英語聽力文摘 防火氣球

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防火氣球

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.

想看變戲法嗎?

當(dāng)然。

給你，吹氣球吧!

然后呢?

我會(huì)點(diǎn)燃一根蠟燭，你覺得把氣球放在火焰下，會(huì)發(fā)生什么事情呢?

氣球當(dāng)然會(huì)爆了哦，橡膠在火焰的熏烤下會(huì)達(dá)到一個(gè)無法承受氣球內(nèi)氣壓“傷不起”的極限。

沒錯(cuò)，但是并非一貫如此哦。

什么意思?

看著，首先，我將1/4杯的水倒入氣球中，再吹好氣球，你幫我拿著氣球，我再點(diǎn)燃一支蠟燭。

你在捉弄人嗎?氣球會(huì)爆，我全身都濕透了。

不，相信我，氣球不會(huì)爆的。

最好你說的是真的。

看看吧。

你說的是真的，但是，為什么氣球不爆呢?

氣球里的水會(huì)吸收火焰絕大部分的熱量，如此橡膠不會(huì)過熱。但是，水會(huì)不會(huì)太熱了，橡膠炸了呢?

水變熱需要段時(shí)間吧，水的比熱容很大，也就是說水沸騰需要很多熱量，而更多的熱量才能讓水蒸發(fā)。事實(shí)上，1克水蒸發(fā)需要540卡路里。

你看，氣球還沒爆。

當(dāng)然，不過如果我將火焰移到氣球的另一邊。

我全身都濕透了。

不好意思，我只是太想知道會(huì)怎么樣了。

specific heat：比熱容(specific heat capacity)又稱比熱容量，簡(jiǎn)稱比熱(specific heat)，是單位質(zhì)量物質(zhì)的熱容量，即使單位質(zhì)量物體改變單位溫度時(shí)的吸收或釋放的內(nèi)能。比熱容是表示物質(zhì)熱性質(zhì)的物理量。通常用符號(hào)c表示。

vaporize：蒸發(fā)