字幕列表 影片播放 列印英文字幕 (gas hissing) It's not often that I think of an experiment for Periodic Videos where I really want to know the answer, rather than just having a regular demonstration. Today, I thought of one of those experiements. There is quite a well known effect -known as a fuel-coolant interaction- where a molten metal drops, say, into water and causes an explosion. And we've sort of had some of those in some of our videos with caesium and sodium in water; But there you have a metal which gets hot through a chemical reaction. I had suddenly thought: Why don't we try dropping mercury into liquid nitrogen because mercury is hot, it's room temperature liquid nitrogen is at -196° C The hypothesis is that because the metal conducts heat so well, the heat will come out very quickly and there'll be a *woosh* as all the nitrogen vaporizes. The experiment was done in three stages. So first of all: Liquid nitrogen with solid copper. Same temperature as the mercury but because it's solid there's no change in surface area, it can't break up into small droplets so it should be safer. So you put it in and much to our pleasure it really looked quite nice. (ethereal music) So, that part at least was safe. But just to be doubly sure he then dropped the mercury the first time into a really big vessel so that even if there was splashing and so on none would come out. (ethereal music) The thing that particularly pleased me was that when the mercury froze, because the freezing point of mercury is -40° C, there were quite nice crystals that you could see formed on the surface of the metal and there was this great lump of mercury at the bottom solid. But Neil had quite a lot of difficulty getting it out at the end and it began to melt and so on. But that wasn't part of the experiment Then came the real experiment: A small vacuum flask where you can see inside without condensation on the surface from the vapor pressure in the air and he dropped in the mercury. What's interesting is that as it goes into the liquid nitrogen, the nitrogen boils, and so the mercury falls almost encased in this little bubble of nitrogen gas. Then, when it gets to the bottom it goes *splat* on the bottom, spreads out, and then the transfer of heat becomes much more violent and there is a huge upwelling. Mercury going this way, and bubbles of gas going up. (gaseous boiling sounds) As an experiment it was really nice it looked really good. I'm not completely convinced that it demonstrates how well heat is transferred from a liquid, but you can see that it's far more violent than when you drop in a lump of copper because the surface area of the mercury is much, much larger and the heat travels out through the surface through the interface between the liquid nitrogen and the metal So, I think in the end, we have demonstrated the point. That even with a difference of just over 200° C you can get quite a violent boiling Imagine now if you put in really hot mercury if you put in boiling mercury into liquid nitrogen, then you might get really quite a bang. You might even break the vessel. (Brady: Are you going to let us do that one, professor?) Well I'd be happy for you to do it Neil would not be happy, we would smash his vessel and he'd have to clear up the mess as well.