Transcript Topic 3 - Thermal Physics

Thermal energy transfer
3.1.7 Describe qualitatively the processes of
conduction, convection and radiation.
Mike Dickinson 2002
1
Figure 3.4 - Three ways to score a goal!
The ball is passed
from player to player,
from one end of the
pitch to the other. The
players do not move.
A single player
dribbles the ball
himself from one end
of the pitch to the
other through the other
players.
Mike Dickinson 2002
A player, in his own
half, kicks the ball to
score the goal. No
other players are
involved.
2
Conduction, Covection & Radiation
Heat moves by molecular
collisions from one end of
a thermal conductor (large
molecular oscillation) to
the other due to a
difference in temperature
between the two ends
When a fluid is heated it
expands and becomes less
dense. The part of the
fluid where the molecules
have a lower density will
“float” up above the part
with the higher density.
Energy from a hot object can
be radiated in the form of an
electromagnetic wave.
Radiation from the sun
consists of many different
wavelengths. The infra-red
(IR) wavelengths are
responsible for heating the
Earth
Conduction happens in
solids (mostly).
Convection happens in
fluids (liquids and gases)
Radiation happens in the
absence of matter.
Mike Dickinson 2002
3
Thermal energy transfer
3.1.8 Describe examples of conduction,
convection and radiation.
Mike Dickinson 2002
4
Conduction
Let’s consider a metal spoon in a pan of hot
liquid. When a good conductor (the spoon) is
placed in contact with something hot (the liquid),
then the molecules in direct contact with the
hotter substance start to vibrate more vigorously,
increasing their kinetic energy and therefore, by
definition, increasing the temperature at that end
of the conductor.
Mike Dickinson 2002
5
Figure 3.5 - Conduction
Movement of heat
Cold end
Hot end
Mike Dickinson 2002
6
These increased vibrations cause collisions
between neighbouring molecules and the
vibrations are passed on from molecule to
molecule (like a football being passed from player
to player). The molecules in the entire conductor
will soon be vibrating vigorously and the other
end of the spoon will feel hot.
Mike Dickinson 2002
7
Convection
In fluids (liquids and gases), the particles are free
to move about and change places. If the
temperature in a hot oven is measured, it is found
that it is hotter near the top of the oven than at the
bottom. As the air molecules gain more energy
through heating, the air expands and becomes less
dense than the cooler air around it. This causes
the warmer air to rise above the cooler air.
Mike Dickinson 2002
8
Convection can also be made to work in reverse.
The ice-box in a refrigerator is placed at the top so
that air that has been cooled by the ice-box
contracts and therefore becomes more dense. The
more dense, cool air then falls through the cavity
of the fridge (chilling the food as it goes).
Warmer air is displaced upwards to replace the
cool air and is in turn chilled by the ice-box. This
circulation of air in a system due to differences in
densities is known as a convection current.
Mike Dickinson 2002
9
Figure 3.6 – Convection current
ice-box
Mike Dickinson 2002
10
When cooking vegetables in a pan of water, the
vegetables can be seen to circulate around the pan
as they move with the convection current that has
been set up in the water.
Mike Dickinson 2002
11
Before the advent of forced ventilation systems,
mine shafts were ventilated using a convection
current. A fire was built under one opening from
the mine. The warm air above the fire would rise
and be replaced with cool fresh air from an other
opening in the mine.
Mike Dickinson 2002
12
Figure 3.7 – Convection current
HEAT
Mike Dickinson 2002
13
Figure 3.8 – Mine ventilation
Mike Dickinson 2002
14
These convection currents can be created naturally
also. Consider the different winds that blow near
the coast in the evening compared with the
morning.
Figure 3.9 - Sea breezes
Mike Dickinson 2002
15
At sunset, after a day
where the sun has been
heating the land, warm air
rises above the land and is
replaced by the air above
the (relatively) cool sea.
This causes a breeze
blowing from the sea to the
land.
In the morning, after the
land has had time to cool
down, warm air rises above
the (relatively) warm sea
and is replaced by air from
above the land. This
causes a breeze blowing to
the sea from the land.
Mike Dickinson 2002
16
Radiation
Stand by a bonfire and it is difficult to get close to
it because of the radiant heat that the fire is
emitting. The hotter an object is, the more radiant
heat is given out from it.
On a sunny day, if you go outside wearing a black
tee-shirt, the shirt will absorb more radiant heat
than a white tee-shirt and you will feel very hot
because of this.
Mike Dickinson 2002
17
Black surfaces absorb radiant heat better than
white or shiny surfaces. Radiation is an
electromagnetic wave (like light), in the infra-red
region of the electromagnetic spectrum and
therefore it has similar properties to light. White
or shinny surfaces will reflect these waves better
than black and therefore will absorb less heat.
For example on a cloudy night, radiant heat
emitted from the earth is reflected back to the
earth from the clouds. The night temperature does
not fall as low as on a clear night, when this
radiant heat can simply escape.
Mike Dickinson 2002
18
Similarly, black surfaces are good emitters of
radiant heat, while white and shiny surfaces do
not emit radiant heat well. Kettles and saucepans
usually have shiny outer surfaces so that heat is
not wasted by radiation from the pan surface.
Mike Dickinson 2002
19