Heat Energy Transfer by Convection
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Video Lecture 23 of 46
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Date Added: February 5, 2015

### Lecture Description

In this mechanism of heat energy transfer there is an actual motion of heated stuff - a motion which results from differences in density and by the action of gravity. A look at the Latin origins of the words conduction and convection will make their meanings clearer. So - as I like to say - knowing Latin - and Greek - is good for knowing Physics!

A. We show two smoke stacks in a glass-walled box. Under one stack
is a lighted candle. By observing the behavior of smoke in the box
we see clearly the flow of heated stuff. Thus we see too why it is
that huge smoke stacks have such a good "draft:. And - strangely
enough - we learn that "wet" air is lighter than dry air. Which
is NOT what we might first think!

B. In a framework holding water which we color a bit with food dye
we see how great the convection forces due to change in density.
Thus in A and in B we encounter convection in FLUIDS - the
generic name for liquids and gases. We have an actual transfer
of STUFF. ~

C. A candle resides in a dish. We bring down over the burning candle a glass cylinder. In a short time the candle flame must certainly
"expire" - the flame is wanting air to breathe. Now we introduce a partition which provides TWO channels - one for the cold
air to fall down into - and another for the hot air to come up.
Remember: it is not good to say that HOT AIR RISES. Nothing
can RISE! It must be pushed up.

D. We show more of DEMONSTRATION L in LESSON 7 - with a
wheel equipped with hub and spokes. An interesting inquiry a-
rises: TWO bars are of the same stuff - iron - but one is twice the
diameter of the other. So some geometry Js necessary. Double
d - we make A four times. We make S - the surface twice as much. So although conductivity is made four times greater the heat loss due to radiation is doubled. Hence the net gain is only twice.

E. A "gag" can be done with a clean silver coin and a handkerchief.
But be careful. The fit must be tight and the coin clean or you'll
end up with a hole in the handkerchief.

F. Since the action of the mercury column in a mercury-in-glass thermometer is not too easily seen we can witness things better by fitting up a "whiskey" flask with a stopper and glass tube. And a bit of color in the water gives the matter some prettiness. Now immerse the flask in hot water. What do we see FIRST? The column FALLS!

The mechanisms of conduction and convection constitute TWO important actions for the transfer of thermal energy. For these we need some STUFF. The question arises: How can heat energy get from place to place with nothing in between?

### Course Description

Demonstrations in Physics was an educational science series produced in Australia by ABC Television in 1969. The series was hosted by American scientist Julius Sumner Miller, who demonstrated experiments involving various disciplines in the world of physics. The series was also released in the United States under the title Science Demonstrations.

This program was a series of 45 shows (approximately 15 minutes each) on various topics in physics, organized into 3 units: Mechanics; Heat and Temperature / Toys; and Waves and Sound / Electricity and Magnetism.

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