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Ideas at Work Module 6 Exploring the Photoelectric Effect The idea: It’s not every day that you get to perform an experiment that won the Nobel Prize in Physics, or that you get to use a computer to take control of another computer, or that you use a mix of 1940’s technology with 1990’s and today’s, all together. But this is your day! Suppose you own a small boat and you keep it moored at a seaside marina. You worry when a storm comes, because a large wave could swamp or smash your boat. Everybody knows that the height of a wave determines it energy. That’s what makes the 2012 film The Impossible so terrifying: a tsunami wave coming in above the rooftops will carry astounding destructive power. Scene from The Impossible, Apaches Entertainment, 2012 But suppose – just suppose – that not the height of the water, but something else – say, the temperature of the water – was what conferred its power. What if a wave as tall as this one pictured, but of cold water, did no damage at all, but a wave only a couple feet high but of warm water destroyed the whole village? Of course I do not mean to make light of the tsunami that killed 300,000 people around the Indian Ocean in 2004. The Impossible tells how one family survived that event. The point is only that our bitter experience tells us that the height of a water wave, and not its temperature, or color, or salinity determines its destructive potential. That’s why Max Planck’s prediction about the nature of light, almost exactly 105 years before that tsunami, seemed absurd. He claimed that the frequency or color of a light wave determined its energy – not its brightness, as everyone thought they knew. He suggested that light is emitted in separate bursts or spurts or drops that he called quanta, and that the energy of a quantum depends on the frequency of the light. (That’s why his work led to what we call Quantum Theory.) A stunningly simple formula expresses the relationship: E=hf where E represents the energy of a quantum, in joules, and f measures the frequency of the light, in hertz or waves per second. The number h is a constant to make the units work out, and it now always bears the name, Planck’s constant. Along with the speed of light and the charge of an electron, it stands as one of the few basic constants of the universe. Einstein won the Nobel Prize in Physics in 1921 for his experimental verification of Planck’s theoretical supposition. What was prize-worthy then is easy now, with an odd collection of electronic parts. Central to it all is a 1940’s vintage 1P39 high-vacuum phototube, the kind that was found in every movie projector, from theaters to schools. “Talking pictures” – movies with sound – require a way to produce sound from the light streaming through the moving film. A thin strip down the side of the film, called the sound track, varies in width as the sound of the actors and orchestra varied in loudness. The term ‘soundtrack’ now means the CD of the music from a film, but the term originally meant that wavy strip along the movie film itself. Side view of a phototube A sharply focused light beam directed through the sound track flickers in brightness as the film runs, and that varying beam falls onto the curved metal surface of the phototube. The wave of incoming light knocks electrons off the plate that are collected by the vertical wire standing in front of it. The pulsing brightness of the beam makes a pulsing stream of electrons. That feeble signal is amplified and sent to the speakers in the theater. Top view In projectors now, tiny light-sensitive transistors read multiple sound tracks for full Dolby stereo sound, and the phototubes are only found in antique shops. For this lab, an old phototube comes out of retirement for a new purpose. You will control an array of light-emitting diodes (LEDs) of varying colors that shine on the tube in a dark box. The LEDs are rigged to produce different levels of brightness as well. Which will determine their energy – their intensity, as with a water wave or sound wave, or their color, as Planck predicted? A bit of a simple soundtrack What you’ll need: The electrons are of course invisible, but you will measure their energy by applying a small voltage to the tube to stop the flow and hold them on the metal plate of the phototube, like ushers at a concert working a rope line to hold back fans. A meter will measure the current and give you an approximate sense of how many electrons are slipping by. A small camera in the box lets you see the lights. And you make all of this happen from anywhere in the world, using a computer and interface controlled by a program in the LabVIEW language. Einstein would be pleased! Your computer with an internet connection The lab apparatus in a room in on campus – but your computer will take you there! Grapher tool from Learn What you’ll do: Î If you are working on campus with either a Windows or OSX computer (not a Chromebook) then go on to step Ï. • If you are working from home or off campus with a Windows or OSX computer you must install a simple, harmless, free application called the FortiClient VPN. Go to and sign in with your Flashline credentials. Scroll down to the Network > Fortinet > FortiClient VPN entry and click View Details. Follow the directions to install this software that will make the link between your computer and the campus network. • If you using a Chromebook, please follow these instructions: to install a similar client application. Ï To start your work, simply click on the link in Learn that says Click Here to access the experiment for Ideas at Work 6. Follow the additional few instructions in that item in Blackboard. Ð Wait a moment for your computer to connect to the on-campus server. You are now in control! The lab in progress. Note the photocurrent value just to the right of the meter, the stopping voltage display just below the slider, and the increment up and down buttons just to the left of the voltage display. This is how your monitor will look during the experiment. The camera image at the top right shows the back side of the phototube with, in this case, the orange LED energized. As you begin, though, all of the LED switches should be off and the camera will only display the darkness inside the box that contains the equipment. Always be patient during the lab. Your commands are being sent and returned through the internet, so expect a brief time lag. Ñ On the left side of the screen, click the Light Source button for the orange LED. You will see it turn on in the