Heat Transfer
Heat always moves from a warmer place to
a cooler place.
Hot objects in a cooler room will cool to
room temperature.
Cold objects in a warmer room will heat up
to room temperature.
Heat Transfer Methods
Heat transfers in three ways:
Conduction
Convection
Radiation
Conduction is a process that describes the transfer of heat
through a solid by the COLLISIONS of higher energy particles
with lower energy particles
Metals are different
The outer e______
lectrons of metal atoms
drift, and are free to move.
When the metal is
heated, this ‘sea of
ineti
electrons’ gain
c
k_____ energy and
transfer it throughout
the metal.
Insulators, such as w___
oo and p____,
lasti do not
have this ‘sea of electrons’
d which
c is why they
do not conduct heat as well as metals.
Why does metal feel colder than wood, if they are
both at the same temperature?
Metal is a conductor, wood is an insulator. Metal conducts the heat
away from your hands. Wood does not conduct the heat away from
your hands as well as the metal, so the wood feels warmer than the
metal.
In solids, lattice vibrations transfer heat by conduction.
Discrepant event
activity
In which case the paper will burn first?
Why?
The metal half conducts the
heat energy away, ‘protecting’
the paper, but the wood and
the paper are both insulators.
Steady state - when the temperature at any point in
the object does not change with time.
What is happening?
What is happening?
Watch from 1.30 min - 2.15 min
Convection: heat transfer in fluids
Fluids are liquids & gases and when HEATED they flow.
Hot fluid becomes less dense when HEATED
A decrease in density occurs because particles gain
kinetic energy and spread out taking up more volume
Cold denser fluid sinks DUE TO FORCE OF
GRAVITY and displaces the warmer less dense
fluid.
Warmer fluid rises upwards and cools.
An increase in density then occurs because particles
lose kinetic energy and get closer together, occupying
less volume
Cool fluid falls downwards ( due to gravity) and the
process continues so long as the fluid is heated.
THE FLOW OF HEAT AS DESCRIBED IN
THE PREVIOUS SLIDE IS CALLED A
CONVECTION CURRENT.
THIS NEEDS BOTH A HEAT
SOURCE AND A GRAVITATIONAL
FORCE.
Why are the Americas, Europe and Africa are
moving further apart every year? (p222)
Convection currents in potassium
permanganate
Convection currents in a heated pot
Would it be a good idea to raise the
heater? Why?
Radiation
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False
Radiation
Radiation travels in straight lines
True/False
Radiation can travel through a vacuum
True/False
Radiation travels at the speed of light
True/False
Emission experiment
Four containers were filled with warm water. Which container
would have the warmest water after ten minutes?
Dull Shiny black
metal
Shiny metal Dull
black
The __________ container would be the warmest after ten minutes
because its shiny surface reflects heat _______ back into the
container so less is lost. The ________ container would be the
coolest because it is the best at _______ heat radiation.
Emission experiment
Four containers were filled with warm water. Which container
would have the warmest water after ten minutes?
Dull metal Shiny
black
Shiny metal Dull black
shiny metal container would be the warmest after ten minutes
The __________
because its shiny surface reflects heat _______ back radiatio
into the
container so less is lost. The ________ container
dull would n be the
coolest because it is the best at _______ heat radiation.
black emittin
g
Absorption experiment
Four containers were placed equidistant from a heater. Which container
would have the warmest water after ten minutes?
Dull Shiny black
metal
Shiny metal Dull
black
The __________ container would be the warmest after ten minutes
because its surface absorbs heat _______ the best. The _________
container would be the coolest because it is the poorest at
__________ heat radiation.
Absorption experiment
Four containers were placed equidistant from a heater. Which container
would have the warmest water after ten minutes?
Dull metal Shiny
black
Shiny metal Dull black
dull
The __________ container would be the warmest after ten minutes
becauseblack
its surface absorbs heat _______ the best. The _________
radiatio
container
shinywould
metalbe the coolest because it is thenpoorest at
__________ heat absorbing
radiation.
Convection questions
Why does hot air rise and cold air sink?
Why are houses painted white in hot countries?
Convection questions
Why does hot air rise and cold air sink?
Cool air is more dense than warm air, so the
cool air ‘falls through’ the warm air.
Why are houses painted white in hot countries?
White reflects heat radiation and keeps the house cooler.
By looking at the furnace can you tell in which
part of it the temperature is the highest, coolest?
https://www.youtube.com/watch?v=BBwULYIOkfk
https://www.youtube.com/watch?v=Psvo_XEc784
https://www.youtube.com/watch?v=_0tkbp8yk-w
We know that any object that absorbs energy will heat up; it will
increase in temperature. The object then has an opportunity to radiate
some of that absorbed energy away. Blackbodies not only absorb
energy, but they also radiate it, and they do this in a very specific way.
Consider the electric heating element shown below.
This heating coil is emitting radiation. We see that part of the coil is
literally “red-hot.” Other parts of the coil are not quite so hot and appear
dull red in color. As the coil heats, the wavelength (and therefore the
energy) of the light emitted changes. This heating coil is a good
approximation of a blackbody. Blackbodies also emit light depending on
their temperature. Keeping in mind that many wavelengths of light are
invisible to our eyes, consider a graph showing how light is emitted by
blackbodies at different temperatures.
At low temperature (3000K) the maximum wavelength emitted from the
blackbody is not in the visible range, so we can see the heat radiation
(colour) coming from such object. As the temperature increases
(4000K), the max. wavelength is still not in visible range but we can
some red and even less mix of other colours of spectrum. At high
temperature (6000K) we can a mix of colours (wavelength) of spectrum
which together combine into a white colour. So a start at this
temperature would appear white.
Change the temperature and observe what happens
to the intensity.
https://phet.colorado.edu/sims/html/blackbody-spectrum/latest/blackbody-spect
rum_all.html
