Teaching the photo-electric effect

The photo-electric effect is one of those demonstrations that can be tricky to get to work, so for my first few years teaching I used to just demonstrate what should happen. Somewhere along the line I got it to work properly and have been gradually building up ways to make it more fool-proof and more interesting.
This can all be done in a single class:

  1. Make sure electroscope is charged negatively
  2. Make sure to use a zinc plate
  3. Sand the zinc to remove the zinc oxide surface layer
  4. Use a normal lamp to show that this has no effect.
  5. You need to use a UV source of the required frequency – I bought a cheap hand-held battery-powered one and it doesn’t work, so presumably it’s emitting in the lower end of the UV spectrum. The one which does work is a larger plugin model bought from a science supplier for this purpose.
  6. Use two normal lamps to show this still has no effect
  7. Use a normal lamp with a higher power rating than the UV lamp to show that this has no effect (but surely this should work – more power means more energy per second, so why wouldn’t this work? – There’s more energy per sec because there are more photons, but each photon of visible still has less energy than a photon of UV. At least that’s my interpretation).
  8. Put a piece of glass on top of the zinc and now shine UV. Leaves do not collapse because glass absorbs UV light – can’t get a suntan in a glasshouse (well, some UV gets through, but relatively little).
  9. Get every student to do the demo. While they’re waiting get them to repeat the diffraction grating demo; it means that in the one class they have proved that light is both a particle and a wave – they will never do anything else in Science as significant or awesome (in the original sense of the word) as what they have just done in this class.

Note that in terms of ‘wow’ factor almost nothing happens; divergent leaves collapse – that’s it. I sell it as the most significant, least impressive demonstration in all of Science.

It’s definitely worth sticking with. If you want to try again do let us know how you got on.

And then show then show this one again – it looks like it’s exaggerating what’s happening, but it’s not.
It really is that strange

I remain convinced that those responsible for making out the science syllabus at junior cert and leaving cert level end each and every meeting with the following question:
“Right, are we absolutely sure that we have removed every possible reference, no matter how indirect, to wonder in this document? We are? Okay then let’s publish it.”

Analogy for the Photoelectric Effect
The energy of the incident photon is like the money in your pocket at the start of a night out.
The work function energy is analogous to the admission charge at the nightclub.
It’s an all or nothing thing; even if you’re only just one penny short, you will be refused entrance.
Different clubs may charge different entrance fees, which is analogous to the differing work functions of different metals.
The maximum Kinetic Energy bit represents the money left over after paying for entrance, which can then be spent on lemonade 

A nice analogy for work function, but as with all analogies it’s worth discussing the limitations.
For instance if one of the group does not have quite enough money, but the rest have some to spare, they can all get in.
Photons, on the other hand, cannot re-distribute energy between themselves.
Even if a photon with insufficient energy arrives at the same time as others with excess energy, it still cannot eject an electron.

You can find more resources on thephysicsteacher website here: http://thephysicsteacher.ie/lcphysics29theelectron.html

and my notes used when teaching The Electron are here:
http://thephysicsteacher.ie/LC%20Physics/Student%20Notes/29.%20The%20Electron.docx

 

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