Video: GCSE Physics Higher Tier Pack 1 • Paper 2 • Question 11

GCSE Physics Higher Tier Pack 1 • Paper 2 • Question 11

09:46

Video Transcript

The motion of a water wave on the sea is shown in Figure one. The direction in which a particle in the wave moves is also shown. Which of the following statements is correct? Tick one box. The water wave is a transverse wave. The water wave is a longitudinal wave. The water wave is not purely a transverse wave or purely a longitudinal wave.

Looking at Figure one, we see it shows us a wave in the ocean moving left to right as well as a particle that moves in the water, while the wave passes by. Interestingly, though the wave moves left to right, the particle we see is moving in a circular path. And based on the information in this figure, we want to figure out which of these three statements is correct.

The statements have to do with what type of wave the water wave is: whether transverse, longitudinal, or not entirely either one. To figure this out, let’s recall what transverse and longitudinal waves are in the first place. When we think about waves, these are the two broad categories we divide them into: transverse and longitudinal.

A transverse wave is one in which the displacement of the wave is perpendicular to the wave’s direction. In other words, if our wave was moving left to right, then in a transverse wave the wave displacement would be up and down, perpendicular to the wave’s direction.

On the other hand, for a longitudinal wave, the displacement of the wave is parallel to the wave’s direction. For this kind of wave, if the wave direction was left to right, then the wave displacement would be along that same axis forward and backward.

Knowing all this, if we look back at Figure one, we see that we have a wave, which is moving left to right. This means that if this wave was purely transverse, we would expect our particle which is in the water to be moving up and down or if this wave was longitudinal, we would expect the particle to be moving left to right only.

But as we look at the path of the particle’s motion, we see that it’s necessary for the particle to move in all these directions at various times. Each one is required for the particle to move in a circular arc. This indicates that the water wave in the sea is some combination of transverse and longitudinal. It’s not purely either one.

And therefore, we tick the third box. Based on the particle’s motion, the water wave is not purely a transverse wave or purely a longitudinal wave.

Next, let’s consider how the distance between a fixed point on our travelling wave and the particle varies over time.

The peak of a wave is initially at the position of the particle shown in Figure one. Describe how the distance between the particle and the peak of the wave varies over time.

Looking again at Figure one, we see that a peak of this travelling wave is colocated with the particle at this instant in time we’re shown. We knew though that the wave is moving left to right. So this particular point on the wave will become more and more distant from the particle as time passes.

As we consider this distance and how it changes over time, we see though that there’s a second dynamic at play. And that’s the fact that this particle is moving in a circular arc. This tells us that sometimes the particle is moving towards the point on the wave, sometimes it’s moving away, and at other times it’s moving neither closer nor farther away.

We want to describe how the distance between this particular wave peak and the particle changes with time. And since both the wave and the particle are in motion, our description will involve both elements.

First, let’s describe what it means that the wave is moving left to right, while the average position of our particle is constant. Based on that, we can say that in general the distance from this particular peak on the wave to the particle increases over time. That makes sense since the wave is in steady motion, while the average position of the particle doesn’t change.

But as we’ve seen, this isn’t the only factor that affects the distance between the particle and the peak of the wave. There is also this cyclical effect of the particle’s motion, moving sometimes closer sometimes farther away from the wave peak.

Even though the distance between the peak and the particle is always increasing, the rate at which that increase happens is not constant thanks to this cyclical motion. As a second part of our description then, we can say this: we can say, “The rate at which the distance increases changes cyclically.” It’s not constant, but it is affected by the particle’s circular motion.

Taken together, these two statements describe how the distance between the particle and the peak of the wave changes with time. Finally, let’s consider how water waves can help drive the motion not just of particles, but of power generators.

Figure two shows an electrical generator that is powered by wave motion. Explain how the generator uses the motion of water in waves to generate and distribute electrical power.

Looking at Figure two, we see it starts off with a wave of water which comes in contact with a flat plate which is part of this power generator. Connected to the flat plate is a permanent magnet. And this magnet is wrapped around with a conducting coil, which itself is connected by cables to the national grid where everyone commercially and residentially gets their power.

Our task is to explain just how this generator actually works — in other words, just how is it that waves may be in the ocean help to generate and distribute electrical power through this generator. For our explanation, we’ll simply walk step by step through just how it is that this generator operates.

If we begin at the beginning, we see that there are water waves, which push down on this flat metal plate. So let’s write that out as step one. Having written that out, we might wonder just how do we know that the waves actually do push down on the plate. What if the wave is a longitudinal wave and it only pushes objects left to right?

It’s a good question and let’s consider it this way. Let’s assume that the waves in this diagram are moving either left or right; that is, like virtually all waves we experience on a large body of water, these waves are moving horizontally. So that means the wave direction is horizontal. But we see from this diagram that the wave displacement — at least a component of it — is vertical.

This is the hallmark of a transverse wave. This tells us that our water wave has at least some transverse wave element to it. And for a transverse wave, the wave displacement is perpendicular to the wave’s direction. That tells us that this water wave indeed will at times press downward under this flat plate.

Okay, so after the plate is pushed down, what happens next? Looking at our diagram, we see that the plate is connected to this permanent cylindrical magnet. And since they’re connected, the two must move together. That means that when the plate moves down, so does the magnet.

We’ll write that out. We’ll say that when the plate moves down, the magnet also moves down since after all the two are connected. We see from our diagram that our permanent cylindrical magnet is surrounded by this conducting coil. And the coil is called a rigid conducting coil.

That tells us that the coil and the magnet move independently or specifically that the coil is fixed in one place. We can say then that because the coil is fixed in place, the magnet moves past the coil. That may not seem particularly profound, but it actually makes a big difference for the power generation in this generator.

Here’s where the process gets very interesting. When we think about this permanent magnet that’s part of our generator, we know that a magnet will create magnetic field lines between its north and its south poles. It will look a little bit like this if we draw those field lines in.

As the water wave pushes down on the flat plate and causes the centre or south pole of this permanent magnet to drop, that magnet will be put in motion, which will change the magnetic field. The rigid coil which we’re told does not move will be exposed then to a changing magnetic field due to the moving permanent magnet. This tells us that the loops in this coil will be exposed to a changing magnetic flux.

And what happens when a loop of conducting coil has a changing magnetic flux through it? We know from Faraday’s law that what will happen is an EMF will be induced across this rigid conducting coil, which will cause the motion of current. If we put this in writing, we’d say that when the magnet moves past the coil, this induces a current in the coil. This is the critical step in the process where electricity is actually generated. It’s through the induction of current in this rigid conducting coil.

Now that we’ve generated electricity, we move on to the question of how we will distribute this electrical power. Looking again at our diagram, we see that our rigid conducting coil, which at this point has induced current running through it, is connected by cables to the national electrical grid. This is the means by which the induced current is spread out or distributed.

Writing out this step, we could say that the current induced in the coil is transmitted to the national grid by cables. At this point, we’ve covered electrical generation and distribution. The only question is “How does the system actually reset itself so we can do all this over and over and over again?” This is where we get to the last step in the process. And it involves this spring, as shown in the bottom of Figure two.

We know the water wave is capable of pushing down on the flat plate, but it’s not capable of pulling the plate back up. That’s where the spring comes in. The spring’s job is to lift this whole magnetic apparatus back up so that another wave can come along and press it back down, creating an up and down motion which continuously generates electricity.

These six steps in this order explain just how it is that this generator both generates and distributes electrical power.

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