Wednesday, 27 August 2008

Angular momentum with sellotape and the speed of light with microwaved chocolate.Two months worth of science in one week.

So, what have we been up to?

Well, last week we went to an hour-long workshop titled 'The Science of Magic' at our library and run by staff from a local science 'hands-on' centre. Having two scientist parents in the family means that the kids have a pretty good grounding in science. Our experience of science workshops aimed at children is at worst they are badly presented or give inaccurate scientific knowledge, and at best they are oversimplified and patronising. Fortunately this presentation was neither.
Although the bicarb and vinegar (in this case, water) in a film capsule experiment was very familiar to us (see previous blogs), the other experiments/demonstrations were less familiar. One involved an optical illusion, another involved angular momentum and another magnetism. Even I learnt several things from it; I didn't know much about angular momentum and had forgotten that zinc and cobalt were attracted to magnets. I also learnt that of the three things: movement, magnetism and electricity, any 2 of these combined would make the third. How simple! Now why didn't they teach me physics like that at school?
Surprisingly, the kids seemed to take quite a bit in. Ds2 was repeating some of the information about 'blind spots' as we crossed the road the following day, and both boys tried to repeat the experiment with a roll of sellotape tied to a string and held up, while a mug with an egg was suspended on the other end of the string over a rod. Ok, difficult one to explain...I did try and do a google search on 'angular momentum' and 'sellotape', but, not surprisingly it just came up with some very weird links. I resisted the urge to click on the one about locusts...don't think that would have been useful. If anyone finds a link to a better description of this experiment let me know and I'll put in it.

Dd1 finds a cabbage white butterfly in the garden and carries it around all day in her pop-up bug box. It even went for a car ride!

So what else have we been up to? Well ds1 played cricket for the first time, at a birthday party. I looked up from my glass of wine to find him actively participating in a group sport (shock horror!) and then taking advice on bowling (!). Maybe sometimes I underestimate my kids. Anyway, he obviously doesn't have any sport genes from his parents as he managed to bowl reasonably successfully AND hit a ball with a cricket bat, neither of which are skills his parents posess.

Then of course more time at the sailing club. Will probably get there this week some time too. Not a huge amount of actual sailing has been taking place, but as usual dd1 has been freezing her wotsits off, swimming in the lake. Have just ordered her a wetsuit (pink of course) in the hope that this might allow her to stay longer in the water (i.e. prevent her lips from turning blue during the first 5 minutes). Just hoping she likes it enough to wear it. Fussy? That's an understatement.

Ds2 takes dd1 out for a sail in an Oppie. I ask them not to go too far out, just in case they need rescuing!


For the past two mornings ds1 has been late out of bed because he has been reading. Yes..READING! READING! READING!I hardly dared to get him out of bed this morning, but he had a workshop to go to and was being picked up quite early. Looks like he's worked his way through the first 5 chapters of a Goosebumps book.

The museum workshop ds1 went to this morning was 'Flintnapping'. So, two hours later he came home with a sharp - but apparently very functional - bit of flint. According to him it would be ideal for making arrows. Great. Might keep that in the kitchen drawer: it could come in handy if I mislay the potato peeler.

At the museum there was an exhibition on the 70s. It's a bit scary to see your familiar childhood items displayed in a museum. Am I really THAT old? I did love the hard plastic tupperware-style snack tray that had swivel dishes on a kind of 'tree'. Orange, too. Nice. Still it's better to have your childhhod labelled as 'retro' rather than 'antique' I guess.

The kids clean up a bowl of chocolate.

Using leftover chocolate from a Krampf experiment (see we covered some chocolate brownies that I'd made. The experiment involved melting chocolate in a microwave to see how a microwave works and to work out the speed of light. We calculated it to be around 4.4 billion thingies, so we were only a couple of billion thingies out. I think we probably didn't have a big enough try to put the chocolate in, so some of the hot spots were missing and this led to inaccuracies in the calculations. Hey, what's a few billion between friends?

