pipe that goes up, like a siphon tube that's built into the pitcher. However, since there's no water in there, no water is being siphoned out of the bowl. However, when you flush, you empty the tank into the bowl. The increase in water fills the bowl until the water makes it over the apex of the siphon tube behind it, and down into the pipes below. Once that happens, the rest of the, oh, let's say "water," in the bowl gets siphoned out, and the bowl can begin to refill.

As for why a toilet doesn't seem to work if the flow is low or slow, the water doesn't fill the tube of the siphon. Instead it pours over the side in a meager, steady trickle that doesn't allow the siphon to kick into gear.

As to why they break on holiday weekends, that's between you and your dietician.

Via MIT and Straight Dope.