Sunday, 2 June 2013

Van Der Graaf - Miniature Lightning Bolts.

This earlier post outlines what you need to consider before doing any Van Der Graaf experiments.

This current post is the second in a series that describes some demonstrations that can be done with Van Der Graaf machines, including theoretical explanations.

The first demonstration is the simplest, throwing miniature lightning bolts.

Set the Van Der Graaf up, place the discharge sphere in it's holder (This can be as simple as a hole in the back of the machine, and some rubber holders.) and turn the machine on.

You'll know if the machine is working as the electrostatic charge will start to attract your body hair as you move near it, and you'll feel a prickly sensation on your arms/head/etc. Given perfect conditions, the machine will throw a spark every 1-2 minutes about 5cm long. If you can do this in a darkened room, it's even more impressive.
If conditions are less than perfect, try the following until something works:

  1. Wait for longer. Sometimes the first spark won't happen for 5 minutes or more. During this time, describe what's happening in the machine (see below). Surreptitiously use the arm-hair test mentioned above to gauge how well the charge is building up.
  2. Take the discharge sphere out of it's holder and bring it closer to the sphere until you get a spark.
  3. If you don't get a spark, shut down the machine, discharge the sphere and readjust the upper electrode. Too hard a contact will slow the rubber belt down, to large a gap prevents charge transfer.
  4. Check the weather and this post. If it's raining outside or everyone is sweating up a storm, pack the machine up and go onto something else. Honestly, it's not worth trying to get a Van Der Graaf machine working in humid weather.

Points of Theory

Discuss these with your students - this is one of those situations where Inductive Learning just doesn't work as well as Deductive - students have to be given some basic concepts before they can start thinking about what is going on. Otherwise you'll get some "oohs" and "aahs", but no learning will occur.
  • The static charge is generated as the rubber band passes over the plastic roller - the different materials hold onto their electrons to different degrees. A little heat and friction will cause the electrons to jump from the plastic to the rubber. (It doesn't actually matter which way you describe the electron transfer, and can differ from one brand of machine to another - the students only need to know about the movement of electrons).
  • The belt transfers the electrons to the top of the Van Der Graaf machine where it is picked off by the upper electrode and placed on the sphere.
  • The discharge sphere is connected to the bottom of the machine.
  • When the voltage (emphasize this term, even if you haven't defined it yet) gets high enough, the air is converted from a gas to a plasma, which allows the electrons to jump from the main sphere to the discharge sphere, back to the bottom of the machine.

Demonstrate how the same thing can happen by unplugging the discharge sphere, touching the plug end to a metal tap (without touching any metal yourself), and bring the discharge sphere near the main sphere again.
  • You will need to describe how the massive size of the Earth means that it can soak up unbalanced charge very easily, and the tap is in direct contact with the Earth. This is where the term ground or earthed in electronics comes from, and (In Australia) why we have three prongs on our electronic devices. The third is connected to the ground and allows dangerous electricity to flow to the ground if something goes wrong with an appliance. Then you can describe how the excess charge slowly leaks back to the Van Der Graaf machine through an Earth Discharge Path.

Extra points to mention if you have time:

  • The spark colour is a rough (very rough) guide to how hot the spark is. Blue is over 10,000K, violet is much higher. (In reality, the colour of the spark depends on the degree of humidity and the excitation of different atmospheric gases.) The main emphasis is that the spark is hot!
  • You can demonstrate that the voltage needed to produce a spark is dependent on the length between the main sphere and the discharge sphere - show the students that the sparks are more often when the discharge sphere is held closer (less voltage needed to jump the gap, less time needed to build up enough charge).
  • If you use a student volunteer to hold the discharge sphere, they will probably go "Oh" on the first spark thrown. Not because the spark surprises them, but because they feel the collapse of the electrostatic field through their arm hairs. Take the time to point this out and get a couple of other students to try this. If you leave anyone out from going near the machine, they will be disappointed. If you're really crowded, get three students to hold their arms about 20 cm away from the sphere while your volunteer discharges the sphere.

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