The Photoelectric Effect Purpose To use a simulation to determine the following properties of a fictitious photocell: the threshold wavelength the threshold frequency the work function experimental value of Planck’s constant Variables Independent (manipulated) variable: _____________________ Dependent (responding) variable: _____________________ Controlled variable: _____________________ Procedure Open the Modern Physics program. Choose The Photoelectric Effect from the Simulations Navigator. Pre-set the simulation controls 1. Notice in the lower left corner that the light intensity - I (also known as brightness) is set to 0.5 mW (milliwatts). This controls the quantity of light shining on the cathode (negative end of the photocell) and should remain at 0.5 mW for now. 2. Move the scroll tool below the light spectrum to the far right (at the red end of the spectrum). This controls the wavelength of light shining on the cathode. 3. Leave the voltage (U) across the photocell set to 1.5V. 4. Oddly enough, the letter I is also used as the symbol for electric current. It is measured in milliamperes (AKS milliamps) in this simulation and is abbreviated mA. The energy of the incoming photons is given by h and is measured in electron-volts (eV). You are about to begin the simulated experiment. 1. Slowly move the scroll tool below the spectrum toward the shorter wavelengths. Notice that the color of the light changes as you proceed. Find the lowest wavelength that causes particles (photo-electrons) to cross the evacuated space between the cathode and anode. Use the arrows on either end of the scroll bar to change the wavelength one nanometer (nm) at a time. The shortest wavelength which causes the photo-electric effect is known as the threshold wavelength. Record the COLOR and the value of the WAVELENGTH in nanometers in the first row of your data table. 2. Using the scroll bar at the lower right side, lower the voltage until it just stops the flow of photo-electrons. This will be indicated by a reading of I = 0.000 mA in the box just above the voltage controls and it is cleverly called the STOPPING VOLTAGE. When multiplied times the charge of the electron (taken as -1 e) it gives the maximum kinetic energy of the electrons crossing the tube. Record this voltage in your data table. 3. Change the wavelength (color) of light striking the photocell to yellow. Record the wavelength. Reduce the voltage and record the value which first stops the flow of current where I = 0.000 mA again. 4. Repeat steps 2 and 3 using green, blue, indigo and violet, recording the wavelength of light and the corresponding stopping voltage. Raw Data and Processing Raw Data COLOR WAVELENGTH STOPPING FREQUENCY KINETIC (nm) VOLTAGE (Hz x 1014) ENERGY (eV) (max) Graphing 1. Plot a graph of maximum kinetic energy (y axis) versus frequency (x axis). 2. Assuming a linear relationship, generate a best fit line and determine the slope, including the units. Conclusion 1. State the value of the slope, including units and state its meaning. 2. State the threshold wavelength and frequency for this photocell and explain their physical meaning. 3. State the work function for this photocell and explain how you determined it.
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