# Newton’s G-ball⤴

from @ stuckwithphysics.co.uk

'Newton's G-ball', marketed by Swedish company Mollic, is a simple electronic timing device which can be used to measure the the freefall time from its point of release to impact on a surface below.

It is available from a number of third party suppliers, including djb microtech and Better Equipped in the UK and Arbor Scientific in the US.

The ball has an integral centisecond timer, which is primed by pressing and holding the button on the face of the timer. Releasing the ball starts the timer, which stops when the ball impacts upon a surface below.

If the height, h, through which the ball falls is known, and the time for the ball to fall, t, is measured, then g can be calculated using the formula -

Taking multiple measurements of the freefall time, t, over a range of heights, h, allows a range of values to be obtained for g.

The results below were obtained by my Higher Physics class on 9th June 2016.

 h (m) t1 t2 t3 mean t (s) g (ms-2) 0.2 0.23 0.22 0.27 0.240 6.94 0.4 0.30 0.27 0.30 0.290 9.51 0.6 0.39 0.35 0.38 0.373 8.61 0.8 0.42 0.39 0.41 0.407 9.67 1.0 0.46 0.48 0.46 0.467 9.18

The results obtained are reasonably good, giving a mean value for g = 8.79 ms-2. Whilst this is in reasonably close agreement with the quoted value of 9.8 ms-2 given in the SQA data tables, discounting the obviously low value obtained for h = 0.2 m gives an improved mean value for g = 9.51 ms-2.

A quick analysis of the uncertainties in this data give the following -

Uncertainties in height, h (approximate reading/position uncertainty = ± 0.02 m)

 h (m) uncertainty in h (m) % uncertainty in h 0.2 0.02 10% 0.4 0.02 5% 0.6 0.02 3% 0.8 0.02 3% 1.0 0.02 2%

Uncertainties in time, t -

 h (m) t1 t2 t3 mean t (s) random uncertainty in t (s) % uncertainty in t 0.2 0.23 0.22 0.27 0.240 0.017 7% 0.4 0.30 0.27 0.30 0.290 0.010 3% 0.6 0.39 0.35 0.38 0.373 0.013 4% 0.8 0.42 0.39 0.41 0.407 0.010 2% 1.0 0.46 0.48 0.46 0.467 0.007 1%

Uncertainties in g -

 g (ms-2) mean g  (ms-2) random uncertainty in g (ms-2) 6.94 8.79 0.55 9.51 8.61 % uncertainty in g absolute uncertainty in g (ms-2) 9.67 8% 0.70 9.18

This gives a final value for g using this procedure as -

g = (8.79 ± 0.70)  ms-2

However, an alternative graphical analysis allows an improved result to be obtained from the same data.

For this approach, the formula above was rearranged for h, giving -

A graph was plotted of h against t2, giving a good approximation of a straight line through the origin, as expected.

 t2 (s2) h (m) 0.0576 0.2 0.0841 0.4 0.1394 0.6 0.1654 0.8 0.2178 1.0

Using the trendline function in Excel, a best fit line was added with its function included. The gradient of this straight line, which is equal to ½ g, is 4.91, giving a value for g from this graph - g = 9.82 ms-2.

Further analysis of the graph, using the LINEST function in excel, gave the following uncertainties -

 g (ms-2) absolute uncertainty in g (ms-2) 9.82 0.69

This graphical treatment of the data gives a final value for g using this procedure as -

g = (9.82 ± 0.69)  ms-2

I have included the raw data, graphical treatment and uncertainties in the in the excel file below.

g ball

from @ stuckwithphysics.co.uk

Radioactivity is an exciting, mysterious and scary part of physics for many students, often because their ideas about ‘radiation’ are closely associated with their ideas about superheroes. Frequently my class discussions with students to gauge their prior knowledge about radiation and radioactivity come round to Stan Lee’s Marvel Comics creations – ‘Spiderman’ and  ‘The Incredible Hulk’.

After explaining that these are works of fiction, with little basis in fact, it was for many years traditional to then introduce students to the realities of radioactivity using Geiger-Muller (GM) tubes, scaler-timers, a range of radioactive sources and absorbers. These allowed teachers to demonstrate the nature of the three types of radiation, their random nature, range in air, and the materials required to absorb them.

These days fewer schools have radioactive sources – they are expensive to buy, require careful storage and handling and must be regularly checked to ensure they are safe to use. As a consequence of this many students never see this equipment being used, and are more likely to see an interactive virtual demo, such as ‘RadiationLab’ from Visual Simulations.

Of course, there are plenty of YouTube videos that will also get these points across, but they’re still not the same as students seeing these effects with their own eyes. As I work in a school with no radioactive sources, I have had to make extensive use of videos and software simulations to demonstrate the properties of the three types of radiation. I always go to the trouble to show students the apparatus, as much to allow them to see the real kit, but also because it allows us to detect and measure background radiation.

In the last week I performed an experiment I read about a number of years ago in a SSERC bulletin, which uses an electrostatically charged balloon to attract radioactive particles from the air onto its surface. This experiment is detailed HERE on their website.

(reproduced with permission of SSERC)

The charged balloon accumulates decay products from the Uranium/Radium decay sequence, which in turn decay as shown in the table below.

(reproduced with permission of SSERC)

Whilst the balloon is collecting radionuclides, the apparatus – a djb Microtech ALBA interface and GM tube – was used to measure the background radiation. The graph produced clearly shows the random nature of the detected background radiation, which averaged out at a level of 16.3 counts per minute.

The balloon is burst and placed below the GM tube, then left to log the radiation over a period of a few hours. The background level is subtracted from the count obtained for the balloon and a graph of the balloon’s activity can be plotted.

Again the raw data fluctuates greatly, reflecting the randomness of the decay process, but it clearly shows an elevated level of activity, around ten times greater than background. Using the software’s built-in line fitting algorithm, an exponential decay curve was fitted to the data.

A copy of this graph was given to each pupil, and used to determine the half-life of the radiation detected from the balloon.

Analysed graph

Having measured the time for a number of sections of the graph where the measured activity had reduced by half, pupils then calculated a mean value for the half life of 37.3 minutes. This is most likely a combination of the decays of Pb-214 and Bi-214.

For those with ALBA interfaces and software, but without the GM-tube, I would be happy to provide you with the raw .atb files for use with your own classes. These could not be uploaded to this blog, but can be accessed HERE via Google Drive.

# 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…..