Photoelectric+Effect

Part 1
toc **10.****French Physicist Louis de Broglie questioned, “If light exhibits dual wave-particle behavior, why can’t any particle of matter, such as an electron, exhibit a wave nature?”** **11.****The Heisenberg Uncertainty Principle reflects the wave-particle duality of light and matter: The more we know about matter as a particle (well-defined position), the less we know about its momentum (wavelength) and vice versa.** **12.****What did each of these scientists contribute to Quantum Physics?** **13.****How does a solar-powered calculator work?** The light that strikes the calculator causes the electrons to flow in a circuit that powers the calculator. When photons hit a solar cell, it excites electrons, which allow them to flow through in between layer p (positive energy) and n (negative energy) which creates a field of electricity. **14.****How does your cell phone’s digital camera work?** When a picture is taken, the photons hit the lens and different frequencies and intensities, which create an image with colors that correspond. CCD and CMOS image censors convert light into electrons. Once the sensor converts the light into electrons it reads its accumulated charge of each cell in the image. **15.****How does a laser work?** A laser causes photons to be released, with the intent of only hitting a certain amount of electrons. This causes a narrow beam of light to be seen. The lasing medium is "pumped" to excite the atoms to two or three levels above the ground state. The excited electrons emit energy, which comes in the form of photons. The photon emitted has a specific wavelength that depends on the state of the electron's energy when the photon is released. **16.****How do solar panels work?** A solar panel absorbs light energy, which causes the electrons to gain energy. This energy can be converted and stored as electricity. When light strikes the cells of the panels, a portion of it is absorbed within the semiconductor and the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. This causes current, and by placing metal contacts on the top and bottom of the photovoltaic cell, we can draw the current for external use. The current and voltage defines the power that the solar cell can produce.
 * 1) **Upon what physical quantity does the color of any glowing body depend?**
 * 2) Temperature
 * 3) **As the cells in a flashlight get weaker, the filament appears redder. Why?**
 * 4) The dimmer red light of the filament has an equal amount of energy as the the brighter red filament.
 * 5) As the batteries get weaker, this in turn causes a smaller current and less energy. When there is less energy, the filament doesn't get as hot so it produces a smaller frequency (brighter color).
 * 6) **The surface temperatures of Vega, our Sun, and Barnard’s star are 10,000 K, 6,000 K, and 3,000 K respectively. Which of these appears blue in color; red in color; yellow in color? Why?**
 * 7) Sun= Yellow
 * 8) Vega= Blue
 * 9) Barnard= Red
 * 10) As something gets hotter its color goes from yellow--> red--> blue.
 * 11) Frequency goes down as temperature increases.
 * 12) **What is the photoelectric effect?**
 * 13) The photoelectric effect is an experiment that proves that energy is released in small finite packages called quanta. This is proved by shining a light on a metal plate which causes photons to be bombarded and current to flow.
 * 14) One photon is absorbed by each photoelectron that the metal ejects. When the energy of the incoming light is greater than the work function, electrons will be emitted and the surplus energy will be in the form of KE.
 * 15) **As a mechanical analog of the photoelectric effect, consider a ball at rest in a depression. If a sufficient amount of energy, E, is given to the ball of mass, m, by the push of the hand, it will roll up the hill and escape with velocity v. Write an equation of energy conservation for this situation and explain each term in the equation by analogy to Einstein’s photoelectric effect equation.**
 * 16) W=1/2mv^2
 * 17) Work is the light emitting photons on the metal screen, or the hand striking the ball.
 * 18) The mass is the mass of the electron, or the mass of the ball.
 * 19) The velocity is the speed of the current, or the speed that the ball moves as a result of the inflicted force.
 * 20) E=mgh + 1/2mv^2 ( E is the total energy, the potential energy is analogous to the work function, and the 1/2mv^2 is analogous to the KE of the released electron.
 * 21) **An ultraviolet light discharges a negatively charged electroscope.**
 * 22) **The effect is known as**
 * 23) The photoelectric effect.
 * 24) **Why isn’t the effect noticed when a glass plate is inserted between the zinc plate and the UV light?**
