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Stand in the SunShine: you feel warm. Go for a fast run: you will get hot.
the warmth of sunshine comes from heat generated in the center of the sun.
Your body also produces heat all the time, and this heat keeps you alive. heat
is important to us in many ways. the sun’s heat causes the weather, making
winds blow and rain fall. earth’s interior contains great heat, which causes
volcanoes to erupt and earthquakes to shake the ground. engines in cars,
aircraft, and other forms of transportation use the heat from burning fuel
to produce movement. Power stations change heat into electricity that
comes to our homes. heat is a form of energy.
everything, even the coldest object, contains heat—a cold object simply
has less heat than a hot object. all things are made of tiny particles called
molecules. heat energy comes from the vibrating movement
of molecules. hot objects have fast-moving molecules; molecules
in colder objects move more slowly.Heat travels through solid objects by
a process called conduction. Metal conducts heat
well. For instance, a metal spoon in a cup of
coffee gets hot quickly. Other substances, such as
wood and plastic, do not conduct heat well. They
are called insulators and are used to make items
such as saucepan handles.All objects give out heat rays
that travel through air and
space. The heating element of
an oven cooks food with heat
rays. The transmission
(movement) of heat by heat
rays is called radiation. It is not
the same as nuclear radiation.A liquid slowly
changes into a gas at a
temperature lower than
its boiling point. This is
called evaporation.
The steam from this
hot cup of coffee is
evaporated water.Heat 251-
BoilinG Point
at a temperature called
the boiling point, a
liquid changes into
a gas. Below the boiling
point the gas changes
back to a liquid again.
the boiling point of
water is 212°F (100°C).SolidS, liquidS, and GaSeS
a substance can be a solid, a liquid, or a gas,
depending on how hot it is. Changing the
temperature can change the substance from
one state to another. For instance, liquid water
becomes a solid—ice—when it is cold and
a gas—steam—when it is hot.MeltinG Point
heating a solid
makes it melt into
liquid. this happens
only at a certain
temperature, which
is called the melting
point. Below this
temperature, the
liquid freezes to
a solid again. the
melting point of ice
is 32°F (0°C).White-hot steelA liquid, such as water, has
molecules that are close
together. The molecules can
move around more easily than
in a solid, so a liquid can flow.A gas, such as steam, has
molecules that move around
freely so that the gas spreads
out to fill its
container.A process called convection spreads heat through
gases and liquids. For example, hot air above
a heater rises. Cold air flows in to take its
place, becomes hot, and rises. In this
way, a circular current of air moves
around a room, carrying heat with it.heat enerGY
heat is just one of many
forms of energy. Sources
of heat change one type of
energy into heat energy.
a burning fire, for example,
changes chemical energy in its
fuel into heat energy. electric
heaters change electrical
energy into heat.The digestive system
of an animal or a person
changes chemical energy
from food into heat
energy inside the body.This is a thermogram (heat picture) of a
person’s face. It was taken by a special camera
that uses infrared rays instead of light rays.
The hottest parts are yellow in the picture.Infrared rays
heat rays are also called infrared rays.
they are invisible rays very similar to
red light rays, which is why the rays are
called infrared. all objects give out
these rays, and hot objects produce
stronger infrared rays than cold objects.
Some electric heaters have curved
reflectors that send heat rays forwards
just as a mirror reflects light rays.Cool
incoming airConvection heaterA solid, such as the ice on
this window pane, has rows of
molecules that vibrate back and
forth. The molecules are
locked together, so solids
are often hard and
cannot be squashed.Warm rising airUS_251_Heat_1.indd 251 21/01/16 5:00 pm