PHOTOELECTRIC EFFECT

This exercise will help you to more fully understand the importance of the photoelectric effect by experimenting with a simulation that graphically shows you the effect of one photon of a given energy hitting a photocathode material (in a vacuum).

The applet is pretty simple to use.  There are three parameters that may be chosen:  The type of material for the photocathode, the energy of the incident photon, and the value of the stopping voltage.

The material for the photocathode is chosen from the 'drop down' list on the left hand side of the applet.  Each material has its own value for what is called the work function of the material.  The work function is the energy a photon must possess in order to eject a photoelectron.   It may be helpful to think of the analogy of a young child making a purchase in a store to understand the photoelectic effect.  The child enters the store, checks the price on the desired item (the work function).  The child then checks to see how much money (energy of the photon) she has and leaves the store if it is insufficient.  If the amount is exactly what is needed the child leaves with her new item, but no change (zero kinetic energy).  If she had more than the purchase price there will be 'change' left over (the kinetic energy of the ejected electron).

The stopping voltage is used to convert the kinetic energy into electrical potential energy.  When the stopping voltage is properly set, the fastest photoelectrons will stop just before they hit the anode.  If the stopping voltage is too high, the ejected electron never gets close, and if it is too low the electron arrives at the anode and causes a current to flow.  A similar prospect is to throw a ball straight upwards from differing heights with the goal of getting as close as possible to the ceiling without actually touching it.  The speed of the toss is analogous to the kinetic energy of the ball.  The starting point determines how much gravitation potential energy must be swapped for the ball's kinetic energy in order to have it stop.   Hitting the ceiling causes a current.

There are several different ways to use this applet:

•   Choose a  photon energy and see how many of the the different materials will eject for the energy that you choose.  A value between 4 and 5 eV is a good starting point.  This means that you want to check all of the possible photocathode materials and make a note of which ones will emit a photoelectron when the photon strikes the photocathode.  In the computer exercise these will be your data for one of the experiments.

• Choose a material and adjust the photon energy to cause the electron to be emitted.  You can get to within 3 significant figures in no more than 10 guesses if you guess with strategy rather than random guesses.

• Choose a material and a photon energy then adjust the stopping voltage so that the electron stops at the point of almost reaching the anode.  It is suggested that you set the photon energy near 10.0 eV (the maximum possible value).  Then determine the MINIMUM stopping voltage which will prevent current from flowing.  In your computer exercise, you will use these two values to determine the work function of the material.

The one aspect of the photoelectric effect that this simulation cannot show is how the photocurrent depends on the intensity of the incident light (when photoelectrons are ejected).  This should be fairly easy to understand if you think of the light as a stream of photons.  The more intense the light, the more frequently a photon strikes the surface and the more rapidly photoelectrons are emitted - causing a larger value of photocurrent.

Sorry, you must have Java enabled to perform the simulation

The wavelength of the light in nanometers (nm) is related to the energy in electron volts (eV) in that their product is always equal to 1240. Thus a 2 eV photon has a wavelength of 1240/2 = 620 nm. A 313 nm photon has an energy of 1240/313 = 3.96 eV. This is a useful result.