## Standing waves on a spring

Posted: October 29, 2014 in Institute of Physics
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You might have seen/done this one before, but the teachers I was showing it to found it quite useful, so I thought I would post it on talkphysics. The nodes and anti-nodes are a lot clearer when the amplitude of the oscialltions is higher, but I was told the signal generator broke the previous vibrator when set too high, so I was a bit cautious with this one.

There are several teaching and learning points with this demo. For example, you could get the students to calculate the speed of the wave along the spring, as we know the frequency from the signal generator and can measure the wavelength (for example by measuring the length of the stretched spring with a ruler).

Once we know the speed of the wave, we could ask the students to predict the next frequency in which a standing wave will form.

What else would you use this demonstration for? What other questions would/could you ask?

## Phases of the Moon (with an infrared twist)

Posted: May 24, 2014 in Institute of Physics, Thoughts and ideas
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I recently was asked about the phases of the Moon and why the Moon appears to change in shape in its orbit around the earth. So I thought a good point to start from was to establish what misconceptions people have on this topic and I found this great video by Veritasium (but I would stop it at the end of the interview without revealing the explanation initially).

The most common misconception in the video seems to be that the earth blocks the sunlight reaching the Moon, hence, we see the phases of the Moon when it is behind the earth with respect to the sun. At this point I would slide two circles of card, a white and a black one (with the black one bigger than the white one) past each other (black on top). If the black card is the shadow cast on the Moon by the earth (essentially what happens in a lunar eclipse), the shapes of the Moon due to the light reflected back to earth are considerably different than the shapes we observe from the phases of the Moon. So, we have establish that this model is a misconception.

Now we can introduce a better model and I use the pingpong ball in the image below with the students in the centre of the room. They are the observers on the earth and the light from the sun is coming from the left hand side in the photo.

If you go around the learner in the middle of the class making sure the “lit” face of the pingpong moon is always facing the wall on the left in this case, your students will see the same shapes we observe during the phases of the moon.

To reinforce this demonstration you could show the students this great animation by Keith Gibbs (also available in HTML5 if using an iPad).

Ok, now for the icing on the cake that you’ve all been waiting for! Check out the video below that shows a cresent moon through an infrared camera. You can see the crescent really bright, but you also see the other parts of the side of the moon facing the earth. I believe that is what is emitted by the moon in the infrared spectrum and that gets picked up by the IR camera. It is really awesome.

## LED bridge rectifier

Posted: February 15, 2014 in Institute of Physics, Thoughts and ideas
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I have always found it is quite hard to show the path of the current in a bridge rectifier to A-level students using diodes alone. The diodes are tiny, for a start, and you end up following the wire with your finger around, but students seem to get lost in the process. I still introduce the rectifier using diodes and one thing I show them is that even using a DC voltmeter doesn’t change the sign. This is convincing for some, but it is still nice to be able to give further proof of what’s going on.

The diagram might also help, because it is easier to follow the path around.

However, I have started building rectifiers with LEDs alongside the diode version and it works a treat. The first thing I show them is the circuit on DC current. Only two of the four LED light up, so I can ask “What would happen, if I reverse the polarity?” They now seem to get it and they often answer correctly that the other two LED will light up. I change the polarity several times to simulate the two half-waves, as in the images below.

Then, I get the spinning wheel we use to observe ripples in the ripple tank (the one with gaps, I can’t remember the name) and put the LED rectifier on AC. The result can be seen in the video below.

## A nice Pressure in Liquids question

Posted: December 6, 2013 in Institute of Physics, Thoughts and ideas
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I first was introduced to this really nice question by Neil Atkin (@natkin) and ever since I have tried to find a good way of showing it. So, look at the question and the explanation that I think is correct, as far as I can tell (but please point out any faults in my reasoning). Then, check out the simple demo I used to show this.

“If I am on a boat in a pond and I hold a 10 kg rock in my hand, what will happen to the level of the water if I drop the rock inside the pond? Will the water level increase, stay the same, or be lower?”

It’s all to do with Archimede’s Principle that states that the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid displaced by the body.

