Physics - Modern Physics - The Photoelectric Effect | Corey Mousseau | Skillshare

Physics - Modern Physics - The Photoelectric Effect

Corey Mousseau

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3 Lessons (26m)
    • 1. Modern Introduction

      1:08
    • 2. Modern Physics The Photoelectric Effect AP Physics (College Physics)

      15:16
    • 3. Modern Physics The Photoelectric Effect AP Physics (College Physics) Part 2

      9:53

About This Class

Physics is all about the world around us.  We live and experience physics every moment of our lives.  For most, we take physics for granted and never really truly understand how this amazing world works.  

I am a high school physics teacher by day.  I create the majority of my videos for my own students.  My courses all follow NY State high school physics entirely, as well as the nationally renown AP Physics 1 & 2 curriculum.  AP physics is exactly the same thing as any algebra based college intro physics course.  

This course provides a in depth look at The Photoelectric Effect.  This course continues with the quantum realm of physics. Each of my Modern Physics courses cover blackbody radiation, the Photoelectric Effect, wave-particle duality, uncertainty principal, atomic models, the Bohr model, nuclear physics, and the standard model of particle physics.  

The order in which you should complete these courses are below:

Physics - Modern Physics - Quantum Physics and Blackbody Radiation

Physics - Modern Physics - The Photoelectric Effect

Physics - Modern Physics - Photon Momentum, Particle-Wave Duality, Electron Diffraction, Uncertainty

