Objectives: The purpose of this Laboratory is to investigate the phenomenon of the Photoelectric Effect using a computer simulation software. Equipment: Scientific Calculator . Computer with MS Excel (see posted instructions for downloading/using Excel) PhET Photoelectric Effect Simulation Software Theory: . Light shining on a metal surface can result in the ejection of electrons. This "electricity from light" effect is known as the "photoelectric effect' and was observed in the late 1880s by Physicist Heinrich Hertz. In 1905, Albert Einstein explained the photoelectric effect as the transfer of energy from the light to do the work to eject an electron. What was new about Einstein's explanation was that light was proposed to come in "packets' of energy called "photons", instead of considered as a continuous electromagnetic wave. The energy of a photon (E) is directly proportional to the frequency (f) multiplied by Plank's Constant (h = 6.63 x 10 ) J.s) (Equation 1): E = hf (1) Using conservation of energy, the energy of the photon (hf) is equal to the minimum amount of work needed to eject the electron (W.) plus the maximum kinetic energy of the ejected electron (KE.) (Equation 2): hf = Wo+ KEmax (2) Where W. is also called the "Work Function" and physically is interpreted as how strongly the least bound electron is held by the metal atoms. For this reason, W. is different for different metals.Rearranging Equation (2), the kinetic energy is equal to the photon energy minus the Work Function (Equation 3). Note that the kinetic energy increases proportional to the increasing frequency: KE.... - hf - W. (3) . The minimum frequency of the photon that just ejects an electron at zero kinetic energy is called the "threshold frequency" (f.) and Equation 3 can be re-written as Equation 4. Importantly, photons with frequency less than the threshold frequency do not eject electrons. hf. = W. (4) The Work Function (W.) can be expressed in terms of the "threshold wavelength" (.) using the relationship between wavelength (2), frequency (f), and speed of light (c) (Equation 5): (5) . Substituting Equation 5 into Equation 4 and rearranging gives Equation 6: W. = hc/ho (6) Equation 6 can be used to calculate the Work Function if the threshold wavelength can be determined experimentally. An important and unexpected observation was that as the photon energy was increased, the kinetic energy of the ejected electrons increased (Equation 3), but the electrical current (i.e., the number of ejected electrons) remained the same. The electrical current was found to be directly proportional to the intensity of the light, where the intensity was interpreted physically as the increasing number of phocons. Practical Uses: The Photoelectric Effect has many and widespread uses and in fact it is likely that you already used a device today that functions due to the photoelectric effect. For example, every time you make a phone call, the optical fiber network starting from the cell-phone receiver-tower, use "photodiodes" (based on the photoelectric effect) to detect and direct your call. Photodiodes are also used in pollution monitoring to detect concentrations of pollutants and even your garage door safety to prevent injury if you are under the door when it starts to close. Another important application is the Charge-Coupled Device (CCD) that performs as "electronic film" to capture photographs in your cell phone camera. Finally, solar-power "solar cells" are photoelectric effect-based devices that directly convert solar energy "photons" into electricity, Note that the direct conversion of light to electricity is the only commercial method to generate electricity that is not a result of Faraday's Law of Induction (that includes hydroelectric, coal, gas, oil, nuclear, wind, and tidal electrical energy generation.)Procedure: There are three parts to this experiment. Part A focuses on the determination of the Work Function for four different metals. In Part B, the relationship between electrical current and light intensity is explored, and in Part C, the effect of an external voltage applied to the plates on the resulting current will be observed. Part A - Determination of the Work Function (W.) for Different Metals: . Open PhET Photoelectric simulation software: https://phet.colorado.edu/en/simulation/legacy/photoelectric > Figure 1 is a screen shot of the simulation screen with the initial parameters shown. The simulation shows a lamp (upper middle-left of simulation screen) that is shining a light on a metal plate to the left of the screen. The