Unit 3

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Activity 2: “MEASURING TEMPERATURE”

A) Videoclip projection: Heat and temperature

Decide which of these three baths you'd like to step into:

It is an easy choice, isn´t it?

So the middle one with my cold foot is too hot, and with my hot foot it is too cold.

Knowing if something is cold or hot is not easy after all. This is because the human body can only know the difference in hotness or coldness: how hot or cold something is, in comparison with something else. Your own feet measure the temperature outside, comparing it with their own temperature.

There must be a better way to measure temperature.

What do we know about hotness?

Well, when the temperature of something goes up, its molecules go faster.
The molecules in the boiling hot bath water are going too fast for comfort.
The molecules in the freezing cold bath water are going too slowly for comfort.
If we can make sure that the molecules in the water are going at the right speed for the human body, you can take your bath.

But how can we measure their speed?

There is something that we know about hotness: it not only causes the speed-up of molecules, but this speed-up causes matter to expand, so we could measure molecule speed indirectly by measuring the effect of that speed.
That's exactly what a Swedish man called Anders Celsius decided to do in 1742.
He took a liquid that expands quite a bit when it gets hot, mercury, and poured it into a little tube, then put the tube into some freezing cold water and the mercury contracted to here, he decided to label this freezing point of water 0º.
Next he put the tube into the boiling hot water and the mercury expanded up to here. He marked this boiling point of water 100º.
Now we have a means of measuring the real speed of molecules, in other words, the relative temperature or degree of hotness of things.
Celsius could take his own temperature, about 37ºC, which is enough for your bath water.
The name of the instrument is themometer.

Is your bath water at the right temperature now?

The story so far...

When matter - in any of its three states - gets hot, its molecules go faster and take up more space, and the solid, liquid or gas expands.

When matter gets cold, its molecules go slower and take up less space, and the solid, liquid or gas contracts.

And now...

MEASURING TEMPERATURE

Can you tell whether something is hot or cold? In that case, decide which of these three bathtubs you'd like to step into.

This one, which is freezing cold? Or this one, which is boiling hot? Or this one, which is neither too hot nor too cold?

It's an easy choice, isn't it? So go on, step into the middle one. It feels just a bit too hot, doesn't it? Try the other foot. Oh! Now it feels a bit too cold.

Well, perhaps judging whether something is hot or cold isn't so easy after all. This is because the human body can only notice differences in hotness or coldness: how hot or cold something is in relation with something else.

But even your own feet can't agree on how hot or cold the water in the middle bathtub is. There must be a better way to measure "degree of hotness", or "temperature", as we usually say.

What do we know about "hotness"? Well, when the temperature of something goes up, its molecules go faster. The molecules in the boiling hot bath water are going too fast for comfort. And the molecules in the freezing cold bath water are going too slowly for comfort.

If we could only make sure that the molecules in the in-between bath water were going at just the right speed for the human body, then you could take your bath. But how can we measure that speed? Molecules are so tiny, we can't even see them, let alone clock how fast they're going.

Wait a minute. There's something else we know about hotness. It not only causes a speed-up of molecules, but this speed-up causes matter to expand. So we could measure molecule speed indirectly by measuring the effect of that speed - the expansion itself.

And that exactly's what a Swedish scientist by the name of Anders Celsius did in 1742.

He took, a liquid that expands quite a bit when it gets hot, mercury, and poured it into a little tube. He then put the tube into some freezing cold water, and the mercury contracted down to here. He decided to label this freezing point of water "zero degrees" (Celsius).

Next he put the tube into some boiling hot water, and the mercury expanded up to here. He marked this boiling point of water one hundred degrees Celsius.

Now he had a means of measuring the relative speeds of molecules. In other words, the relative temperature, or degree of hotness, of things.

Comfortable room temperature is twenty degrees. But thirty degrees is a bit too hot. And minus twenty degrees is much too cold.

Celsius could even take his own temperature, and find it to be about thirty-seven degrees, which naturally enough is the ideal temperature for your bath water: normal body temperature.

All you have to do is adjust the water to thirty-seven degrees, and you'll finally be able to take your bath, courtesy Anders Celsius, and his "hotness meter", or if you prefer the Greek word for "hotness", which is "thermos", as in "thermos bottle", his "thermos-meter", or "thermometer".

Is your bath water at the right temperature now? Oh, my, my. Perhaps you should've taken a shower!

Activity 3
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