Transcripts

1. Modern Introduction: Hi, My name is Corey Moose. Oh, and I have been teaching high school in AP physics for over 10 years. Are you interested in learning all about physics? Do you want to master the high school in AP curriculum? Whether you're a high school student or an entry level college student, this course is for you. I have developed a series of video lessons from my own students Toe Watch to master the high school in AP physics curriculum. These videos are broken up into multiple Siri's toe allow you to manage your time properly and to pick the course best suited for your needs. Each syriza dresses every topic thoroughly with lessons, demonstrations, an example problems. While this Siri's does include lessons for both high school and AP physics, every lesson specifically within the AP curriculum will be clearly marked. College students and algebra based introductory courses should also benefit from these series of videos. This particular course covers the topics of modern physics 2. Modern Physics The Photoelectric Effect AP Physics (College Physics) : All right, let's continue our discussion on modern physics. If you recall last video, we discussed black body radiation and ultimately plunks quanta ized energy description. And that ended up really more or less opening up the window for all kinds of quantum effects and physics. And so the next one we talk about is the photo electric effect on the photo logic effect itself was understood kind of prior to plunks work in black body radiation. So let's describe what it is, and I'm gonna then talk about how classical physics modeled it, and then how reality is. And so, for electric effect is when light or really any e m radiation strike certain materials. It actually causes electrons to be ejected from the material. And ah, that's why it's called a photo electric effect because we're using a photo Thanh, which is give you later defined by Einstein to move electrons to create electric effect. And really, in this nice, simple diagram, we've got our incident light ray radiation, Remember, doesn't need to be visible late striking material that has free electrons available. And if there's enough energy, those electrons get admitted, we will call these electrons photo electrons simply because they're the electrons that were emitted by a photon. They still behave just like any electrons, and they will still be behave just like all circuits if we try to take advantage of this. Classical physics suggests that this incident light ray the energy itself will be related to the amplitude or the intensity the late So you shine the light and basically suggesting if you shine light of some wavelength or frequency on a material. If that light is unable to eject the electron from the material, classical physics says all you need to do is make the light brighter. Make the light brighter, you to give it more energy, and you're more likely going to eject electrons. Well, well, as you're probably expecting, classical physics was wrong. Instead, experimentation shows that it turns out that instead of it being the intensity or the brightness that increases the energy, it's simply the frequency that was increasing the mission of the electrons. So if we shine red light on the material and it did not emit electrons instead of making that red light brighter, we need to change that to a higher frequency light source. And that's where Einstein took it and ran with it because they know everyone understood, or at least most scientists the time understood the effect was true. Changing frequency cause 40 mission changing intensity didn't seem to do much. But then, when Einstein looked at Mark Max plunks work on that whole idea of Kwan ties energy, he was then able to connect it to the photoelectric effect. And for that connection, he earned the Nobel Prize in 1921. In fact, that's the only thing in which he won. That's what he earned the Nobel Prize for none of his work on relativity or any of that other stuff that's probably more famous that equals M M C Square stuff. No, it was his work in the photoelectric effect. And so I'm gonna get to this equation a second. But I should be familiar. It's like this diagram over here. What we've got going on here is we're shining a particular light source on this material, and it did nothing. And so classical model says, increase intensive. So maybe double the amount of energy of light that's hitting the material, and it turns out that didn't seem to do much now following experimentation. Instead of increasing the intensity, they increase the frequency. And when they did that, they notice electrons actually started to eject from the material, and they ejected with some amount of energy to them. So they say, Okay, well, what happens now when I increase the energy intensity, remember, increasing frequency caused the actual emission of electrons. So what happens if I increase the intensity at that same frequency? Well, it doesn't change the ability to eject the electrons. It doesn't seem to change the energy in which the electrons have once they've been ejected . But it does increase the number of photo electron e mission continuing. If we increase the frequency even more, we're gonna have electrons emitted, just like we did prior. But now these electrons will become emitted with higher energy values. And once again, if I increase the intensity, we're not gonna increase the energy of each individual photo electron nor the ability to eject the electrons. But we will increase the number of electrons that we are injecting. I'll come back to that idea in a minute. And so Einstein took plunks equation on quantities. Energy level E equals age F and he basically said that That same thing is true for the photoelectric effect. Not only that, this goes and flips everything around on physics. This one particular experiment calls light a particle. Mind you throughout most of the 18 hundreds and prior science was showing that light behaves like a wave. Don't forget about Thomas Young's experiment about that whole diffraction and interference pattern that takes place when Einstein comes in and says, No, really, What is happening is this light itself carries energy in discrete quantities packets that he calls photons and light is really a bundle of photons, each carrying a certain amount of energy based on their frequency. And so this does imply this experiment that Israel, not just some thought, does imply. Light actually behaves like a particle, which is weird and confusing, and it will lead to future impact that we will discuss. Let's break down the foot electric effect a little bit more in terms of an equation, and this is an equation that you will need to master. It says that the amount of energy that we put into the materials this would be the incident photons with light that is about to hit the material. It will carry energy based on its frequency. H f. So if I have blue light, we're gonna use the frequency for blue multiplied by Planck's Constant. We have the incident energy or the input energy We then and I'm gonna skip this for a second. We look at this thing called the work function Greek symbol Fi. This work function represents the amount of energy required to simply eject the electron. So if we give it less than the work function, the electron doesn't eject at all. If the input energy is equal to the work function, the electron will will leave the metal, but at a very low speed or low kinetic energy. And as we continue to give mawr Maurin put energy. So long as we've met that work function, we will now have electrons moving with certain kinetic energy. So this is really a conservation of energy equation. This is the input side and this is the output side. And so if we give it more than it needs, it will absorb that energy in the form of kinetic energy. If we look at a Catholic tube, this is really how cathode Ray works. Energy comes in a photons of some certain frequency