mrmackenzie

Geiger-Müller tube

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We examined the operation of a Geiger-Müller counter today.

image by Theresa Knott

The Geiger-Müller (GM) counter is used to detect ionising radiation such as alpha and beta particles or gamma rays.  The radiation enters through a very thin window at one end of the tube.  This window is usually made of mica.

Mica flakes.  Photo by Rpervinking

Mica is a mineral that forms in layers called sheets.  These sheets can be split apart into very thin layers, so thin that even an alpha particle can pass through it (remember that alpha particles can be stopped by something as thin as your skin or a sheet of paper).  The mica window prevents the argon inside the tube from escaping and also stops air from getting into the tube.

When radiation enters the tube and collides with an argon atom, an electron may be knocked off the atom – we call this process ionisation.  When ionisation occurs, a positively-charged argon ion and a negatively-charged electron are produced.  The argon ion is attracted to the outside wall of the tube, which is connected to the negative terminal of the power supply, while the electron is attracted to the central electrode, which is kept at a high positive voltage – typically 500V.

A small pulse of current is produced each time an electron reaches the central electrode.  These pulses can be counted by an electronic circuit and a displayed on a 7-segment display.  Sometimes a small speaker is added to the system to produce a click for each pulse.  On its own, the GM tube cannot tell the difference between alpha, beta and gamma radiation.  We need to place different materials (e.g. paper, aluminium, lead) in front of the mica window to discover which type of radiation is responsible for the reading.

Here is a short video demonstrating the use of a Geiger-Müller tube.

Finding the distance to a thunderstorm

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The thunderstorm this morning reminded me that I was going to post something about calculating the distance to a storm.

Play this video and then replay it, paying close attention to the time index at the bottom.  You will need to move your cursor over the video box to keep the time visible.

Lightning from Daniel Dingemanse on Vimeo.

Can you calculate the distance between the flash of lightning and the camera?

Hint: take the speed of sound in air as 340 m/s.

pressure and high heels

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We deviated from the script a little this week to look at how choice of shoes can affect the pressure put on the ground beneath your feet.  If we know the size of the force (F) and the area over which the force is applied (A), then we can calculate pressure using

P = \displaystyle {F \over A}

So when we start talking about shoes, we need to figure out the surface area of the footwear.  Luckily, Caitlin had flat-soled shoes on and volunteered to provide data for our calculation.  Here is the outline of her shoe, it’s drawn on graph paper so that the area can be calculated quickly by counting the large squares.  Each large square is 1 square centimetre – we counted the approximate area by considering only whole squares inside the black outline of the shoe.

shoe outline

Assuming a mass of 50kg, the pressure when wearing the flat shoes is

P = \displaystyle {{(50 \times 9.8)N} \over {0.0306m^2}} = 16000Nm^{-2}

The red shaded area of our photo shows the reduction in area when heels are worn.  With heeled shoes, the area is reduced to approximately 1 square cm per shoe.  The change in the pressure is staggering:

P = \displaystyle {{(50 \times 9.8)N} \over {(0.0002)m^2}} = 2450000Nm^{-2}

The Science Babe has made a video on this topic.  Good physics in here, although I’m not sure she can claim the equation shown above is Newton’s 2nd law.   What do you think?

Higher webtribe solutions site

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I had a comment that the Higher solutions on the webtribe site were no longer available.  I can’t get in to them either but I do have a copy of the site that I use in school when there are filtering issues.

I have uploaded my version of the site to

http://www.mrmackenzie.org/webtribe/ie.html

I hope the person who created the original site does not mind me mirroring the content. Please get in touch is this is a problem and I will remove it.  Obviously I am only interested in making this available until Friday.