color of the lamp-light corresponds to the wavelength selected. A second metal plate is shown on the right side of the screen. A battery and a current-meter (lower middle of screen) connect both plates. The two plates are assumed to be contained in a vacuum chamber (sketch of cylinder around plates). In "Options" (upper left on tool bar) select "Show Photons". Select "Sodium" (upper right) as the first metal to be tested. Set the Intensity (upper middle window) to 80% and the wavelength into the Red color at about 700 nm. Do not activate any of the check boxes at the right-hand side or the battery at the bottom of the screen. PHET Figure 1: Screenshot of the PhET Photoelectric Effect initial parameters for Part A.Slowly move the wavelength selector to shorter wavelengths (from red towards blue) and observe that at first the photons coming from the Lamp in the simulation do not eject any photons. . Continue to slowly move the wavelength selector until you observe electrons being ejected from the metal plate on the left (Figure 2). This wavelength is the "Threshold Wavelength" for that metal. Note: When you are close to the threshold wavelength, you may need to move the wavelength selector a small amount towards the red wavelengths until no electrons are ejected and then more slowly move towards the threshold wavelength. Finding the threshold wavelength is very sensitive to the size of the wavelength selector movements and the time waited. . Record your value for the threshold wavelength in the below Table 1. . Repeat finding the threshold wavelength for the metals Zinc, Copper, and Platinum (select metals in upper right). . Record your value for the threshold wavelength in the below Table 1. . Save a screenshot (screen capture) of the simulation showing the parameters for one of the trials to include in the graphs section of your report. Part B - Electrical Current verses Light Intensity: . Open PhET Photoelectric simulation software: https://phet.colorado.edu/en/simulation/legacy/photoelectric Figure 2 is a screen shot of the simulation screen with the initial parameters shown. In "Options" (upper left on tool bar) select "Show Photons" Select "Sodium" (upper right) as the first metal to be tested. Set the Intensity (upper middle window) to 10% and the wavelength into the UV region at about 221 nm (well beyond the threshold wavelength). Do not activate any of the check boxes at the right-hand side or the battery at the bottom of the screen. At 10% intensity, note the current indicated in the ammeter box at the lower middle-right of the simulation screen. The initial value at 10% intensity is 0.136 A. . Record the current value in the below given Table 2.Increase the intensity by 10% increments until 100% intensity and record the current at each increment in the below given Table 2. . Now, select "Zinc" metal (upper right) as the metal to be tested. . Maintain the values of 10% initial intensity and 221 nm wavelength. . At 10% intensity, note the current indicated in the ammeter box at the lower middle-right of the simulation screen. Record the current value in the below given Table 2. Increase the intensity by 10% increments until 100% intensity and record the current at each increment in the below given Table 2. Save a screenshot (screen capture) of the simulation showing the parameters for one of the trials to include in the graphs section of your report. hash Figure 2: Screenshot of the PhET Photoelectric Effect initial parameters for Part B.Part C - Electrical Current verses External Voltage: Open PhET Photoelectric simulation software: https://phet.colorado.edu/en/simulation/legacy/photoelectric Figure 3 is a screen shot of the simulation screen with the initial parameters shown. In "Options" (upper left on tool bar) select "Show Photons". > Select "Sodium" (upper right) as the first metal to be tested. Set the Intensity (upper middle window) to 50% and the wavelength into the UV region at about 221 nm (well beyond the threshold wavelength). In "Graphs", select "Current vs Battery Voltage". Figure 3: Screenshot of the PhET Photoelectric Effect initial parameters for Part C. . Move the voltage on the external battery (lower middle of simulation screen) to positive voltage. Record your observations of the current and the speed of the ejected electrons the voltage is increased in the below Table 3. . Move the voltage to negative voltage. Record your observation of the current and the speed of the ejected electrons, as the voltage becomes more negative, in the below Table 3. Can you reach a voltage where the current becomes zero? Think of the technological significance of this when you record your observations. Discuss in the conclusion of your lab report. . Save a screenshot (screen capture) of the simulation showing the parameters for one of the trials to include in the graphs section of your report.Analysis: . Part A - Determination of the Work Function (W.) for Different Metals: 1) Use Equation 6 and the data in Table 1 to calculate the Work Function for each metal. 