and therefore energy. H f strikes the metal. If that energy is enough to match or exceed the work function, electrons will eject and they will seek out a point of higher potential. Remember electrons or low potential? The Morrell energy that falls incident on the metal, the higher the energy each of these electrons on average, will receive. Now, I'm gonna come back to this diagram in a second. We could graph this if you look over here to the right as we increase the frequency that is emitted once we get to the work function. So that's this right here. By the way, that work function is equal to H f, not F not stands for the threshold frequency or basically the smallest frequency of light required to get photo electron emission. So, um, we are shining a particular lightless AIDS infrared on the metal. We're getting no emission. We increase that to red light. No emission. And I'm making the frequencies up right now. Then maybe we get to green light. Now we're at Green Light. We have now reached the threshold frequency, and we're gonna now get electron emission as we continue to increase the frequency of light . So instead of being read, maybe it's green. And then if we keep cranking it up instead of it being green, it's blue. We can even keep going toe violet or even ultraviolet or higher. If we keep increasing the frequency of the incident photons. The electron will now just move, but with higher and higher kinetic energies. So I want to take a look at this, and I want to talk about this and a little bit greater depth right here. Let's understand. Increasing frequency will not increase the number of the electrons that we admit. Increasing frequency will increase the energy each individual electron receives. We look at this equation H f equals K E max plus five. We're using the max here for a reason, because not every single electron will receive that same amount of energy. I like to think of it as the electrons are embedded in the material right, and as a faux thanh falls on it, the electrons that are closer to the surface, the material or more freely available, they will easily reject, and so they might get that maximum kinetic energy. Some of these other electrons that are a little bit more tightly bound or deeper within the material they're getting reject, but with less kinetic energy. So we more or less look at either the average energy of the particles or, in this equation, the maximum energy the particles understand. They don't all necessarily have the same energy. And so, by increasing frequency, we're gonna increase the kinetic energy of each individual electron being admitted. But we will not increase the quantity of electron emission to increase the quantity of electron emission. That's where we need to increase the intensity. Intensity is related to amplitude, which is related to frequency and brightness. By increasing the intensity, we're not increasing the amount of energy each individual photons carry. So I'm gonna refer to that as photon this particular photons, or bundle of photons as energy of HF. If I increase the intensity, I'm not increasing this HF. Instead, I'm just increasing the number of photons. I like to think of the photons little bullets. So by increasing the intensity, I'm increasing the number of bullets that air hitting the material. I will then be able to increase the number of electrons being ejected. Honest summarizes in the next slide here is we make sure we get it all. First photo lecture effect shows that light sometimes can act like a particle definition. When you shine light on certain metals, they give off certain photo electrons. Now, here's where things got a little mucky When you bright in the light or the incident energy , you're gonna get mawr photo electrons, but you're not going to change their energy. When you increase the frequency of the light, you will increase the number. I'm sorry, the energy of the photo electrons, but you will not change the number of photo electrons. Lastly, that threshold frequency bit. I just want to make sure you understand that if the incident energy is below the threshold frequency, you will get no emission of electrons at all. When it's above the threshold frequency, you do get a mission. Photoelectric effect actually explains more or less how photovoltaic cells work, like thes solar panels that are powering your calculators or that power plenty of businesses and homes. It's also can explain how infrared goggles can work. If you want to try to see something in dark, the infrared goggles are actually seeing the infrared particles. What does that mean? Well, a body that's in a dark room will still be emitting infrared heat. They're warm. We can't normally see infrared radiation, so that person will appear to not be there cause it's a dark room. We were infrared goggles. The material in the goggles or photo sensitive to infrared particles settles. Infrared particles are above the threshold frequency of the material in the goggles. And then once that those goggles start receiving a photo electric emission, we have a similar effect here we have a circuit, and based on the current flowing through the circuit, it will more or less enhance the image so you can see it in the dark. Last thing I want to talk about. And that's why I came back to the slide. Is this thing called a stopping potential? Uh, remember that whole electric potential can also be related to energy quandary. Um, understand that if you increase if I were to reverse the polls on this battery, So instead of this being the positive potential here, if I make this negative, that if I make this the negative side and that's the positive side. The electrons don't want to go to the negative plate. If I increase that negative to a specific amount, I will get to the, uh, spot where the amount of energy getting put in will equal the amount of energy of the electrons. So I'm gonna redraw this over here. Negative plate, positive plate. It's positive because electrons have left it. So these photo electrons wanted travel so left because they have energy. But we now have a negative plate that basically slowing the electron down. You could get to a scenario where these electrons are moving really slowly or even kind of just chilling here. That energy that we put in is what we'll call the stopping potential. And thats stopping potential is equal to energy overcharge. So if we can measure that voltage in a lab, we know how much energy each unit of charge carried, and that's how we're able to revolt. Ultimately measure the threshold frequency of the material itself. Okay, photo logic effect. Pretty profound. Pretty big experiment at blew open physics a lot more than what I'm alluding to here. It helped confirm plunks idea of quantities energy levels. It showed that light actually can behave like a particle. When we always thought it was a wave and more or less, it helped cement the idea of quantum physics that will continue to grow as I advanced through these videos. Thank you. 3. Modern Physics The Photoelectric Effect AP Physics (College Physics) Part 2: when Einstein comes in and says, No, really, What is happening is this light itself carries energy in discrete quantities, packets that he calls photons and light is really a bundle of photons, each carrying a certain amount of energy based on their frequency. And so this does imply this experiment that Israel, not just some thought, does imply. Light actually behaves like a particle, which is weird and confusing, and it will lead to future impact that we will discuss. Let's break down the foot electric effect a little bit more in terms of an equation, and this is an equation that you will need to master. It says that the amount of energy that we put into the materials this would be the incident photons with light that is about to hit the material. It will carry energy based on its frequency. H f. So if I have blue light, we're going to use the frequency for blue multiplied