Use any heavy object to fix one end of a metal
rod, as shown in Figure 5.9. The metal rod
could be a knitting needle, a bicycle spoke or a
Heat
ruler. Fix four or five pins or matches along the
rod, using wax. Now heat the free end of the
rod, using a candie. In a while, the wax will melt
and the pins will start falling off. The pin closest
to the flame will fall first, while the one farthest
from the flame will fall last. This shows that heat
travels gradually from the hot end of the rod to
the cold end,
Fig. 5.9
If you had placed another metal rod in contact with the first, it would also have become
hot after some time. The flow of heat from a hot part to a colder part of a body, or from a hot
body to a colder body in contact with it, without any movement of the body itself is called
conduction. In other words, conduction is the flow of heat through a substance without the
movement of the substance itself,
Heat stops flowing when the two bodies (or different parts of the same body) are at the
same temperature. This is exactly what happens when you place a thermometer in your
mouth. Heat flows from your mouth to the bulb of the thermometer, until the two are at the
same temperature. Thus, for heat to be conducted from one body to another, they must be
(a) in contact with each other, and
(b) at different temperatures.
Conductors and insulators All substances do not conduct heat with the same ease. Do the
following activity to see this yourself.
Fill tumblers or cups made of steel, glass, china, paper and plastic with hot water. Hold each (tumbler or cup) in
your hand after a few seconds. Do they all become equally hot? The steel tumbler will feel the hottest because
metals conduct heat the best. Which material feels the least hot?
Substances through which heat flows easily are
called good conductors, or simply, conductors. Most
metals are good conductors, silver being the best. That
is why we use metals to make cooking utensils. That is
also why we do not generally drink tea or coffee in a
steel cup.
Substances through which heat does not flow easily
are called bad conductors, or insulators. Paper, glass,
asbestos, wood, plastic and rubber are examples of bad
conductors. You can hold one end of a piece of paper in
Fig. 5.10 Metals are used to make
cooking utensils.
your hand while the other end is burning because
paper is a bad conductor. The same goes for a match, which is made of wood. Liquids and
gases, in general, are poor conductors of heat. Mercury is an exception.
Guess why we use plastic for making the handles of cooking utensils. So that the heat
from the utensil does not reach our hands. And why is it better to wear two thin sweaters
than one thick sweater? Because the air trapped between two sweaters is an insulator. It
does not allow the heat from your body to escape easily.
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