 * 25) The glass filters the UV light and stops the discharge of photoelectrons.
 * 26) photons of visible light don't have enough energy to eject electrons from zinc (the energy of the electrons is not greater than the work function of zinc)
 * 27) **What does the work function usually denoted by** **// Ψ //** **represent?**
 * 28) The amount of energy an electron must 'pay' in order to be released. This work function is subtracted from the total kinetic energy.
 * 29) **Rewatch the video at about the 7 minute mark. The upper half of the screen shows an electron trying to escape from the surface of the metal. The bottom half is a graph. The vertical axis represents the electric potential energy of the electron. As you watch the video, notice that the electron slides along the bottom line and slips up the edge to the bottom of the work function line. This represents the most energetic electrons, the ones that will escape from the surface if photons strike the surface with energy equal to or greater than the work function. If an electron absorbs a photon of ultraviolet light with energy** **//hf//** **greater than** **// Ψ //** **, what will happen to the electron?**
 * 30) The electron will be released and there will be kinetic energy. The kinetic energy will be equivalent to the difference btween hf and the work function.
 * 31) **Use the table of work functions of various metals on page 1.**
 * 32) **If green light will cause the photoelectric effect to occur in sodium metal, for which other metals listed will it definitely also eject photoelectrons?**
 * 33) Potassium and Cesium
 * 34) **What would be the effect of doubling the intensity of the light used?**
 * 35) The number of photons would double, but the KE would remain the same.
 * 36) **What would be the effect of changing the color of the light used, for example, from green to blue or ultraviolet?**
 * 37) Changing the color, or the wavelength would cause a change in kinetic energy. Kinetic energy would increase as the color changed from green-->blue-->ultraviolet
 * 1) **How did de Broglie relate a particle’s momentum to its wavelength?**
 * 2) He combined the equations p=h/lamda and E=mc^2. He came up with the equation E=hf, which states that the kinetic energy is directly equal to frequency. Since frequency and wavelength are related, in turn momentum and wavelength are related too ( inversely .) Only particles with very small masses have wavelengths that are observable.
 * 3) **In the de Broglie model of the atom, if electrons are viewed as waves circling the nucleus, why do they have to exist in orbits that increase a whole wavelength at a time?**
 * 4) The crests and troughs of each wave must match up therefore constructive and destructive interference can take place. The only way for this to happen is if the orbits increase by a whole wavelength at a time, and not a fraction of a wavelength.
 * 5) **Waves produce an interference pattern. How can the pattern be explained in terms of particle of light ?**
 * 6) The particles of light hit the screen and project an alternating bright and dark pattern. The bright spots are where constructive interference is occurring, and the dark spots are where destructive interference is occurring.
 * 7) There is a higher probability that more photons land in some places than others.
 * 8) **To what do the white light spots correspond where the wave pattern hits the screen?**
 * 9) The white spots are the areas where constructive interference occurs.
 * 1) **How can a particle’s location be made more definite?**
 * 2) By measuring the moment less accurately.
 * 3) Location can be made more definite by adding waves of various wavelengths
 * 4) **As the position of the wave becomes more definite, what happens to the momentum?**
 * 5) The momentum of the wave becomes less definite.
 * 1) **Planck**
 * 2) discovered that light was discovered in bundles of energy called quanta
 * 3) discovered Planck's constant
 * 4) **Einstein**
 * 5) Used the photoelectric effect to prove Planck's theory
 * 6) **De Broglie**
 * 7) Discovered that light acts as both a wave and a particle
 * 8) He united particle properties and wave properties in the single equation p=h/lamda. This enabled particles to move in what are called "matter waves".
 * 9) **Schrodinger**
 * 10) Discovered that when you add together multiple waves of the same wavelength, light can be studied as a particle
 * 11) Described the electron cloud model by explaining the motion of electrons as probabilities
 * 12) **Heisenberg**
 * 13) Discovered that when wave properties are measured precisely, particle properties are measured less precisely and vice versa

