Waves (GCSE Physics)


Uploaded by freeeschool on 04.09.2012

Transcript:
GCSE Physics - Waves
Hello! Welcome to a video about Waves. We are going to look at two different types of
waves. We are going to put some labels on them and we are also going to work out how
to calculate the velocity of a wave.
It starts off by breaking the waves down into two different and distinct kinds. There are
transverse waves and there are longitudinal waves. These are two very important key words.
In terms of transverse waves, these are probably the ones that you are more familiar with.
These are the ones that you might see on a rope or water waves. But also, in addition
to that are other kinds of waves that are transverse as well. All the electromagnetic
waves are transverse waves. For example: radio waves and light waves. These are both types
of transverse waves.
In terms of longitudinal waves, the types of waves that you get for this are sound,
and certain types of seismic waves. These are waves that are created by earthquakes
and they can also be longitudinal as well.
Now, the thing to remember about the different types of waves is that these four at the top
here (rope, water, sound and seismic) are what we call mechanical waves. That means
that they need a substance to travel through. They need what we call a medium. Medium is
just a science word for substance, whereas these ones down here on the transverse side,
these two (radio and light) are types of electromagnetic waves. They actually don’t need a medium;
they don’t need a substance to travel through. So radio waves and light waves and in fact,
all the electromagnetic waves can travel through a vacuum.
That’s just a few key ideas behind the two types of waves and what we are going look
now is the labels on the two different kinds of waves. What we have at the top here, this
is our transverse waves and there are a couple of things that I want you to be able to label
on this. The first one is the points at the top of each wave. We call those the peaks
or sometimes called the crest. On the opposite side, on the bottom there, we have the trough.
As well as the peak and the trough, we have a distance between one wave and another. The
distance between the two peaks is called wavelength. Let’s just put an arrow in there to make
it a bit clearer.
The distance between one peak and the next is called the wavelength. The label in here
is the distance between this middle section here and the top of the wave or even the middle
and the bottom. That’s called the amplitude. It is probably worth putting in some arrows
there so you know what referring to.
Those are the different labels I’d like you to know. There is also one more that you
can clearly see in the diagram and that’s called the frequency and frequency just means
the number of waves per second. The number of waves per second is measuring frequency
and it has a unit. The unit is Hertz or Hz which is an abbreviation.
Frequency is the number of waves per second. You can’t tell because you got no timing
in here. But that’s how you measure that. The unit is Hz. There is one thing probably
worth mentioning here. If you want to count the number of waves, we can go to a peak to
peak, but starting at this end here, we can go from there all the way down and back up
to the top as we said. If we are starting at the midpoint in here, this is one wave.
From there, all the way down to the bottom and back to the middle line again. That’s
one wave there. So in this diagram, there are six waves and that can help us in a little
while when we do the calculation for frequency.
That’s the transverse waves and a few definitions there. The wave we have at the bottom here
is called a longitudinal wave. This is the longitudinal wave and it is the kind of wave
that we said that sound produces.
Now the difference between these two waves is the transverse waves go up and down while
the longitudinal wave goes back and forth. So if the wave is travelling in this direction,
the vibration to create that wave goes backwards and forwards.
It is important to know that the transfer of energy is parallel to the vibration. So
vibration is going backwards and forwards and the transfer of energy is going in a parallel
direction. With the transverse wave, it is slightly different. The wave is travelling
in this direction here. But the vibration travels up and down and we say that the vibration
is perpendicular to the direction of transfer of energy.
So a couple of important points there. Now, in terms of labeling our longitudinal wave,
there are two different parts. We have a bit of kind of squashed there and that’s called
a compression and the stretched out between two compressions is called a rarefaction.
If you are to measure the wavelength on the longitudinal wave, you’d go from middle
of one compression to the middle of the next one or you could go from the middle of rarefaction
to the middle of the next rarefaction. But that’s how you measure wavelength on the
longitudinal waves.
A couple of key points or a couple of key labels on there, which will help us with the
next bit. Here are two bits of information that we are going to use that we are going
to use to do the following calculation. The first one is we said before, frequency means
waves per second. But we also need to know this formula here: Velocity = frequency x
wavelength. Velocity is measured in meters per second. Frequency in Hertz and wavelength
in meters. But before we can do anything else, we need to have a look at an example.
Here, we have an example. I got a timeline and a couple of ways here in which we can
work with. Now in terms of frequency, the waves per second, I got eight seconds along
the bottom and I got a certain number of waves and if you remember from the previous slide,
the number of waves that you can count is as follows, so there two waves and the time
is eight seconds. It is 2 divided by 8 equals 0.25 waves per second or 0.25 Hz.
It follows then that we try to calculate the velocity of this wave. I need to know the
frequency which we have and the wavelength of which actually, you can’t tell from there.
If I just add that in, you got a wavelength of 10 meters. How do we calculate the velocity?
It is a simple case of putting in the numbers into this formula over here. So we have frequency
from our previous calculation is 0.25 Hz. The wavelength is 10 meters and so our velocity
is therefore 0.25 x 10 = 2.5 meters per second.
So there, we have it. A few key labels on the two different kinds of waves, how to calculate
the frequency and how to calculate the velocity.
[end of audio – 08:19] GCSE Physics - Waves
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