Tag Archives: practical work

Cloud chambers⤴

from @ stuckwithphysics.co.uk

Thanks to Dr Aidan Robson from Glasgow University I was able to borrow a set of cloud chamber kits which I have used with my Higher physics classes this week. This facility is now available to any physics teachers in the central belt, via an on line booking facility, with kits available from the both Glasgow and Edinburgh University. (Teachers interested in borrowing kits from the scheme can register by contacting Aidan via the email given in the link above).

Like many commercially available cloud chamber kits, these use dry ice in order to cool the alcohol vapour in which the trails form due to ionisation caused by energetic particles. Both the dry ice and the alcohol are provided with the kits. However, unlike other ‘home made’ cloud chambers, which often use small fish tanks, these kits are very compact, allowing pupils to easily build them and observe interactions within a single 50 minute period. Their size also allow pupils to get in very close, making the whole experience more personal for them.

The kits use very simple items to construct the cloud chambers – small aluminium pie cases, plastic pint tumblers, a few strips of black tape, expanded polystyrene and a small torch. The instructions are clear and easy to follow, so that with a little preparation (pre-cutting lengths of tape speeds things up a great deal) a class of 20 pupils could build 10 cloud chambers in as many minutes.

The booking website also links to resources which include a short presentation explaining a little about the invention of cloud chamber by CTR Wilson, who won a Nobel prize for its invention, and an overview of how it operates. It includes a number of annotated images of cloud chamber interactions, which lead on to information about particle accelerators and their detectors, including the Stanford linear accelerator and Large Hadron Collider.

Without the need for a source of radiation, the working cloud chambers can be used to detect particles created by the interaction of cosmic rays with the upper atmosphere. These particles include muons, protons, alpha particles, pions, electrons, and neutrons.

The majority of particles detected at ground level are muons, which are an excellent example of the relativistic effect of time dilation. Muons travel at velocities very close to the speed of light (0.999 c) but due to their very short lives (~2.2 µs), and according to classical mechanics should only travel around 500 m through the atmosphere. However, because they travel at such high velocities, special relativity allows this time to be increased for stationary observers (us on the surface of the Earth, some 10 km below the upper atmosphere) so that in our frame of reference the muon has a significantly longer life, allowing it to reach us.

 

Cloud chamber detecting cosmic ray muons

Although my school no longer has any sealed radioactive sources, we do have a small collection of radioactive minerals. Ensuring it was handled appropriately, a small piece of Cornish pitchblende was placed inside one of the cloud chambers.

 

Cloud chamber with pitchblende source

This video clearly shows the tracks created by particles which stream from the small piece of rock. These are alpha particles, which though highly ionising are very easily absorbed in air (and in the alcohol vapour in the cloud chamber), travelling only a few centimetres. Their short range and careful handing ensures that the risks from exposure to the radiation from this source is minimised.

[It should be noted that mineral sources should not normally be removed form their packaging. This ensures that any small fragments which fall off and may be active, will remain with the main sample. A readily available alternative is a thoriated TIG welding rod – these can be bought from many hardware and DIY stores relatively cheaply and should produce visible trails when placed inside a cloud chamber.]

If in any doubt concerning the safe handling of radioactive materials, refer to the SSERC website in Scotland or CLEAPSS in the rest of the UK.

 

Newton’s Rings⤴

from @ stuckwithphysics.co.uk

I’ve been trying to show my AH pupils all of the experimental work for Unit 3 during this week, as it’s the last week of the course before their NAB next week.

Having gone over much of the theory before Easter and encouraging them to cover the theory on Scholar, I set up a few of the interference experiments – Young’s Slits with microwaves and using a He-Ne laser, which are both nice and obvious and relatively reliable (for physics demos). We took a few measurements and used them to find the wavelength for the microwaves and the slit separation, d, for the laser experiment.

We also used the travelling microscope to measure the slit separation, using a flexi-cam and projector to show both the view down the scope and the readings on the Vernier scale.

Optimistically, I decided to try the same set up for Thin Wedge Fringes and Newton’s Rings – demos which are not so nice and not so obvious and, as I’ve found in the past, can be awkward to set up. Worse still, they must be observed through a microscope, ideally a travelling microscope to allow measurements of fringe spacing to be taken.

The thin wedge fringes worked pretty well and we measured the fringe spacing, using it to calculate the thickness of the wedge. And it all worked!

Continuing to ride my luck, I had a go at Newton’s Rings, using our ancient, somewhat chipped Griffin apparatus. After setting it up, I had a look through the eyepiece and, to my very great surprise, saw the brightest, clearest Newton’s Rings fringes I have ever seen.

To my further surprise, it all looked great through the flexicam-projector, so much so that I took a picture and tweeted about what I’d been doing. One reply, from John Burk (@Occam98) asked how I’d set it up.

So, here goes…..

Griffin Newton’s Rings apparatus -
plano convex lens placed convex side down on glass plate
Beam splitter (sloping glass plate) reflects light from sodium lamp (in blue lamp holder) down on to lens
Travelling microscope above for viewing interference pattern through beam splitter.

The images below show how the flexicam was connected to the travelling microscope, using a collar to align the camera and eyepiece lenses, and in turn connected, via the S-Video input, to a Sony LCD projector.

It’s a very rare physics lesson where all of the experiments work, let alone first time. Luckily, when I needed to get through a lot of experiments to gather up the loose ends of the unit, that’s exactly what happened. After all the effort of getting all the apparatus together and set up, getting such excellent images for Newtons’ Rings was a great way to finish my lesson, and coincidentally the Advanced Higher course.

All downhill to the exam now…..