Photoelectric+Effect

The Photoelectric Effect - Virtual Lab

flat =Part 1=

>> positive energy and n for negative charge that creates a field of electricity. >> energy electrons). It is necessary to have a large collection of atoms in the excited state for the laser to work efficiently. In general, the atoms are excited to a level that is two or three levels above the ground state. Once the lasing medium is pumped, it contains a collection of atoms with some electrons sitting in excited levels. The excited electrons have energies greater than the more relaxed electrons. Just as the electron absorbed some amount of energy to reach this excited level, it can also release this energy. The electron can simply relax, and in turn rid itself of some energy. This emitted energy comes in the form of photons (light energy). The photon emitted has a very specific wavelength (color) that depends on the state of the electron's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths. OK... Got it. Some of these responses are perfunctory. I hope you know the details better than these explanations demonstrate. ~EB
 * 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) More energy is required to create light at higher frequencies. As the cells die, there is a higher frequency, which requires more energy.
 * 5) The battery produces a smaller current which means less energy being received by the filament. There is now a lower frequency and the filament is redder.
 * 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) Vega - blue
 * 8) Sun - yellow
 * 9) Barnard - red
 * 10) The frequency goes down as the temperature decreases due to Wien's Equation.
 * 11) What is the photoelectric effect?
 * 12) The photoelectric effect is the phenomenon of atoms releasing electrons when they are hit by energy in the form of light.
 * 13) 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.
 * 14) E=mv^2. In this equation E is energy from the hand, m is mass of the ball, and v is the velocity it escapes with.
 * 15) E = mgh + (1/2)mv^2, where E is the total energy, the potential energy is the work function, and the last term is the kinetic energy of the released electron.
 * 16) An ultraviolet light discharges a negatively charged electroscope.
 * 17) The effect is known as...
 * 18) <span style="font-family: Arial,Helvetica,sans-serif;">the photoelectric effect.
 * 19) <span style="font-family: Arial,Helvetica,sans-serif;">Why isn’t the effect noticed when a glass plate is inserted between the zinc plate and the UV light?
 * 20) The glass restricts the amount of light waves allowed to pass through it, causing a lack of energy reaching the ainc plate and releasing electrons.
 * 21) <span style="font-family: Arial,Helvetica,sans-serif;">What does the work function usually denoted by **// Ψ //** represent?
 * 22) <span style="font-family: Arial,Helvetica,sans-serif;">It represents the minimum energy required to remove an electron from a certain atom.
 * 23) <span style="font-family: Arial,Helvetica,sans-serif;">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?
 * 24) <span style="font-family: Arial,Helvetica,sans-serif;">The electron would be released because there is more energy than the work function, allowing it to leave the material and fly off.
 * 25) Use the table of work functions of various metals on page 1 to answer these questions.
 * 26) If green light will cause the photoelectric effect to occur in sodium metal, for which other metals listed will it definitely also eject photoelectrons?
 * 27) Cesium and Potassium. They both have work functions less than sodium.
 * 28) What would be the effect of doubling the intensity of the light used?
 * 29) Photoelectric effect would not change.
 * 30) More photoelectrons, each having the same energy as before.
 * 31) What would be the effect of changing the color of light used, for example, from green to blue or ultraviolet?
 * 32) When the color of light changes, frequency changes. This also changes the amount of energy needed to remove the electron.
 * 33) 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?”
 * 34) How did de Broglie relate a particle’s momentum to its wavelength?
 * 35) He created this equation.
 * 36) [[image:Screen_shot_2012-02-27_at_11.33.42_PM.png]]
 * 37) 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?
 * 38) Electrons orbit the nucleus in orbitals and orbitals must decrease by some measure of length when moving up and down orbitals. A fraction of a wavelength is not possible, so they must increase and decrease by one at a time.
 * 39) Waves produce an interference pattern. How can the pattern be explained in terms of particle of light ?
 * 40) The double slit experiment explains this well. Light is shown through two slit and it shows up in a pattern, alternating from spots of destructive interference and constructive interference.
 * 41) To what do the white light spots correspond where the wave pattern hits the screen?
 * 42) Constructive interference
 * 43) 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.
 * 44) How can a particle’s location be made more definite?
 * 45) By decreasing the accuracy of momentum.
 * 46) By adding waves of different wavelengths.
 * 47) As the position of the wave becomes more definite, what happens to the momentum?
 * 48) The momentum becomes less definite.
 * 49) What did each of these scientists contribute to Quantum Physics?
 * 50) Planck - light came in bundles of energy
 * 51) Einstein - photoelectric effect explanation
 * 52) De Broglie - light behaves as both a particle and wave
 * 53) Schrodinger - schrodinger equation - electron cloud model
 * 54) Heisenberg - uncertainty principle
 * 55) How does a solar-powered calculator work?
 * 56) The photovoltaic effect occurs and voltage is created by the absorption of energy in the form of photons.
 * 57) When photons hit a solar cell, it excites electrons, which allow it to flow through in between layer p, which stands for a
 * 1) How does your cell phone's digital camera work?
 * 2) Light photons hit tiny panels which capture the intensity of light and wavelength of the photons. This information is packaged and is taken to the screen where the picture is shown.
 * 3) Camera that captures images electronically rather than on film. The image is captured by an array of charge- coupled devices (CCDs). Instead of film, a digital camera has a sensor that converts light into electrical charges. Some cameras use complementary metal oxide semiconductor (CMOS) technology instead. Both CCD and CMOS image sensors convert light into electrons. A simplified way to think about these sensors is to think of a 2-D array of thousands or millions of tiny solar cells. Once the sensor converts the light into electrons, it reads the value (accumulated charge) of each cell in the image.
 * 4) How do lasers work?
 * 5) Lasers stimulate the emission of photons by exciting a certain number of electrons. These electrons emit light in photons, which creates a beam of light in this case.
 * 6) "Laser" is an acronym for light amplification by stimulated emission of radiation, which describes very succinctly how a laser works. In a laser, the lasing medium is “pumped” to get the atoms into an excited state. Typically, very intense flashes of light or electrical discharges pump the lasing medium and create a large collection of excited-state atoms (atoms with higher-
 * 1) How do solar panels work?
 * 2) Solar panels absorb light which causing electrons to gain energy, enough energy to be released by the material flow and freely. These electrons are used in the huge circuit of your home.
 * 3) Also called photovoltaic (PV) cells, which convert sunlight directly into electricity. Photovoltaic cells are made of special materials called semiconductors such as silicon. When light strikes the cell, a certain portion of it is absorbed within the semiconductor material and the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely. PV cells also all have one or more electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off for external use. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