2) The Work Function is expressed in units of "Electron Volts" with the symbol "eV". One electron volt is the energy required to move an electron across a 1 Volt potential. 3) Convert the Work Function energy in Joules (Equation 6) to Electron Volts using the following conversion: 1 ev = 1.60 x 10-19 J. 4) Note that Plank's Constant (h = 6.63 x 10 3 J.s) and the speed of light (c = 3.00 x 10 m/'s). 5) Remember to express the wavelength in meters and not nanometers. 6) Compare your "Calculated" values with the "Accepted" values by calculating the percentage error. 7) Record your calculated values in the below Table 1. 8) Discuss your results in the conclusion of your lab report. . Part B - Electrical Current verses Light Intensity: 1) Using MS Excel program (see posted instructions for downloading/using Excel), use the data in Table 2 for Sodium and Zinc to plot Electric Current versus Light Intensity by entering your Light Intensity (x-axis) and Electric Current (y-axis) into an Excel spreadsheet. Then, determine the slopes by fitting a straight line (linear fit) to the data in the Excel graph. Record the slopes in the below Table 2. 2) Plot the data for both Sodium and Zinc on the same graph. 3) What do the data plots look like - linear, curve, same slopes? Explain in the conclusion of your lab repot. 4) Discuss the relationship between light intensity and electric current in the conclusion of your lab report. 5) Save your Excel graphs showing the data points, linear fit, and slope to include in the graphs section of your lab report.. Part C - Electrical Current verses External Voltage: 1) Explain your observations of the current and the speed of the ejected electrons the voltage is increased to be more positive. Use your knowledge of electrical forces and understanding, of the Photoelectric Effect from the Theory section. Discuss in the conclusion of your lab report. 2) Explain your observation of the current and the speed of the ejected electrons, as the voltage becomes more negative. Did you reach a voltage where the current becomes zero? How could you use this fact of finding a voltage to stop the current at a specific light wavelength to develop a new technological device? This is an open-ended discussion and it is how new devices can be made from previously well-known physics principles! Explain in the conclusion of your lab report. Lab Report: . When writing the lab report, you must review and follow very carefully the Physics Lab Report instructions handout. In your lab report, include the Cover Page, Objectives, Theory, Equipment, Data, Graphs, Calculations, Conclusions, Sources of Error, and References. Remember to show all equations and calculations in detail and to round the results to the correct number significant digits and precision, In the conclusions section, be sure to summarize the final results, comment on the agreement or disagreement of the results with the theory or expectations, and discuss what you personally learned from this experiment and your observations/comments Submit your complete lab report electronically by the due date!. Part A - Determination of the Work Function (W.) for Different Metals: Table 1: Threshold Wavelength and Calculated Work Function Threshold Calculated Work Accepted Work Metal Wavelength (nm) Function (ev) Function (ev) Sodium 2.28 Zinc 4.30 Copper 4.70 Platinum 6.35 Accepted Values from CRC Handbook of Physics and Chemistry Table 1: Table for entering the threshold wavelength and calculated Work Function. . Part B - Electrical Current verses Light Intensity: Table 2: Intensity as a Function of Light Intensity Intensity (X) Soclum Current (A) Zire Current (Al 10.0 20.0 30.0 40 0 :0 0 70 0 BO.0 90 0 100.0 Slope Table 2: Table for electrical current as a function of light intensity.. Part C - Electrical Current verses External Voltage: Table 3: Observations of Current and Ejected Electron Speed Verses Applied Voltage Observations for Increasing Positive Voltage Observations for Increasingly Negative Voltage Table 3: Observations of current and ejected electron speed verses applied voltage