英文科學讀本 第五冊·Lesson 55 Specific Heat

Lesson 55 Specific Heat

We may as well begin our lesson to-day with an experiment, said Mr. Wilson. "It will be a very simple one, but it will start us on our way. I have here a basin of cold water. We will test it with the thermometer to find its temperature. Now take the poker, red-hot as it is, and plunge it into the basin. After stirring it about for a short time, you may remove it from the water. Its heat has all gone; it is cool. But what has become of the heat? The poker has given up its heat to the water, as we soon find, with the aid of the thermometer. This instrument at once tells us that the water is warmer than it was. It has taken in the heat which the poker gave out. The poker and the water are now at the same temperature.

Suppose we proceed a little further. I have suspended in this kettle of boiling water a ball of iron, and another, exactly the same size, of lead. What is the temperature of the boiling water?

It is 212℉., sir.

Quite right; and if we test the balls with the thermometer, they too will show the same temperature, 212℉. We will now remove both balls, and plunge them into equal quantities of water from the pipe. What will be the effect of the heated metal balls on this water?

I suppose in each case the heat will be given out by the ball and absorbed by the water—the water will be heated, replied Fred.

Let us see, continued Mr. Wilson. "The thermometer, as usual, will quickly tell us of any change in the temperature of the water.

Yes, he went on, after testing it, "the water in both basins has been heated, but, strange to say, the iron ball has made the water in the one basin hotter than the lead has made that in the other. This proves that the iron must have given out more heat than the lead.

But the iron and the lead both obtained their heat from the same source—the boiling water. Hence it is clear that the iron is able to take in and hold more heat than the lead. The result would be very similar if we substituted any other kind of matter for the iron and the lead. We should find that some bodies have a greater capacity for holding heat than others. This is exactly what we mean when we speak of the specific heat of different bodies.

We can now take another step. I have here some ice-cold water; the thermometer standing in it registers 32℉. I am going to measure equal quantities of this, and of boiling water from the kettle, and mix the two together in this basin. Let us see what the thermometer has to tell us about the mixture. The temperature of the mixture is 122°. Now the difference between 212° and 32° is 180°. The hot water gives up, and the cold water receives, exactly half of this, i.e. 90°. Hence the ice-cold water increases its temperature from 32° to 122°, and the boiling water lessens its temperature from 212° to 122°. It is important to remember that the result would be exactly the same if we mixed equal quantities of any liquid—oil with oil, or mercury with mercury. The quantity of heat which the hot liquid gives out in cooling through a certain number of degrees is just what is required to raise the temperature of the cool liquid through the same number of degrees.

Let us next take equal quantities of different liquids, say mercury and water, the mercury being heated to a temperature of 212°, the water standing at 32°. On mixing these, and testing the mixture with the thermometer as before, we shall find the new temperature to be a little below 38°. That is, the water, in this instance, will have gained only about 6° in temperature, although the mercury has been lowered from 212° to the same temperature as the water, viz. 38°. That is to say, the mercury has lost 212°－38°=174°, and all this heat has been absorbed by the water for the purpose of increasing its own temperature from 32° to 38°(6°).

If, on the other hand, we took water at 212°, and mercury at 32°, and mixed them in equal quantities, we should find the resulting temperature of the mixture to be about 206°. The water would part with only about 6° of its heat, but this would be sufficient to raise the temperature of the mercury from 32° to 206°, so that the quantity of heat which is sufficient to raise the temperature of mercury 174° will raise an equal amount of water through only 6°. This shows, by another method, that water has a greater capacity for taking up and holding heat than mercury. We say that the specific heat of water is higher than that of mercury."