Photoelectric effect.

http://phet.colorado.edu/en/simulation/photoelectric

The purpose of this webpage is to provide you with the simulation to complete this lab activity.

This lab activity has two (2) sections. The purpose of each of these two (2) sections is to provide you with an opportunity to learn the principles of physics and apply them to solve relevant problems. Upon completion of these two (2) sections, you will be better prepared to tackle future physics problems and the examinations in this course (the midterm examination and the final examination). Section 1 requires you to produce a lab report and the section 2 requires you to answer questions.

The second step is to scroll down to each of the two (2) sections and follow the instructions given in each section. Once you have completed each of the sections, move onto the third step.

The third step is to ensure that you have completed all the relevant questions and sections for this lab activity, and then submit your work to the dropbox within the course.

OBJECTIVES:

Objectives
At the end of this lab activity, the student should be able to 
Discuss photoelectric effect.
At the end of this lab activity, the student should be able to 
Solve problems involving photoelectric effect.

At the end of this lab activity, the student should be able to  Devise experiments and write satisfactory laboratory reports

SECTION ONE (1) –

INSTRUCTIONS FOR COMPLETING SECTION ONE (1):

  1. Click on “Run Now” within the simulation. The link to the simulation was provided in the “DETAILED DIRECTIONS FOR LAB ACTIVITY COMPLETION.”
  2. Run the simulation.
  3. Use the simulation to develop the laboratory report.
  4. Ensure that you have completed the lab report, and then move to “Section Two (2).”

DIRECTIONS TO DEVELOP LAB REPORT:

  1. For this section, you must write a laboratory report of an experiment to determine the work function of a metal. Use the simulation to conduct your experiment and gather data for this section. Under the heading, “Supporting Activities,” some possible activities are given (These activities are to give you an understanding of the experiments you can use for your lab report).
  2. Follow the general outline of a lab report as provided in the link below:
    http://www.utm.edu/staff/cerkal/report.html
  3. Remember to show your calculations where necessary.
    These laboratory reports will prepare you to do well in the future lab activities and in the project.

Supporting Activities (Provided only to support the development of your lab report – Do not submit the activities given here with the lab report. In this section one (1), you only have to produce the lab report covering all the headings of a lab report with data and calculations):

The photoelectric effect is one of the key experiments that supported early quantum theory. Light, prior to the early 20th century, was considered to be a wave phenomenon. In most ways, this idea reflects reality well—for instance, light is bendable when passed through a lens. The energy of a wave is given by amplitude of the wave squared, so a light wave of a certain frequency should be able to have any value for energy as long as there is a bright enough light source.
However, when red light was shone on a metal surface, no electrons were ejected even when the brightest red light sources were used. On the other hand, when blue light was shone on the same metal surface electrons were ejected even when the source of light was weak (and brighter blue lights ejected more electrons) How could this be? The energy didn’t seem to depend on the amount of light hitting the metal but instead the frequency of light that hit the metal.
Planck put us on the path leading out of this thicket of confusion when he theorized that light and other forms of energy comes in “packets” or discreet “bundles”. Light, in this theory, is considered to be a particle, which we now cahttp://phet.colorado.edu/en/simulation/photoelectric
ll a photon. The photoelectric effect was explained by Einstein when he conjectured that Planck’s bundles of energy (i.e. photons) were “knocking loose” the electrons—but only if the photons had enough energy to do the job (a two year old isn’t able to knock a football player off his feet, but a bull undoubtedly could). Einstein’s ideas gave further support to the theory that light energy really is not continuous with infinitely small increments of change (a wave), but is in fact “chunky”.
Today’s lab involves a simulation of the photoelectric effect. You will be checking various metals for the point at which they begin to shed electrons, based on a specific threshold frequency—the exact point when the photons have enough energy to knock the electrons loose. This energy is called the work function (W) for the metal. Different metals hold on to their electrons more strongly or weakly due to atomic structure, so the work function for various metals varies. The formula for calculating W is as follows:

h = Ek + W

Where
 h is Planck’s constant
  is the frequency of the light
 Ek is the kinetic energy of the ejected electron
 W is the work function

The kinetic energy of the electron refers to its actual movement once ejected. Ek can effectively be ignored if we just reach the amount of energy to loosen the electron but not get it moving (Ek in these circumstances will essentially have a value of zero). You will be trying to achieve the lowest possible speed for the electrons you eject from the virtual metal surface. W can be obtained by calculating frequency and using Planck’s constant. The work function will be in joules, so in order to compare to published lists of work function values—which are in electron volts—your final value will require conversion into this unit.

The “equipment” you will be working with looks like this:

  1. Bring up the internet and go to the following site: http://phet.colorado.edu/index.php
  2. Look for “Run our Simulations”, click “On Line”, then “Light and Radiation” (in the left hand column), then “Photoelectric Effect”, then “Run Now”.
  3. Keep battery voltage at 0. Turn light intensity up to 100%. You will be testing sodium first (metals are changeable in the upper right hand box).
  4. Adjust wavelength to a value which just allows electrons to leave the surface at a lowest possible speed.
  5. Calculate W in electron volts using the following values and formulas: c = , where c = 2.998E17 nm/sec,  is in nm and  is in s-1
    h = 6.626E-34 Js
    1 electron volt = 1.60217646 × 10-19 joules
    h = Ek + W
    ***remember—Ek is set at zero
  6. Calculate work function (in eV) for all other metals, including the mystery metal.
  7. Look up work function values on the internet. Identify the mystery metal, and check the values obtained for the other metals.
  • You only need to submit a comprehensive lab report for the section one (1). The questions given under the supporting activities are not necessary to be submitted with the lab report. If you have any questions regarding the answers to the above questions, you may contact the professor.

Once you have completed each of the prior directions for Section One (1), save your work and move onto Section Two (2)

SECTION TWO (2):
INSTRUCTIONS FOR COMPLETING SECTION TWO (2):

  1. Answer the following questions:
  2. Protons in an accelerator at the Fermi National Laboratory near Chicago are accelerated to a total energy that is 400 times their rest energy.
    (a) What is the speed of these protons? (Round your answers to six decimal places.)

(b) What is their kinetic energy?

  1. When light of wavelength 210 nm falls on a gold surface, electrons having a maximum kinetic energy of 0.81 eV are emitted. Find values for the following.

(a) the work function of gold

(b) the cutoff wavelength

(c) the frequency corresponding to the cutoff wavelength