This should help us think about this problem. In fact, if the boat is floating it means that the weight of the water displaced by the rock when it is inside the boat must be the same as the weight of the rock itself. That is because the upthrust balances the weight of the boat, myself and the rock, or the boat would sink. So the rock displaces a volume equivalent to the space occupied by 10kg of water, i.e. 10 litres.

When I throw the rock inside the pond the water displaced by the rock is only the volume of the rock itself, which is most likely not 10 litres, but much less. So, the level of the water in the pond decreases!

I took these two photos before and after to convince you of this (the measuring cylinder we used in another attempt was to big to appreciate the difference). Click on either photo to enlarge them and see them in Gallery view.

It is quite amazing what you can learn by a simple visit to your old school (well I am on a Secondment, so it is still my school…). And it is quite scary, because I got this really cool demonstration by the guy who is covering me for this year and I am starting to fear they will want to get rid of me to keep him :-S

His name is Jonathan Wallace and he is an NQT at Croesyceiliog School (Cwmbran in Sunny Wales) you can contact him at jonny.wallace@live.co.uk

Anyway, have you ever seen the trick of the jelly marbles disappearing in water? Well that happens because these marbles are superabsorbent polymers that get filled with water when the come in contact with it, so when you put them into water they seem to disappear, because, being filled with water they have the same refraction index as the water surrounding them, i.e. light goes straight through them without being refracted (bent)! There is a really nice explanation of this phenomenon on Steve Spangler’s blog and you can buy these jelly marbles quite cheaply here.

But what William (Oops, I meant Jonathan) showed me a really nice twist, especially because it uses items that are a bit more familiar to the kids than some superabsorbent polymers, although they are really cool! William (Blow! I’ve done it again, I meant Jonathan) pours glycerine in a Pirex beaker and an empty (and very clean) test tube inside.

At this point you can still see the test tube inside the beaker, because the air inside the tube refracts the light going through it! But what would happen if we add Glycerine inside the test tube too?

Magic! The test tube disappears in the Glycerine! So, has the Glycerine dissolved the glass of the test tube, is it real Magic, or just another wonder of Physics? What does really happen here?

The answer is quite simple and it is very similar to the jelly marbles. The Pirex and Glycerine have the same (or at least very similar) refraction index and, therefore, light is not refracted at their boundaries and carries on through its path undisturbed by refractive effects, which means that the test tube appears to be invisible!

Thanks to William Wallace (again? Sorry, I meant Jonathan; I know it’s not funny if you are not a member of staff at Croesy, but I have to take the mick) for this great demonstration!

Here is a lovely classroom demonstration that I saw at the ASE Conference 2010 in Nottingham. The demo was part of the Physics Education Lecture, which displayed the best of the PhysEd magazine. I really learned a lot and was well impressed by the quality and creativity of the demonstrations, activities and workshops proposed by the Institute of Physics. As one of the IoP Network Coordinators I was very proud to be part of the Institute and see how many outstanding workshops and lectures they put together for the event. Apparently, the IoP did the majority of workshops and they were all free of charge, although the conference was organised by the ASE.

Anyway, coming back to our demonstration. At the lecture it was shown using two small glasses, so when I went back to my lab I thought; “What would happen, if I use two very tall columns of water? And this was the result!

So, why won’t the two liquids mix?

I put cold water in the bottom column with some blue food colouring and boiling hot water in the top column with some red food colouring. The tricky bit is how to turn the top column upside down, as it is really hot and heavy, but it was well worth it! So, I put a sheet of paper on the top and then carefully turned it upside down (you might need a helper to do this). Then, I placed the top column on bottom one and as you can see, and unlike what the kids would expect, the red and blue water don’t mix. They actually stay unmixed for a very long time (over an hour at least).

But how do we explain such an effective phenomenon? Well, the hot water is less dense than the cold water at the bottom, as its particles have more kinetic energy, hence moving further apart from each other. The result is that we have two liquids of different density, with the less dense one at the top, which therefore will float on top of the denser cold water. It is a bit like having oil and water, you can tell your students!

This is a really nice demonstration that will really help your pupils to understand that hot liquid rises and cold liquid falls. It’s not only very memorable, but it also shows quite clearly that in heat convection currents it’s not the “heat” that rises, but the hot liquid, or gas.