《那些古怪又讓人憂心的問題》第125期:攪拌加熱(1)
STIRRing TEA
Q. I was absentmindedly stirring a cup of hot tea, when I got to thinking, “Aren't I actually adding kinetic energy into this cup?” I know that stirring does help to cool down the tea, but what if I were to stir it faster? Would I be able to boil a cup of water by stirring?
Q．有次我在漫不經心地攪拌一杯熱茶時突然想到：“咦，我不是在往這杯茶裏輸入動能嘛。”我知道攪拌能夠加速茶的冷卻，但如果我攪拌得快一些呢？我能不能通過攪拌的方式把一杯茶燒開呢？——Will Evans
——威爾•埃文斯
A. NO.
A．不能。The basic idea makes sense. Temperature is just kinetic energy. When you stir tea, you're adding kinetic energy to it, and that energy goes somewhere. Since the tea doesn't do anything dramatic like rise into the air or emit light, the energy must be turning to heat.
不過你說的基本原理是有道理的。溫度其實就是（分子）動能的體現。當你攪拌茶的時候，你就在增加它的動能。這些動能總得有個去處，既然這杯茶沒有從桌子上升起來也沒有發光，那麼這些動能最後肯定變成熱了。Am I making tea wrong?
一定是我泡茶的方式不對！The reason you don't notice the heat is that you're not adding very much of it. It takes a huge amount of energy to heat water; by volume, it has a greater heat capacity than any other common substance.1
至於你爲什麼沒有覺察到這些熱量，是因爲這些熱量並不多。加熱水需要大量熱量，按體積來說，水的比熱容比其他任何一種常見物質都要大。1If you want to heat water from room temperature to nearly boiling in two minutes, you'll need a lot of power:2
如果你想要在2分鐘內把水從室溫加熱到接近沸點，你需要有很大的功率：2Our formula tells us that if we want to make a cup of hot water in two minutes, we'll need a 700-watt power source. A typical microwave uses 700 to 1100 watts, and it takes about two minutes to heat a mug of water to make tea. It's nice when things work out!3
上面的公式告訴我們如果你想在2分鐘內變出一杯熱茶，那麼你需要有一個700瓦的熱源。微波爐一般來說有700～1100瓦，用它的確只需2分鐘就可以做出一杯熱茶。理論和現實一致的感覺真好！3Microwaving a cup of water for two minutes at 700 watts delivers an awful lot of energy to the water. When water falls from the top of Niagara Falls, it gains kinetic energy, which is converted to heat at the bottom. But even after falling that great distance, the water heats up by only a fraction of a degree.4 To boil a cup of water, you'd have to drop it from higher than the top of the atmosphere.
將一杯水以700瓦的功率加熱2分鐘會使水吸收非常多的能量。當水從尼亞加拉瀑布頂端落下時會獲得動能，到了瀑布底部這些動能變成了熱能。但即使是下落了如此長的距離，水的溫度也只是上升了0.12℃。4要把一杯水燒開，你需要讓它從大氣層頂部以上的高度落下來。
