# Photoelectric Effect Computer Simulation PHET

An excellent computer simulation enabling students  to visualize many aspects of the photoelectric effect experiment. By working with this simulation, by working through the interactive presentation, and working the problems on the student activity sheet students have the opportunity to gain a better conceptual understanding of the photoelectric effect.

Computer simulation availability courtesy of the University of Colorado-Boulder

Change metals, change the frequency/wavelength, change the intensity of light, change to photon of light model, observe number and speed of ejected electrons from the metal surface (photoelectrons).

A set of interactive lecture presentation Power Point slides can accompany this computer presentation.  Clicker Questions are available. See attached file.

Curriculum Notes

A student activity sheet can accompany this computer simulation.  See attached file.

Learning Objectives

1.  Compare classical physics model to quantum theory view in order to understand how A. Einstein explained the photoelectric effect

2.  Each metal has a unique threshold energy - the minimum energy required to begin to eject electrons from the surface of the metal.

3. Classical Physics: Takes time to heat up ⇒ # of e-s coming off start low and should increase with time (energy added).  Quantum Physics: the photoelectric experiment: using frequency/wavelength of light above the threshold frequency, photoelectrons come off the metal surface immediately, no time delay to heat the metal surface (add energy) observed.

4. Classical Physics: With respect to ejecting electrons from the surface of a metal, the frequency/wavelength of light should not matter, only intensity of the electromagnetic radiation.   The photoelectron effect experiment shows strong dependence on frequency/wavelength of light is one of the main factors to eject photoelectrons from the surface of a metal.

5. KE ejected electron = hvincoming energy – BE    where BE is the binding energy of an electron (how strongly the surface electrons are bound to the atoms), also known in physics as the work function

6. Photon model of light: When light interacts with matter light can be represented as separate, discontinuous quanta called photons. Light energy comes in packets. Each photon has an energy of E = hv

7.  Students should be able to explain how only a photon model of light can explain the photoelectric effect.  When light of a low frequency/long wavelength is shining on a metal surface and there are no ejected electrons, increasing the frequency will lead to observing ejected electrons.  When using a low frequency/long wavelength of radiation then increasing the intensity of light, this does not lead to observing ejected electrons.

Students need to receive some information about classical physics, waves, electromagnetic spectrum, the Bohr model of the atom, etc. before learning about the photoelectric effect.

Discussion

Important modeling notes / simplifications: used in the PHET Photoelectric Effect Computer Simulation*

• Electrons are emitted with a range of energies because photons can eject electrons with a range of binding energies. If more of a photon’s energy is used to release an electron, the emitted electron will have less kinetic energy. Note that this behavior is different from the simplified model used by some textbooks, in which all electrons are emitted with the same kinetic energy. If you want to use this simplified model, you can check the “show only highest energy electrons” option. This option does not change the graphs because current is still calculated based on all the electrons.

• Not every photon emits an electron, even if the photons have enough energy to emit electrons. If a photon is absorbed by an electron with binding energy greater than the photon energy, the electron will not be released. Photons with higher energies are more likely to release electrons because a greater proportion of the electrons in the metal have binding energy less than the photon energy. Therefore, as you increase the frequency, the number of emitted electrons (and therefore the current) will increase until all photons are emitting electrons. Note that this behavior is different from the simplified model used by many textbooks, in which every photon with frequency greater than the threshold frequency releases an electron, so the current is constant above the threshold frequency.

* University of Colorado-Boulder PHET guide document

Procedure

A guide to using the PHET computer simulation is available in the menu.  See attached file.

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© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group