by Planck's Constant. We have the incident energy or the input energy we then, and I'm gonna skip this for a second. We look at this thing called the work function Greek symbol Fi. This work function represents the amount of energy required to simply eject the electron. So if we give it less than the work function, the electron doesn't eject at all. If the input energy is equal to the work function, the electron will will leave the metal, but at a very low speed or low kinetic energy. And as we continue to give mawr Maurin put energy. So long as we've met that work function, we will now have electrons moving with certain kinetic energy. So this is really a conservation of energy equation. This is the input side and this is the output side. And so if we give it more than it needs, it will absorb that energy in the form of kinetic energy. If we look at a Catholic tube, this is really how cathode ray works. Energy comes in a photons of some certain frequency and therefore energy. H f strikes the metal. If that energy is enough to match or exceed the work function, electrons will eject and they will seek out a point of higher potential. Remember electrons or low potential? The Morrell energy that falls incident on the metal, the higher the energy, each of these electrons on average will receive. Now, I'm gonna come back to this diagram in a second. We could graph this if you look over here to the right as we increase the frequency that is emitted once we get to the work function. So that's this right here. By the way, that work function is equal toe h f not f not stands for the threshold frequency or basically the smallest frequency of light required to get photo electron emission. So, um, we are shining a particularly lightless AIDS infrared on the metal. We're getting no emission. We increase that to red light, No emission. And I'm making the frequencies up right now. Then maybe we get to green light. Now we're at Green Light. We have now reached the threshold frequency, and we're going to now get electron emission as we continue to increase the frequency of light. So instead of it being read, maybe it's green. And then if we keep cranking it up instead of it being green, it's blue. We can even keep going toe violet or even ultraviolet or higher. If we keep increasing the frequency of the incident, photons. The electron will now just move. But with higher and higher kinetic energies. So I want to take a look at this, and I want to talk about this and a little bit greater depth right here. Let's understand. Increasing frequency will not increase the number of the electrons that we admit. Increasing frequency will increase the energy each individual electron receives. We look at this equation H f equals K E max plus five. We're using Max here for a reason because not every single electron will receive that same amount of energy. I like to think of it as the electrons are embedded in the material, right, and as a faux thanh falls on it, the electrons that are closer to the surface, the material or more freely available they will easily reject. And so they might get that maximum kinetic energy. Some of these other electrons that are a little bit more tightly bound or deeper within the material they're getting reject, but with less kinetic energy. So we more or less look at either the average energy of the particles or, in this equation, the maximum energy the particles understand. They don't all necessarily have that same energy. And so, by increasing frequency we're gonna increase the kinetic energy of each individual electron being admitted. But we will not increase the quantity of electron emission to increase the quantity of electron emission. That's where we need to increase the intensity. Intensity is related to amplitude, which is related to frequency and brightness. By increasing the intensity, we're not increasing the amount of energy each individual photons carry. So I'm gonna refer to that as photon this particular photons, or bundle of photons as energy of HF. If I increase the intensity, I'm not increasing this HF. Instead, I'm just increasing the number of photons. I like to think of the photons little bullets. So by increasing the intensity, I'm increasing the number of bullets that air hitting the material. I will then be able to increase the number of electrons being ejected. Honest summarizes in the next slide Here is we make sure we get it all. First photo electric fact shows that light sometimes can act like a particle definition. When you shine light on certain metals, they give off certain photo electrons. Now here's where things got a little mucky. When you bright in the light or the incident energy, you're gonna get mawr photo electrons, but you're not going to change their energy. When you increase the frequency of the light, you will increase the number. I'm sorry, the energy of the photo electrons, but you will not change the number of photo electrons. Lastly, that threshold frequency bit. I just want to make sure you understand that if the incident energy is below the threshold frequency, you will get no emission of electrons at all. When it's above the threshold frequency, you do get a mission. Photoelectric effect actually explains more or less how photovoltaic cells work, like thes solar panels that are powering your calculators or that power plenty of businesses and homes. It's also can explain how infrared goggles can work. If you want to try to see something in dark, the infrared goggles are actually seeing the infrared particles. What does that mean? Well, a body that's in a dark room will still be emitting infrared heat. They're warm. We can't normally see infrared radiation, so that person will appear to not be there cause it's a dark room. We were infrared goggles. The material in the goggles or photo sensitive to infrared particles, settles infrared particles are above the threshold frequency of the material in the goggles . And then once that those goggles start receiving a photo electric emission, we have a similar fact here we have a circuit, and based on the current flowing through the circuit, it will more or less enhance the image so you can see it in the dark. Last thing I want to talk about. And that's why I came back to the slide. Is this thing called a stopping potential? Uh, remember that whole electric potential can also be related to energy quandary. Um, understand that if you increase if I were to reverse the polls on this battery So instead of this being the positive potential here, if I make this negative that if I make this the negative side on this the positive side, uh, electrons don't want to go to the negative plate. If I increase that negative to a specific amount, I will get to the ah spot where the amount of energy getting put in will equal the amount of energy of the electrons. So I'm gonna redraw this over here negative plate, positive plate. It's positive because electrons have left it so these photo electrons wanted travel till left because they have energy. But we now have a negative plate that basically slowing the electron down. You could get to a scenario where these electrons are moving really slowly or even kind of just chilling here. That energy that we put in is what we'll call the stopping potential. And thats stopping potential is equal to energy overcharge. So if we can measure that voltage in a lab, we know how much energy each unit of charge carried. And that's how we're able to revolt. Ultimately measure the threshold frequency of the material itself. Okay, photo logic effect. Pretty profound. Pretty big experiment at blew open physics a lot more than what I'm alluding to year. It helped confirm plunks idea of quantities energy levels. It showed that light actually can behave like a particle. When we always thought it was a wave and more or less, it helped cement the idea of quantum physics that will continue to grow as I advanced through these videos. Thank you