Simple Photoelectric Effect


Adam Beehler

In each newsletter, Adam Beehler, Lecture Demonstration Specialist, explains one of his demonstrations. This demo won second place at the American Association of Physics Teachers 2009 Summer Meeting in Ann Arbor, MI.

 

When ultraviolet light, x-rays, or other forms of electromagnetic radiation are shined on certain kinds of matter, electrons can be ejected. This phenomenon is known as the photoelectric effect. It made scientists think about light and other forms of electromagnetic radiation in a whole new way. The peculiar thing about the photoelectric effect is the relationship between the frequency (energy) of the light shined on a piece of metal and the number of electrons ejected. The higher the frequency (energy) of the light source, the more electrons and thus electric current, is given off. Just making the same type of light brighter, or more intense, does not give the ejected electrons any more energy.

Demo

Albert Einstein’s Nobel Prize winning photoelectric effect has been demonstrated for many years quite effectively, yet now it can be done with simple household items. Past versions used specialized black lights or carbon-arc lamps to show any effect. This version takes advantage of the currently popular germ sanitizer lights, which are much more affordable and portable. It also uses aluminum pop cans instead of zinc plates and Christmas tree tinsel instead of standard electroscopes.

Use

  • Lightly sand one side of your aluminum soda pop can to remove any oxidation/ outer coating.
  • Rub the PVC pipe with the brown paper.
  • Slide the pipe across the tinsel. The tinsel should now be negative and the strands repel from each other.
  • With the tinsel repelling, touch the soda pop can and discharge it. The tinsel should relax.
  • Charge the tinsel again with the negatively charged PVC pipe.
  • Move the UV germ sanitizer lamp into position (near, but not touching, the sanded portion of the soda pop can). Nothing should happen.
  • Turn it on and watch the tinsel slowly relax as the negative charges leave the aluminum. This is the photoelectric effect. The energy of the short-wave UV light is enough to eject electrons from the surface of the aluminum.

  • If incoming light has enough energy, electrons will be ejected from the middle.
  • Charge the tinsel again with the negatively charged PVC pipe.
  • This time shield the soda pop can from the UV light with a piece of glass. Nothing should happen. The tinsel should remain charged. Why? The UV light is absorbed (or blocked) by the glass. Now remove the glass and watch the tinsel relax.
  • Charge the tinsel again with the negatively charged PVC pipe.
  • This time use the long-wave UV lamp. Nothing should happen. Why? This UV light does not have enough energy (too low a frequency) to eject electrons.
  • Charge the tinsel again but this time by induction so that the tinsel becomes positive.
  • Shine the short-wave UV lamp onto the soda pop can. Nothing should happen. Why? The photoelectric effect ejects electrons from the aluminum, and the tinsel and soda pop can already have a deficiency of electrons.

You can also view this demo, and a complete materials list, here.