Part 2
> **A. Increasing the intensity of the light beam** > **B. Decreasing the intensity of the light beam** > **C. Increasing the wavelength of light** > __ **D. Decreasing the wavelength of light** __ > __ **E. Increasing the frequency of light** __ > **F. Decreasing the frequency of light** > __ **G. Increasing the voltage of the battery** __ > **H. Decreasing the voltage of the battery** > **I. Replacing the target with a material that has a larger work function** > __ **J. Replacing the target with a material that has a smaller work function** __
 * 1) **Suppose you set up the experiment so that the plate is ejecting electrons. Predict which of the following changes to the experiment could increase the maximum initial kinetic energy of the ejected electrons. (Select all that apply) Then test your prediction.**

> **A. Increasing the intensity of the light beam** > **B. Decreasing the intensity of the light beam** > **C. Increasing the wavelength of light** > __**D. Decreasing the wavelength of light**__ > __**E. Increasing the frequency of light**__ > **F. Decreasing the frequency of light** > **G. Increasing the voltage of the battery** > **H. Decreasing the voltage of the battery** > **I. Replacing the target with a material that has a larger work function** > __ **J. Replacing the target with a material that has a smaller work function** __
 * 1) **Suppose now you set up the experiment so that the light intensity is non-zero but the plate is NOT ejecting electrons. Predict which of the following changes to the experiment could make the plate start ejecting electrons? (Select all that apply) Then test your prediction.**

> **A. The force exerted on the electrons by the battery** > __**B. The beam of light shining on the plate**__ > **C. Both A and B.** > **D. Neither A nor B.**
 * 1) **What causes the electrons to be ejected from the left plate in this simulation?**


 * 1) **Light is shining on a metal and electrons are being emitted. You turn the intensity down very very low. What do you observe? What conclusions can you draw about light, and why? how it is or is not consistent with what you would expect to observe if light matched the classical wave model and with what you would expect to observe if it matched the photon model of light.**
 * 2) As I turn the intensity down very low, less electrons are being emitted. This causes me to believe that the intensity of light affects only how many electrons are emitted, not the amount of kinetic energy that is emitted. This is inconsistent with the wave model of light because the intensity of a wave affects the amount of much kinetic energy is emitted. This is consistent with a photon model because at different intensity levels, more or less quanta are released.
 * 3) Classical wave model would say that as intensity increases, the KE emitted should also increase


 * 1) **Light is shining on a metal plate and electrons are being emitted. Without changing the intensity, you make the wavelength longer and longer. What do you observe? What conclusions can you draw about light, and why? how it is or is not consistent with what you would expect to observe if light matched the classical wave model and with what you would expect to observe if it matched the photon model of light.**
 * 2) The same number of electrons are emitted, but they have more KE. This causes me to believe that the wavelength of light affects only the amount of kinetic energy emitted, and not how many electrons are emitted. This is inconsistent with the wave model of light because light has no momentum and therefore has no energy. This is consistent with the photon model of light because wavelength and KE are directly related. The smaller the wavelength the more KE emitted.
 * 3) Classical wave model would say that regardless of how low the frequency was, the electrons would gain enough energy to be ejected.

**a) Explain your reasoning for** __**why**__ **this curve has the shape that it does. In your answer, you should address: Why is current level at V>0, why does current go to zero at some negative voltage and what determines that voltage, and why does current start decreasing steadily at V<0?** The current goes to zero at some negative voltage because the absolute value of this voltage represents stopping voltage. This is the exact voltage where current goes from not flowing to just flowing. This stopping voltage is determined by the work function. The current starts to decrease steadily when voltage is less than zero because there is no pressure difference to cause the movement of electrons.
 * 1) **In the photoelectric effect experiment, the graph of current vs battery voltage for a metal with light of a particular frequency shining on it looks like the curve below. This graph represents** **current vs voltage** **for** **200nm light** **shining onto** **Cadmium (Cd)** **which has** **a work function of 4.07 eV****.**

It depends on the number of charges per second only, not their speed. Since voltage influences KE, the intensity will remain constant. Voltage opposes the direction of the current and causes zero flow rate at a certain stopping voltage. When V is negative it opposes flow rate but as it approaches zero it opposes the flow rate LESS and allows current to increase.