Here are the instructions, cut and pasted direct from Krampf website.



Part 1:

To try this, you will need:

a microwave oven
waxed paper
several chocolate bars

a large plastic, glass, or paper plate. Do not use metal!
Start by looking at the inside of the oven. If it has a turntable to rotate the food (most do), remove it. We want the chocolate to stay in one place, not move around.
Cover the plate with waxed paper, and then place the chocolate bars (unwrapped) on the plate to form a solid layer. You want the layer of chocolate to be as flat and even as possible.
Place the plate of chocolate in the oven and set the timer for 30 seconds. Depending on your oven, you may have to cook it a bit longer, but I learned from experience (see this week’s video) that cooking too long gives you a LOT of smoke and a mess.
After 30 seconds of cooking, check the results. You should find that there are spots where the chocolate is melted, and maybe burned, and other places where it is not melted at all. Why?
Your microwave oven works by producing microwave radiation. No, its not radioactive! This is electromagnetic radiation, which also includes visible light, radio waves, ultraviolet light, radar, etc. Microwaves can cause water molecules to vibrate, producing heat to cook your food. OK, so why does your oven have hot spots, instead of cooking evenly?
Instead of just blasting microwaves around, your oven produces something called a standing wave. The easiest way to imagine a standing wave is to look at one. Get several feet of rope, and tie one end to a doorknob. Hold the other end move back to take up most of the slack. You don’t want the rope tight. Start shaking the rope up and down, and notice the way the rope wiggles. By adjusting how fast you shake the rope, you can find the point where it produces a stable pattern. Some parts of the rope will always be moving up and down, while other points will not move much at all. Its easier to see in the video than it is to describe, but you should recognize the pattern when you see it. That is a standing wave. The points where the wave is moving up and down a lot would be the part of the wave that produces a lot of heating in the oven, producing the burned spots. The part of the wave that does not move much would not produce much heat, giving you the cooler spots in the oven. That is why you need a turntable to move the food through the hot spots, to heat it evenly.

Part 2:

Follow the directions from last week’s experiment, to produce the melted spots. If you still have last week’s chocolate, you can use it. If, on the other hand, you are like me, you ate all the chocolate, and so now you have to start from scratch. Of course, that means more chocolate to eat, so its not a bad thing.
Once you have cooked the chocolate and have your melted spots, the next step is to look at the pattern. Different ovens may have different patterns, but mine produced three evenly spaced spots on one side, and two on the other side. There would have been three on the other side, but one of the spots was beyond the edge of the chocolate.
Measure the distance from the center of one melted spot to the center of the next one. You should get something close to 6 centimeters. Mine measured 6.2 centimeters. Then measure the distance from the center of the second spot to the center of the third. Again, I got 6.2 centimeters.
Now, think back to last week. We said that each hot spot represented the extreme point of a standing wave of microwave radiation. If you froze the wave, the first melted spot would be the top of one wave. The second melted spot would be the bottom of that wave, and the third spot would be the top of the next wave. The distance from wave top to wave top is the wavelength, so the wavelength for my microwave oven is 12.4 centimeters (6.2 + 6.2)
Once we know the wave length, then we multiply that by the frequency. Frequency tells us how many waves per second are passing a point. Looking on the back label, I found that my microwave oven has a frequency of 2450 megahertz. One megahertz is one million waves per second, so 2450 megahertz is 2,450,000,000 waves per second. We know that each of those waves is 12.4 centimeters long, so if we multiply frequency times wavelength, we will get the speed of the microwaves. That gives us a speed of 30,380,000,000 centimeters per second. The actual speed of light is 29,979,245,800 centimeters per second, surprisingly close considering that I was not very precise with my measurements.
Afterwards, I realized that I should have put the chocolate bars on an insulating bed of graham crackers, and then added a top insulating layer of marshmallows, to make light speed s’mores. Now that is a potentially winning science fair project!

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