英文科學(xué)讀本 第五冊·Lesson 17 Convection

Lesson 17 Convection

The other day I promised to explain to you how heat travels through liquids, said Mr. Wilson. "I will do so now by means of a very pretty experiment.

I will fill this large glass flask with water nearly to the top, and drop into the water a few pieces of this blue substance: litmus. The flask shall be fixed upright over the flame of the Bunsen burner, and we will watch the water boil. As the flame plays upon the bottom of the flask, we see an upward central current of water, rendered distinctly visible by the blue coloring matter of the litmus. The current rises immediately above the spot where the flame acts on the bottom of the flask, till it reaches the surface of the water. Here it bends over in every direction, forming a great number of descending currents along the outer wall of the flask. These continue to travel downwards until they reach the heated lowermost portion of the flask, when they again ascend as before. Now let us see if we can discover what is actually taking place. First, what is the great difference between the molecules of a liquid and those of a solid?"

The molecules of a liquid have little or no cohesion, sir, said Fred. "They are free to move about in any direction, while those of a solid are fixed and stationary."

Just so, my lad, replied Mr. Wilson. "The molecules of the water in the flask were at first quiet, but the heat set them in motion in some way, so that the whole of the liquid was soon in rapid circulation. The particles of water themselves moved upwards, outwards, and downwards. We want to find out why this is the case.

We have already learned something about the way in which water acts in cooling. The particles on the surface, of course, are the first to feel the effect of the cooling process. How do they act?

They contract in cooling, sir, and become denser and heavier than the particles below them.

Exactly. But what happens then?

These heavier particles sink, and force the lighter ones upwards towards the surface.

Quite right, but why are they lighter?

They are lighter because they are warmer, sir. It was only the cooling that made those on the surface contract, and so become dense and heavy.

Good! said Mr. Wilson. "Now let us go back to our flask. The flame heats some of the particles near the bottom of the flask. The heating makes them expand, and become lighter than those around and above them. There can be only one result: the heated, expanded particles must rise to the surface. It was the stream of these rising particles that formed the central current, which the coloring matter enabled us to see.

But we have not done yet, he continued. "Think of this stream of heated particles rising through the cold ones all round them. What must happen as they rise?"

I suppose, replied Fred, "they give out their heat to the colder ones, as they pass upwards, and if so, they themselves must be cold by the time they reach the surface."

I am glad to see you reason so clearly, Fred, said Mr. Wilson. "That is exactly what takes place. But all this time more and more particles at the bottom of the flask have been heated by the flame, and these continue to rise and force the others onward. Hence it happens that those which were heated at the bottom, and have reached the surface, are now cooler and heavier than the others, and being heavier, as well as being forced onwards, they move in the only direction possible—that is, downwards along the sides of the vessel, there to meet again, at last, the heat of the flame, and rise once more.

Thus we see that in water the heat is carried or conveyed by the particles of water themselves. The heated particles rise through the whole mass, and as they rise, give out their heat to the rest. This method of carrying or conveying heat is called convection. As the temperature of the whole of the liquid rises, the heat at the bottom converts the little particles of water into particles of steam. These are lighter still than the heated particles of actual water; they are extremely light. They rise in little balls or bubbles through the water. At first the water robs them continually of their heat, and they burst as they rise, but after a time, as the water itself becomes hotter, the bubbles reach the surface without bursting. It is this bursting of the steam-bubbles on the top which makes the commotion as water boils."