Hi. It’s Mr. Andersen and this AP Physics essentials video 101. It is on mechanical
and electromagnetic waves. A good way to contrast these is to figure out, can they be transmitted
through space? Through an area where there is no medium or no matter? An example of electromagnetic
waves that we can see are explosions on the moon. The moon keeps getting hit by meteors
and there are little flashes of light that NASA has been monitoring. One, in 2013, was
so bright that you could have seen it with a naked eye. Now that flash is moving towards
our eyes through space. It is electromagnetic. But it is also producing mechanical waves.
So we would have seismic waves on the moon. If you were on the moon, you could feel those,
the vibration through the moon surface itself. But since there is no air around the moon,
no atmosphere, you would not hear any sound. And in fact, if the moon were to explode,
if you were to look at it in space, it would be totally silent. You could still see it,
because that is electromagnetic waves. And so energy is transferred through waves. Those
waves can be mechanical or electromagnetic. Mechanical are ones that require a medium
or some material or matter to move through. Electromagnetic do not. No medium required.
An example of a mechanical would be sound waves. An example of electromagnetic would
be light. And so mechanical waves require some medium to move through. So as you move
your hands across the surface of water, water is the medium. You are oscillating the surface
and that energy is being transferred through the medium of the water itself. An example
could be ocean waves. Sounds waves work the same way. You cannot see it, but there are
air molecules. And so as I speak I am vibrating air molecules that vibrate the microphone
which vibrates your speaker which vibrates the air around your ear and so you can hear
it. If there is no gas molecules around your ear then you cannot hear it. An example of
mechanical waves are seismic waves, those that are produced after an earthquake. And
this is a really cool study. So this was in 2011, there was an earthquake. You can see
right here in Virginia. And they were monitoring Twitter and looking for people who tweeted
the word earthquake. And they just kept track of where those tweets were and how long it
took for them to reach those people. And so you can see the seismic waves are actually
moving out. You can trace where the earthquake went. Now what is interesting is that twitter
uses electromagnetic waves. So it is using electricity to transfer those messages and
so the U.S. Geologic Service is looking at Twitter as a way that we could have early
warning system so we could pick up and protect ourselves before those mechanical waves reach
us. An example of a mechanical wave that we are all familiar with is sounds waves. So
when you hit a bass drum like this it is very loud. But what you are really doing is vibrating
the surface of the drum and that is vibrating, if we look at it, it is vibrating all the
air molecules around it. And so we have these longitudinal waves that move out, kind of
if a three dimensional sphere, away from that bass drum. So that is vibrating your ear drum
and that is what you perceive as sounds. Now electromagnetic waves are different. If we
look out into space there is no medium there. There is no matter. But we are getting light
from these distant stars. And so light or radio waves can travel through nothingness.
Now they can still move through medium. So for example light is moving though your room
where there is gas or could move through a prism. We could break it into its electromagnetic
waves. So we could see the different colors inside white light. But it can still move
through nothingness. A good example of that would be using a vacuum pump. So if we were
to put an alarm clock inside a vacuum pump and we just have the alarm go off. And so
the way this works is it vibrates back and forth and you could ring the bells back and
forth like this. But let’s say we start to remove the air from inside the vacuum.
What would happen as we pump the air out? You would still be able to see the vibration
in the alarm clock, because that is electromagnetic waves, but pretty soon all of the gas would
be gone. There would be no more sounds waves being produced and you would not hear anything.
And so did you learn to describe sound as a transfer of energy through a medium? And
then can you contrast mechanical, which require a medium and electromagnetic waves which do not?
I hope so. And hope that was helpful.