Heat Topic - Notes
Temperature is a measure of the hotness or coldness of
something and is measured in 0C.
Conduction
Heat travels through solids by conduction. Heat travels from
atom to atom and this is possible because the atoms in solids
are close together.
The heated atoms vibrate more quickly and pass the vibrations
on to neighbouring atoms so spreading the heat.
Conductors are substances that let heat pass through them
easily.
Metals are the best conductors.
Insulators do not let heat pass through them easily. Many
solids are insulators such as rubber, plastic, cork, glass and
wood.
Air is an extremely good insulator.
Our clothes keep us warm by trapping the air between the
fibres – keeping the heat in
Conductors we use in the home are pots – which conduct heat
to the cooking food and radiators which provide heat for our
homes.
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Heat Topic - Notes
Insulators used in the home are pot handles – which stop us
burning our hands. Also oven gloves and insulating jackets for
hot water tanks. Our clothes are insulators to stop our bodies
losing heat.
Convection
Heat travels through liquids and gases by convection.
Liquids and gases cannot conduct heat because the particles
are close enough together to pass heat to one another.
Instead when we heat a liquid or gas a convection current is
set up. The particles that are heated rise up and colder
particles move in below them. The continual movement of hot
and cold particles eventually heats up the whole container of
gas or liquid.
We looked at a convection current by putting a purple dye
crystal in water and observing the movement of the dye as the
water was heated. This clearly showed that the particles of
water were rising when heated and falling when cooled.
The air in our homes is heated in a similar way by radiators and
fires. The moving air near radiators often causes dust to move
and sometimes it sticks to the wall just above the radiator.
Homes with high ceilings are hard to heat as the hot air goes
to the top and it takes longer for warmer air to circulate
through a room.
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Heat Topic - Notes
Convection and Density
When we heat air particles they move about more and spread
out. They become less dense – less closely packed. The air
which is less dense is ‘lighter’ than the colder air so it rises to
the top.
This principle is used in hot-air balloons. The hot air inside
the balloons is less dense than the air outside so the balloon
floats.
Radiation
Lots of heat comes to us through space from the sun. Space
has very few particles in it. We say that space is a vacuum.
Conduction or convection cannot take place in space because
there are no particles that can vibrate or move. The heat is
travelling to earth by another method called radiation.
Radiation travels from the sun in straight lines in all directions.
The suns rays contain many different types of radiation
including ultra violet and infra red. The most important one
for this course is infra red which is another term for heat.
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Heat Topic - Notes
Radiation and Surfaces
We found out that:
Black and dark surfaces are best at absorbing heat
Black and dark surfaces are best at radiating heat
Shiny and white substances are poor at absorbing heat.
Shiny and white substances are poor at radiating heat.
So in warm sunny countries light clothing is worn to reflect the
heat.
Car radiators are black in colour because that makes them
better at taking heat away from the engine.
Uses of Infra-red radiation
Infra-red radiation is another word for heat. Our bodies give
out infra-red radiation. This can be picked up by thermal
cameras and can be used for finding people that are lost up a
mountain at night or are trapped in a smoke filled building.
Infra-red can also be used to operate TV remote controls and
to trigger burglar alarms and security lights.
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