**b) What is the stopping potential in this situation (in eV)? (Remember stopping voltage is expressed as a positive number).** 4.07 eV  **c) In the graphs below, the gray curve is always the same and represents the situation you explained in part a (the current vs voltage for 200nm light shining onto Cadmium (Cd) which has a work function of 4.07 eV). The red curves now represent the current vs voltage after a change in the experiment. Use the graphs to answer the questions that follow.**

**i) If you decrease the wavelength of the light shining onto the metal, what happens to the voltage where the current goes to zero...**

__ **becomes a larger, negative number** __ **becomes a smaller, negative number** **is unchanged**

**ii) Which graph would represent an increase in the intensity?** E because intensity increases the current but not the stopping potential

**iii) Which graph would represent an increase in wavelength to 290nm?** D  C because increasing wavelength decreases the stopping potential and current

**iv) Which graph would represent an increase in wavelength to 500 nm?** I Because there is no current when the wavelength is very large, so no there are no electrons emitted

**v) Which graph would represent a switch to sodium?** G Because the stopping potential of SOdium is smaller than Cadmium the cutoff wavelength for sodium is a little bit higher.

**vi) What change or combination of changes would you need to explain the change observed in Graph H above? (check all that apply)**

__ **decrease in wavelength** __ **increase in wavelength** **decrease in intensity** **increase in intensity**


 * 1) **If you have the experiment set up so that electrons are being emitted from the metal plate, which of the following are true and which are false?**
 * 2) **As long as conditions do not change, all emitted electrons have the same initial kinetic energy.**
 * 3) False
 * 4) **The work function for the metal is different for different electrons.**
 * 5) False
 * 6) **The energy of the photons hitting the plate must be less than the work function of the metal.**
 * 7) False
 * 8) **The electrons emitted with the largest initial kinetic energy are those that were the least tightly bound in the metal**
 * 9) True


 * 1) **You have a colored spot light, but you don't know its precise wavelength. To find out the wavelength you shine your light on a sodium target placed in a circuit as shown in the simulation. You look up the work function of sodium and find that it is 2.3 eV. If you set the battery voltage to -0.5 V, you find that the most energetic electrons nearly reach the right plate, but turn around just before they get there. What is the wavelength, in nm, of the colored light that you used? (You can answer this question either by doing a calculation or by using the simulation. To get practice for answering the next question, we recommend that you use both methods and check that they give the same answer.) SHOW YOUR WORK/EVIDENCE.**
 * 2) Wavelength= 444 nm on simulation
 * 3) [[image:Screen_shot_2012-02-29_at_6.14.57_PM.png]]
 * 4) [[image:apphysicslevine/Screen_shot_2012-02-29_at_8.32.54_AM.png caption="Screen_shot_2012-02-29_at_8.32.54_AM.png"]]


 * 1) **You have a plate of metal, but you have no idea what kind of metal it is. You come up with the brilliant idea of measuring the work function of this metal by using it as the target in a photoelectric effect experiment. You can perform this experiment virtually by selecting '???' as the target in the simulation. SHOW YOUR WORK/EVIDENCE.**
 * 2) **What is the work function, in eV, of the mystery metal?**
 * 3) 3.7 eV
 * 4) [[image:Screen_shot_2012-02-29_at_6.12.46_PM.png]]
 * 5) **What is the mystery material?**
 * 6) Magnesium

Note the "SHOW YOUR WORK" in all caps for numbers 8 and 9???? That means you need to show your work... equations, numbers plugged in, etc. Thank you for moving this to a new page :)