= 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) Changing the intensity only changes the number of electrons that are emitted from the metal. It does not change the speed of electrons. If light matched the classical wave model, this would not be the case as the electrons’ speed would in fact be affected.


 * 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 speed of the electrons decreases when the wavelength is increased. An increase in wavelength decreases the energy of the light, which ultimately decreases each electrons’ kinetic energy. It is not consistent with the classical wave model because this claims light does not have any energy and it is consistent with the photon model of light because it does.


 * 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.

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? Current is charge per second. Speed does not matter. Since voltage affects kinetic energy, current will not change. Negative voltage opposes the direction of current and causes no flow. As it approached zero from the negative side, 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). eV = hf - work function  eV = (6.626x10^-34)(3x10^8)/(1.6x10^-19)(200x10^-9) - 4.07 = 2.17eV

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?


 * 1) Graph E

iii) Which graph would represent an increase in wavelength to 290nm?


 * 1) Graph C

iv) Which graph would represent an increase in wavelength to 500 nm?


 * 1) Graph I

v) Which graph would represent a switch to sodium?


 * 1) Graph G

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) True. 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?
 * 2) 410 nm

eV = hf - work function e(0.45) = (6.626x10^-34)(3x10^8)/(1.6x10^-19)(299x10^-9) - work function e(0.45) = 4.155 - work function work function = 3.7eV
 * 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?


 * 1) What is the mystery material?

i. Magnesium. Work Function = 3.68 eV

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.

= Part 3 =

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 line on the graph of current to intensity can be described as _
 * 2) The line on the graph of energy to frequency can be described as linear. Slope is planck's constant.
 * 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.
 * 6) 1.63eV
 * 7) 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//.
 * 8) The work function for cesium is 1.96 eV. Find the cutoff wavelength for the metal.
 * 9) 634 nm
 * 10) What is the maximum kinetic energy for the emitted electrons when 425 nm light is incident on #7’s metal?
 * 11) 1.6x10^-19 J
 * 12) 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?
 * 13) 5.3 eV and 8.5 x 10^-19
 * 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 104 Hz
 * 4) ** 6.6 x 1018 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) The wavelength of the light
 * 3) What determines the number of photoelectrons emitted from a metal?
 * 4) The intensity of the light