Infrared Radiation (GCSE Physics)

Uploaded by freeeschool on 04.09.2012

GCSE Physics – Infrared Radiation
Hello! Welcome to a video about Heat transferred. This is the third video looking at Heat Transfer
and specifically, we are going to look at Infrared Radiation.
It starts off by having a look at some key, important points about infrared which you
should be aware about. First one is that all objects emit or absorb infrared. Sometimes,
we just say emit or absorb heat, but what we actually mean is infrared radiation. Hot
objects will emit more infrared radiation. It’s electromagnetic wave and if you think
back to the video on Electromagnetic Spectrum, infrared was in there. Actually, infrared
radiation is one of those waves with various uses.
Different surfaces emit and absorb differently. We will have a look at that in the moment.
Heat can be detected with your skin but you can’t detect it with your eyes. You can
actually look at what we call thermograms, which are special cameras that can detect
infrared and create images based on infrared radiation, but you can’t see it with your
eyes just alone.
Another important point here is there are no particles required to transfer heat by
infrared radiation. If you look at conduction or convection, these require particles for
heat to be transferred, but with infrared radiation, it doesn’t require any particles
at all and that’s a good thing because it can travel through a vacuum and space is a
vacuum and that means that the heat from the sun can reach planet Earth.
Let’s have a look at the 4th point in here in a bit more detail. I’ve got various surfaces
over here. This one is shiny black, dull or sometimes called matt, matt black. I’ve
got shiny white and dull or matt white.
Just to give us an idea of how these absorb infrared, we are going to look at just absorbing
for the moment; we will be looking at emitting in a minute. How do these absorb infrared?
If I use some arrows to give you an indication, these arrows show that the thicker arrow shows
this shiny black surface is absorbing quite a lot of infrared. If you have a dull matt
surface, or dull dark surface, you’ll notice that there is much more absorption or there
is more absorption of infrared than compared to shiny. Although, you got the same color,
the shininess has the effect of reducing the amount of infrared that is absorbed and that’s
partly as a result of some of the infrared being reflected.
Again, if you look at shiny white surface, that will absorb a lot less than the black
or the dull black, but it will also absorb less than if you got dull or matt white. These
arrows are supposed to be slightly thicker but while it is not absorbing very much, a
matt surface will absorb more than a shiny surface.
You should be aware of the kinds of surfaces that absorb best and also the kind of surfaces
that emits best and that’s what we are going to have a look at next. The example that I’m
going to give is a dull black versus a shiny white surface or the two extremes. Imagine
that they are both containers and they are both at 100?C. It could be a container filled
with boiling water. If you were to look at how these two emit infrared radiation and
you draw a graph, you’ve get something that looks like this.
So for the black, because it has a dull surface, not only is it a good absorber of infrared,
but it is a better emitter of infrared, as well. So it gives out more infrared than a
shiny surface. If you look at the graph, it would look something like this. The black
would get rid of that heat much more rapidly or more rapidly and it would cool down more
quickly. Whereas the white would take longer to cool down and it would cool down less rapidly.
The point here is not only is a dull, dark surface better absorbing infrared, but it
is also at emitting and giving out.
Let’s have a look at that as an example. Imagine there is heat being actually place
onto the two containers that we have here. So heat is actually falling on these two containers.
The infrared is falling on those two. What would the graph look like for those two, which
one would warm up more rapidly? Well, we know that a dull, dark surface is better at absorbing
and so if you are to track the temperature over time, you would see that the black surface
would warm more rapidly and get to a high temperature in a certain amount of time. The
white one is slightly more slowly.
The key points here are while dark surfaces are good at absorbing, they are also good
at emitting radiation and they are better at emitting radiation than white surfaces.
Over all, you should be able to compare the different types of surfaces, give a few features
of infrared radiation and just describe possibly graphs such as this.
[end of audio – 05:33] GCSE Physics – Infrared Radiation