Sunday, March 20, 2016

What's wrong with this photoelectric effect lesson?

Let me just say at the outset that this post is Reason I'm a Terrible Person #417.

I was scoping out the YouTubes in search of a nice video lesson on the photoelectric effect. Demonstrating the effect can be troublesome and finicky. So why not enjoy the fruits of someone else's labor?

I watched through Adam Micolich's lesson (below) and... found it wanting. Wanting to be used as a lesson in skepticism and critical thinking!

The story/script is nicely straightforward:
1. A negatively charged electroscope is discharged via exposure to ultraviolet light.
2. The UV light is causing the discharge: a glass plate blocks the UV and prevents the discharge.
3. The UV liberates electrons: a positively charged electroscope is not discharged by UV light.

Let's see how it goes. (The actual photoelectric demo begins at about 2:50.)



Each segment seemed to go according to plan and proved the aspect being investigated. The content as spoken is spot-on correct.

And I have no reason to suspect shenanigans. But in my judgment, the demonstration is flawed to the point it would be fraudulent if done intentionally.

Consider it a PhyzMaster Challenge: Can you find the flaws I found?

I offer this challenge in good faith as an exercise in physics-based critical thinking. These principles are ostensibly valued by Common Core, NGSS, and the reimagined AP Physics courses. I mean no disrespect to Mr. Micolich. He was good enough to produce the video lesson; I—someone who didn't bother to create a video lesson of the photoelectric effect—spent a Saturday night finding fault with it. So who is more deserving of harsh judgment? (That was rhetorical.)

Hints and specific allegations in the comments. I might remove these later if I assign a skeptical critique to my students.

2 comments:

Dean Baird said...

1. A negatively charged electroscope is discharged via exposure to ultraviolet light.

I believe he inadvertently left the electroscope with a positive charge on this first sequence.

The UV lap's shield is not an innocent bystander in this demo. Looks like a solid metal housing he's holding onto to shield those UV rays. It might have been more instructive to see him conduct (!) the experiments without that shield.

2. The UV light is causing the discharge: a glass plate blocks the UV and prevents the discharge.

When the demonstration needs the electroscope to maintain its charge, care is taken to keep the UV lamp (and it's not-so-innocent case) a safe distance from the electroscope plate. When discharging the electroscope is called for... things play out somewhat differently.

3. The UV liberates electrons: a positively charged electroscope is not discharged by UV light.

Once again, the gap between the UV lamp shield and the electroscope is maintained with care.

Again, I'm not accusing Adam Micolich of intentionally misleading viewers. But he might have unconsciously worked too hard to ensure the correct results. If you don't think bias enters into scientific endeavors, you haven't studied science much yet.


Dean Baird said...

While it *can* be done, I don't recommend using an insulator (like plastic) to charge objects (like electroscopes) by conduction. Insulators are, by definition, poor conductors.