# The Naked Scientists Forum

### Author Topic: International Young Physicists' Tournament 2010  (Read 3350 times)

#### Laura_Kelly

• Full Member
• Posts: 66
##### International Young Physicists' Tournament 2010
« on: 25/08/2009 07:22:17 »
I was wondering if the amazing world of physicists out there would be interested in helping me solve some questions given for the International Young Physicists Tournament held in March next year (the NZ part anyway). We are encouraged to get opinions and theories from lots of sources, so your help is perfectly within competition rules.

There are 7 questions I need to investigate and prepare powerpoints about:
1. Electromagnetic cannon:
a solenoid can be used to fire a small ball. A capacitor is used to energise the solenoid coil. Build a device with a capacitor charged to a maximum 50V. Investigate the relevant parameters and maximise the speed of the ball.

2. Grid:
A plastic grid covers the open end of a cylindrical vesselcontaining water. The grid is covered and the vessel is turned upside down. What is the maximum size of holes in the grid so that water doesn't flow out when the cover is removed?

3. Liquid light guide:
A transparent vessel is filled with a liquid (e.g. water). A jet flows out of the vessel. A light source is placed so that a horizontal beam enters the liquid jet. Under what conditions does the jet act as a light guide?

4. Sand:
Dry sand is rather "soft" to walk on when compared to damp sand. However sand containing a significant amount of water becomes soft again. Investigate the parameters that affect the softness of sand.

5. Shrieking rod:
A metal rod is held between two fingers and hit. Investigate how the sound produced depends on the position of holding and hitting the rod.

6. Rotating spring:
A helical spring is rotated about one of its ends around a vertical axis. Investigate the expansion of the spring with and without an additional mass attached to its free end.

7. Kelvin's Dropper:
Construct Kelvin's dropper. Measure the highest voltage it can produce. Investigate its dependence on relevant parameters.

It would be awesome if I could get some theories, as well as how I could go about building some of these experiments, as I will be building them all and researching the results as well.

#### Pyromaniac

• First timers
• Posts: 1
##### International Young Physicists' Tournament 2010
« Reply #1 on: 28/08/2009 00:28:46 »
Well i'm only 16 myself and most of this stuff is way over my head, and i'm sure you could get a much better explanation from someone else, but i'll try to help. The only one i think i really understand is the 'liquid light guide'. This is quite a simple experiment to do, in fact i imagine you've probably seen it quite a few times. i had a look around but these were the best pictures i could find, so i hope you know what I mean.

If it works, the light will follow the curve of the water jet and be 'bent'. If you allow the jet to fall onto a flat surface you will see a spot of light where it hits it. I can't say for certain, but i'm pretty sure that this works because you get refraction where the water and the air meet, and the angle that the light is travelling in relation to the jet means that you get total internal reflection (the refraction actually bends the light back inside the jet at the border between the two media, like a mirror). The main condition for this to work would be the angle of the jet as it bends. Too acute and you won't get total internal refection. This is the critical angle, which is about 48.6 degrees for water. I'll admit here that i'm not great at formulas, but if you wanted to work it out for yourself here it is, courtesy of Wikipedia :)

The critical angle is the angle of incidence above which total internal reflection occurs. The angle of incidence is measured with respect to the normal at the refractive boundary. The critical angle θc is given by:

where n2 is the refractive index of the less optically dense medium, and n1 is the refractive index of the more optically dense medium.

If the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence. If for example, visible light were traveling through acrylic glass (with an index of refraction of 1.50) into air (with an index of refraction of 1.00). The calculation would give the critical angle for light from acrylic into air, which is
.
Light incident on the border with an angle less than 41.8° would be partially transmitted, while light incident on the border at larger angles with respect to normal would be totally internally reflected.

The critical angle for diamond in air is about 24.4°, which means that light is much more likely to be internally reflected within a diamond. Diamonds for jewelry are cut to take advantage of this; in particular the brilliant cut is designed to achieve high total reflection of light entering the diamond, and high dispersion of the reflected light (known to jewelers as fire).

If the fraction:  is greater than 1, then arcsine is not defined--meaning that total internal reflection does not occur even at very shallow or grazing incident angles.

So the critical angle is only defined for .

I hope this makes sense, although i suspect that most of what i have written will already be well known to you.
Good luck!

Luke
« Last Edit: 28/08/2009 12:23:22 by Pyromaniac »

#### Laura_Kelly

• Full Member
• Posts: 66
##### International Young Physicists' Tournament 2010
« Reply #2 on: 29/08/2009 00:39:25 »
Thanks Luke. We did know this already, but that was a good explanation. One of the things I would like to know/find out is why (or how) different wavelengths are refracted by different amounts considering that all light travels at the speed of light and only the wavelengths differ?

#### The Naked Scientists Forum

##### International Young Physicists' Tournament 2010
« Reply #2 on: 29/08/2009 00:39:25 »