window at the top right, and the meter needle on the right side will move up a bit, measuring the current through the tube caused by that light. The camera is much more sensitive to orange light than is the phototube, so the needle does not move much. Begin with the Stopping Voltage slider all the way to the left, at zero volts. The photocurrent is quite small so the computer has difficulty reading it, and the needle will jitter up and down. Decide what seems to be an average value and record that photocurrent from the display just under the meter on the Report Sheet. Ò Use your mouse to move the voltage control slider at the bottom of the screen slowly to the right. You will be cutting off the flow of electrons, and the voltage needed to stop them measures their energy. Click the up and down increment buttons to fine tune the stopping voltage. Raise it until the current drops nearly to zero, as you see in the window under the meter scale. I installed a little yellow LED to help you. It lights any time that the photocurrent is above zero and turns off when it drops below zero. Again, because this is a real lab experiment and not a computer simulation, the photocurrent has some noise or jitter. It will fluctuate or wander a bit on its own. Raise the stopping voltage gradually until the LED seems to turn off for about as much time as in turns on. Please raise the voltage slowly when the photocurrent display is nearing zero, because if you hurry and go too high, the display will still read approximately zero and you will read too high a value for the stopping voltage. When you think you have the current very close to zero with the least possible stopping voltage – that is, when the yellow LED is off about as much as it is on – record on the Report Sheet the stopping voltage from the window below the slider. Click the Light Source button to turn it off and reset the stopping voltage to zero. Ó Repeat the process for each color LED. Turn it on, note the approximate average initial photocurrent from the display, and record in on the Report Sheet. Gradually raise the stopping voltage until the current has fallen to zero – when the yellow LED stays off for about as long as it is on. Record the stopping voltage on the Report Sheet. Ô When you have measured the initial photocurrent and stopping voltage for each of the four colors, be sure to reset the stopping voltage to zero and check that all of the Light Source switches are off. Close your browser so that others may use the experiment. Please note: Only one person can use the equipment at a time, and many people need to do the experiment. If someone else is using it ahead of you, you will see a yellow box on your screen letting you know that the computer is in use, and your mouse and keyboard will have no effect on the equipment. In that case, the lab computer will notify the current user that he or she has ten minutes to finish, after which control will pass to you. If you decide not to wait, please close your browser to ‘step out of the line’ and then come back another time. If you are conducting the experiment and get a notice that someone else is waiting, please work quickly to try to collect your data within the next ten minutes. You may have to come back another time. Thanks in advance for your patience! Õ Open the spreadsheet called Grapher Module 6 from Learn. It will do the heavy lifting of calculating and graphing for you. For each color of LED, enter the photocurrent and stopping voltage that you recorded on the Report Sheet. The Grapher will use the stopping voltage values to calculate the kinetic energy of the electrons, in joules, and then graph both the photocurrent and the kinetic energy versus frequency of the light. Be sure to press Enter or to click out of the box when entering the last of the data at the bottom right so that the graph will include that value. One of the graphs will have no apparent pattern or connection between the two variables, but the other will form a more or less straight line. In physics, a straight line means that you have found a simple relationship between the two variables. Note the slope of the line for the more linear, better looking graph and record that on the Report Sheet. That value represents your best estimate of Planck’s constant! Ö Use Google to find and record the accepted value (that is, the current known value) of Planck’s constant. × Determine the percentage difference between your result and the accepted value. Since both values will have the same exponent (10-34), you should ignore the exponent and just use the first part of the numbers. Use a calculator to find the difference between your value for Planck’s constant and the accepted value, divide that difference by the accepted value, and multiply by 100. Record that result on the Report Sheet as well. For example, let’s say that the accepted value of Planck’s constant, from Google, is 5.2567356 x 10-34 J-s. (It’s not; let’s just use that as an example.) And let’s suppose that your value, from the Grapher tool, is 4.37 x 10-34 J-s. You would first round the accepted value to three digits and drop the exponent, to give 5.26. Chop the exponent off your value, for 4.37, and subtract one from the other. That gives 0.89. Divide that number by the chopped accepted value, and then multiply by 100: (0.89/5.26) x 100 = 16.92 % Round that three digits, or 16.9 %; your result was within 16.9 % of the accepted answer, in this example. Your result will probably yield a smaller percent difference. With care, you can measure Planck’s constant within 5 or 10 percent in this experiment. Pretty cool, for a bunch of LEDs and an antique vacuum tube, run by remote control! Ideas at Work Module 6 Report Sheet Exploring the Photoelectric Effect Name Color Peak Frequenc y,Hz Orange 5.1 x 1014 Green 5.7 x 1014 Blue 6.2 x 1014 Ultraviol et 6.7 x 1014 Photocurrent, mA (Current with zero stopping voltage) Stopping voltage, V(Voltage to force current to zero) V Photocurrent measures V Stopping voltage measures the brightness of the the energy of the photonsincoming light. of the incoming light. Based on the graph with the better fit straight line, what can you say about the relationshipbetween the color (or frequency) of light and the energy of its separate bits or photons? Your value of Planck’s constant (h) from the Grapher: J-s The accepted value of Planck’s constant from Google: J-s V Round this number to three digits, or two decimal places. Percent difference of your value from the accepted value: %

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