Reading
Read the
passage carefully and then answer the questions that follow.
Since
ancient times, man has learnt that he needs heat in order to survive. By rubbing two pieces of dry wood against
each other, primitive man was able to produce enough heat to start a fire. The heat from fire enabled him to cook his
food and also kept him warm on cold days.
Today, we do not need to go through the tedious process of starting a
fire in the same way that early man had to .
We can now easily use a match or lighter to start a fire. However, we still need the heat from fire to
cook our food and to warm us up.
What
exactly is heat? According to
scientists, heat is the result of movements.
Everything around us is made up of minute particles called atoms and
molecules. These atoms and molecules are
far too small to be seen by the naked eye.
However, they can be seen under a powerful microscope. These participles are moving all the
time. They vibrate rapidly even while in
a stationary position, thus causing heat to be produced.
Take
two pieces of metal of the same size and observe them closely. Put one piece of metal in a cool place and
put the other piece of metal in the fire.
After an hour, take a look at the two pieces of metal. You will notice that the piece of metal that
was left in the cool room has remained the same in size whereas the piece of
metal that had been in the fire for an hour would have expanded in size. If that piece of heated metal is put under a
microscope, you will notice the atoms in the metal moving very rapidly further
and further away from one another. That
is why when a substance is heated, it increases in size.
Besides
solids, expansion can also occur in liquids.
We can observe this by conducting a simple experiment. Hold a clinical thermometer in one hand and
shake it vigorously. You will find that
the mercury would have all flowed back into the bulb. Then place the thermometer under your tongue
for a few minutes. Take the thermometer
out and look at it. You will find that
the mercury in the thermometer has expanded.
The heat from your body has caused the mercury to expand.
What
about air? Does it expand? Yes, it does.
Fill a basin with hot water.
Place a flask that has a rubber stopper in its mouth into the basin of
hot water. After some time, the flask
would get heated. The air trapped inside
the flask would also become heated. The
hot air inside the flask will then expand and push the rubber stopper out. This shows that even air expands when heated.
There
are many ways by which man has learnt to produce heat which he needs for numerous
purposes. One way is by converting other
forms of energy into heat. The main
kinds of energy are electrical, chemical and mechanical. When you switch on an electric kettle, you
are using electrical energy to make it hot.
The electricity flows through the heating coils of the kettle and makes
it hot. Chemical energy is derived when
substances such as wool, coal or gas are burned. Once these substances are heated, they are
converted into chemical energy. If you
touch the tyres of a car after it has gone through a long journey, you will
find that they are quite warm. This is
because the mechanical energy released during the movement of the car, has been
turned into heat energy by the friction of the tyres against the road. Whatever type of energy used produces the
same result, in that heat is released.
1. How did primitive man start a
life?
2. How can we start a fire today the
easy way?
3. How is heat formed?
4. What are atoms and molecules?
5. What causes a solid to expand?
6. Why does the mercury in a
thermometer rise up when heated?
7. What are the main types of
energy?
8. Show how the movements of a car
can be turned into mechanical heat energy.
Answer Key
Reading
1. Primitive man started a fire by
rubbing two pieces of dry wood together.
2. We can start a fire today by
lighting a match or using a lighter.
3. Heat is formed when atoms and
molecules vibrate rapidly.
4. They are tiny particles found in
all matters which are constantly moving.
5. The atoms in a heated solid cause
it to expand as they move rapidly further and further from one another.
6. The liquid mercury expands when
it is heated, thus causing it to rise up.
7. They are electrical, chemical and
mechanical energy.
8. The movements of a car are turned
into mechanical heat energy as a result of the friction caused by the tyres
rubbing against the road.
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