Part 3

 * 1) **The line on the graph of current to intensity can be described as _** linear
 * 2) **The line on the graph of energy to frequency can be described as _** linear
 * 3) **At a** __**frequency below**__ **the frequency required to overcome the work function, increasing the light intensity causes the current to** //**increase / decrease / __remain the same.__**//
 * 4) **At a** __**frequency above**__ **the frequency required to overcome the work function, increasing the light intensity causes the current to** //**__increase__ / decrease / remain the same.**//
 * 5) **Old darkrooms (for developing film) were once illuminated with a feint red light (765nm). If this was the limit of the silver compound used in the film, solve for the work function of the silver compound. ___ 1.625 eV 2.6 x 10^-19 ____J__**
 * **Consider the following scenario: On a partly cloudy day you find that a household photovoltaic array outputs 2.4 amps of current. If the clouds part and the sun comes out, exactly doubling the amount of light incident on the PV array, we should expect the array to output** //**2.4 A / __4.8 A__ / more than 4.8 A / less than 2.4 A / __between 2.4 and 4.8 A__ **// __**.**__

**10.Stars vary in color. Which color indicates the hottest surface temperature of a star?** **11.Which of the following ojbects, all moving at the same speed, would have a de Broglie wavelength associated with them that would be larger than that of a proton travelling a the same speed?** **12.When green light shines upon a given metal, it emits phtoelectrons. Which of the following will also produce photoelectric emission, using this same metal?** **13.Ultraviolet light shines upon a sheet of zinc metal, and photoelectrons are emitted. If the intensity of the light is increased,** **14.Consider the following frequencies of electromagnetic radiation. Which photon has the greatest energy?** **15.Compared to a photon of blue light, a photon of red light has** **16.An electron is confined to a box of sides L and it has a definite speed. If the walls of the box were to move inward so that the box shrinks, the electron** **17.The idea of packets or quanta of energy originated with** **18.A matter wave** **19.Which of the following does not demonstrate the wave nature of matter?** **20.When doing the photoelectric effect experiment,**
 * 1) __ **The work function for cesium is 1.96 eV. Find the cutoff wavelength for the metal.** __ 634 nm
 * 2) **What is the maximum kinetic energy for the emitted electrons when 425 nm light is incident on #7’s metal?** 1.544 x 10^-19 **J**
 * 3) **In certain metal, the stopping potential is found to be 3.70 V. When 235 nm light is incident on the metal, electrons are emitted. What is the maximum kinetic energy given to the electrons in eV and J? ___ 2.15 eV__ _**3.44 x 10^-19 **J**
 * 4) eV=KE
 * 5) 3.7eV
 * 1) **Red**
 * 2) **Orange**
 * 3) **Yellow**
 * 4) __ **Blue** __
 * 1) __ **An electron** __
 * 2) **A neutron**
 * 3) **A bacteria**
 * 4) **A baseball**
 * 1) __ **Low intensity blue light** __
 * 2) **Low intensity red light**
 * 3) **High intensity red light**
 * 4) **high intensity yellow light**
 * 1) **The electrons will have less energy.**
 * 2) **The electrons will have more energy**
 * 3) __ **More electrons will be emitted** __
 * 4) **Fewer electrons will be emitted.**
 * 1) **6.6 x 10****-34** **Hz**
 * 2) **6.6 x 10****-4** **Hz**
 * 3) **6.6 x 10****4** **Hz**
 * 4) __ **6.6 x 10****18** **Hz** __
 * 1) **More energy**
 * 2) __ **Less energy** __
 * 3) **Shorter wavelength**
 * 4) **The same wavelength**
 * 1) __ **Would speed up** __
 * 2) **Would slow down**
 * 3) __ **Would move with the same speed** __
 * 4) **Would exhibit none of the above.**
 * 1) **Louis de Broglie**
 * 2) __ **Max Planck** __
 * 3) **Werner Heisenberg**
 * 4) **Erwin Schrodinger**
 * 1) **Applies only to “massless” particles**
 * 2) **Applies only to a photon**
 * 3) __ **Has a wavelength inversely related to its momentum** __
 * 4) **Has a wavelength directly related to its momentum**
 * 1) __ **The cloud model of the electron** __
 * 2) **The two slit interference pattern**
 * 3) **An electron in motion in a conducting wire (circuits)**
 * 4) **Electron diffraction**
 * 1) **What determines the amount of kinetic energy photoelectrons will have?**
 * 2) Wavelength
 * 3) **What determines the number of photoelectrons emitted from a metal?**
 * 4) Intensity