Most classes have done some work recently on improving answers to the kind of questions that ask you to describe/explain/justify.  I haven’t saved all of the mindmaps that have been produced but this one is fairly typical of the responses generated by a class.  There is a pdf version available to download.

Please hand in your HW no later than Tuesday 7th September. Late jotters will not be accepted.

Late HW will not receive credit.

Late jotters will not be accepted.

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.

www.youtube.com/watch?v=s9PZNJ970M8

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.

www.youtube.com/watch?v=ATVbnilxIrs

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

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:

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?