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Originally Posted by Juked07
Suppose you want a glass of 50 degree C water, but all you have is a hot tap and a cold tap that produce typical streams of water at 98 and 2 degree C respectively. So you fill the glass halfway with 98 degree water, stop that tap, then move over to the cold tap. When the stream of cold water hits the hot water, some drops splash out, like when your poop hits the toilet water and water splashes back out.
1. Why does this happen??
2. What is the average temperature you'd expect the water that splashes out to be?
I'm sure the problem is underspecified; feel free to make whatever reasonable assumptions you think are necessary.
Thanks very much in advance.
Lol. I addressed this a while back when someone asked how much of the splash is actually urine when you pee and some splashes back. Turns out nearly all of the splash back is urine.
In your case, almost all the splash back will be the cold water.
1. This is due to surface tension. The bonds in the surface require some energy to break. So when a drop of water falls onto a pool or a cup of water, the objects don't immediately merge. They express forces on each other, like a ball on a stretchy sheet. The drop pushes down on the pool, and the pool pushes up on the drop. This accelerates some of the molecules in the pool downward, forming a depression in the surface, and it accelerates some of the molecules in the drop upward. So the action which causes upward acceleration is from the pool to the drop. Therefore, (some of) the drop is accelerated upward and bounces away. Frequently, the drop splits, and some will remain in the pool, while some goes bouncing away.
It's not like poop hitting water and causing a splash. It that case, the poop is more solid and the upward force will only slow its decent. It's not enough reaction force to tear the poop and send bits of it flying, like you'd presumably see if you dropped that duke out a tall window. It's also relevant that it's more dense than the water and it sinks readily, causing a "big" divot in the water's surface, which refills from the bottom up and can create a jet-like fountain as the up-filling water is compressed from the in-rushing water from the side, too.
BTW, if this happens to you often, do yourself a favor and drop a single square of TP in the bowl before you sit. It will fold around the first turd and basically prevent the splash. It only works on the first one, but the first is usually the biggest one that causes a splash. FYI.
2. I expect the water which splashes out to be nearly 2 C.
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That hot tap needs to be adequately marked as a safety hazard. Much over ~55 C can cause 1st degree burns on contact.
About 49 - 50 C is a respectable, but not immediately dangerous temp. for a household tap.
As such, I recommend just go ask your neighbor for a glass of hot water from their tap.
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