英语听力文摘 防火气球

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防火气球

Yael, Want to see a trick?

Sure.

Here, blow up this balloon.

Ok, now what?

Now, I light this candle. Now what do you think will happen when you put the flame under the balloon?

It’ll pop, of course. The flame will heat the rubber to the point where it becomes weak and can’t hold up under the pressure from the air inside.

Right, expect not always.

What do you mean?

Watch, first, I put about a quarter cup of water in the balloon and blow it up. There, now hold this while I light another candle.

This is a trick, right? You pop it and I get wet?

No, trust me. The balloon won’t pop.

It better not.

See.

Wow, you are right. How comes it not pop?

As long as the flame is beneath the water in the balloon, the water absorbs most of the heat and the rubber doesn’t get very hot. But won’t the water eventually get hot enough to break the rubber?

Eventually, water will take a while. Water has a really large specific heat, meaning that it takes a lot of heat to make water boil, and even more heat to make it vaporize. In fact, it takes about 540 calories of heat to evaporate one gram of water.

Well, it hasn’t pop yet.

Of course, if I move the flame to another part of the balloon.

Don, I’m all wet.

Sorry, I wouldn’t resist.

想看变戏法吗?

当然。

给你，吹气球吧!

然后呢?

我会点燃一根蜡烛，你觉得把气球放在火焰下，会发生什么事情呢?

气球当然会爆了哦，橡胶在火焰的熏烤下会达到一个无法承受气球内气压“伤不起”的极限。

没错，但是并非一贯如此哦。

什么意思?

看着，首先，我将1/4杯的水倒入气球中，再吹好气球，你帮我拿着气球，我再点燃一支蜡烛。

你在捉弄人吗?气球会爆，我全身都湿透了。

不，相信我，气球不会爆的。

最好你说的是真的。

看看吧。

你说的是真的，但是，为什么气球不爆呢?

气球里的水会吸收火焰绝大部分的热量，如此橡胶不会过热。但是，水会不会太热了，橡胶炸了呢?

水变热需要段时间吧，水的比热容很大，也就是说水沸腾需要很多热量，而更多的热量才能让水蒸发。事实上，1克水蒸发需要540卡路里。

你看，气球还没爆。

当然，不过如果我将火焰移到气球的另一边。

我全身都湿透了。

不好意思，我只是太想知道会怎么样了。

specific heat：比热容(specific heat capacity)又称比热容量，简称比热(specific heat)，是单位质量物质的热容量，即使单位质量物体改变单位温度时的吸收或释放的内能。比热容是表示物质热性质的物理量。通常用符号c表示。

vaporize：蒸发