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[MUSIC PLAYING] PROFESSOR: Hello, everyone. Today we'll be taking a look at how light interacts with the surface of a solar cell. Right now I'm standing next to a solar module made up of individual silicon solar cells. If you look closely, these cells actually appear black. And they appear black for a very important reason. Solar engineers work very hard to make their solar cells as efficient as possible. Reflected light is lost energy, so good engineers will want to minimize the total amount of reflected light. To make solar cells absorb as much light as possible, and appear black, solar engineers do two things. First, they grow this very thin film of a dielectric layer on the surface. This layer is aptly called an anti-reflection coating. Second, they texture the wafer. And today we'll demonstrate how texturing is performed, and quantify its enhancement for reducing light reflection. Silicon wafers don't start out black. In fact, they appear gray. Polish...

[MUSIC PLAYING] PROFESSOR: Hello everyone, today we're going to learn how a Solar Cell is able to turn light generated mobile charges into electricity. Today's lesson will use everything we've learned in the past videos to understand this effect. So, make sure you understand the material from the previous videos before watching. First, let's go over the structure of a Solar Cell. Here's a cell that I made. And we can see that a metal ribbon is connected to the top metal contacts, which form a grid. The spaces between the grid lines allow light to enter the cell. If we flip over the cell we see the entire back surface is coated with metal, which allows easy extraction of charge from the back surface. Additionally, we have another metal ribbon that's connected to the backside. Now, let's hook up our Solar Cell to an ammeter to measure the current. So, here we have an ammeter connected to our Solar Cell and our light source which will simulate the...

[MUSIC PLAYING] PROFESSOR: Hello, everyone. Today we'll talk about doping, which is the process of intentionally adding impurities to a semiconductor in order to change its electrical properties. Doping is a critical process in the tech world. It's used in manufacturing almost all semiconductor technologies today. Without doping, the solar industry would not exist, but even though doping is common today, the effects of impurities confused semiconductor physicists in the 1950s, who had trouble reproducing results. Eventually, they realized that contamination levels, as low as 1 in a billion, were vastly changing the electrical properties of their samples. Today, we'll show you how this works with a very simple experiment. We'll be measuring the electrical conductivity of two silicon slabs using an ohmmeter. One is doped with impurities, phosphorus in our case, and the other is ultra-pure, or what we call intrinsic. Let's go over our experiment. We'l...

DAVID JORDAN: Hello, and welcome back to recitation. Today the problem I'd like to work with you is about computing partial derivatives and the total differential. So we have a function z which is x squared plus y squared. So it depends on the two variables x and y. Now the variables x and y themselves depend on two auxiliary variables, u and v. So that's the setup that we have. So in part a, we just want to compute the total differential dz in terms of dx and dy. So u and v aren't going to enter into the picture. And then in part b, we're going to compute the partial derivative partial z partial u in two different ways. First, we're going to compute it using the chain rule. And then we're going to compute it using total differentials. And so we'll substitute in some of the work that we had in a to solve that part. So why don't you pause the video now and work on the problem. We'll check back and we'll do it together. Hi, and welcom...

DAVID JORDAN: Hello, and welcome back to recitation. So today, the problem I'd like to work with you is about taking partial derivatives in the presence of constraints. So this is a pretty subtle business. So take your time when you work these problems. So what we have is we have this function w, and it's a function of four variables: x, y, z, and t. OK? But it's not really a function of these four variables because we have a constraint. So we want to study how w changes as we vary the parameters, except that we have imposed this constraint here. So that really we kind of only have three variables, because we have four variables and one constraint. So that's what partial derivatives with constraints help us do. So let's explain first the notation. OK? So it says partial w partial z, and then we have the subscripts x and y. So what's important about this notation is not what you see as much as what you don't see. What you don't see is the va...

STUDENT: Hello, everyone. Today, I'm going to talk about a very important model, analyze the energy band of a crystal, that is the tight binding model. Before I talk about tight binding model, let's now take a look at free electron model. The potential energy of free electron can be considered as zero, so the Hamiltonian of the free electron system is without the potential term. And the waveform function can be simply written as a plane wave. By substituting the wave function into Schrodinger equation, we can gather energy dispersion relation, which is a parabola in one dimension case. And we usually form the parabola into the first Brillouin zone. And this is the reduced Brillouin zone. And in this figure, we can see that the first band is at the bottom of the parabola. And the second band and third band, fourth, and so on. Similar to one dimension case, the energy of three dimension case is proportional to kx squared plus ky squared plus kz squared. For a given ...

NICHOLAS BURNAND: Hello and welcome to this short video on Mohr's circle. The goal of this video is to show you how Mohr's circle works, how they could be useful for you, and to make some interesting observations as well. So first of all, Mohr's circle is a graphical way to find the principle normal stresses and the maximal shear stress of a given stress state, which could be the one right here, where you have some normal forces acting on sigma x, y, and z, and also some different shear stresses-- the tau's you have here. Your stress state could also be shown using a tensor-- stress tensor-- which is shown right here by this symmetric 3 by 3 matrix. It has to be symmetric because we don't want to have any movement acting on our stress element to prevent any rotation. And also of course, we're using an isotropic material. So I'm going to start with the 2D Mohr's circle-- the simplest one. So now obviously, my stress element is a square. We h...

ALEXIS GERVAIX: Hello everyone, and welcome to this little video presenting you my project on Mathematica. My name is Alex. And I'm currently studying material science in EPFL in Switzerland. And we had this personal challenge to do something you wanted with Mathematica. So I took an interest into crystallography, because this is a quite essential field of material science. Well, actually, it was on the request of my girlfriend, who has quite trouble to see things in 3D that I did this. And my idea was to create a tool that could be used by anyone that doesn't especially know about Mathematica, and that, without coding anything, he could create a couple of structures, have a look at the-- have an interactive part with it, and choose the atom, et cetera, and visualize more easily these structures. So let's have a look first at a bit of theory, and first with the Bravais lattice. So Bravais lattice is one of the fundamentals of crystallography. It defines the di...

[SQUEAKING] [RUSTLING] [CLICKING] CHARLENE: Hello, everyone. My name is Charlene. THERESE: My name is Therese. CHARLENE: And we're here to present INMUSE, a music capturing game. So you're going for a walk, and you put on Bose Air glasses and turn on your phone, and open up the INMUSE app. As you walk with your Bose Air glasses, you will hear a melody to your right. You turn towards the melody to get a better hearing of it. And if you like it, you double-tap on your glasses to capture the melody. If you feel the melody is not something that you like, you can shake your head and let it go. And by the end of the walk, you will have to capture a series of melodies and compose your own little soundtrack that you experienced during your walk. So here's a little video of a [INAUDIBLE].. [LAUGHTER] AUDIENCE: [INAUDIBLE] AUDIENCE: All right. [LAUGHTER] CHARLENE: [INAUDIBLE] [STEADY BEAT PLAYING] [MUSIC PLAYING] AUDIENCE: Yeah, that was great. CHARLENE: Great, so now w...

Hello Kelly. First of all, a very big thank you for accepting to do this little interview. Thanks to you. Can you tell us about yourself and what you're working on? All right, yes. Thank you very much, Liliane. I am Kelly Picard, I am French, a lecturer at the University Jean-Monnet, near Lyon, in public law. I work more specifically in public law. I've been working for about ten years now on the notions of historical prejudice and transitional justice. We will go back to this later. Specifically, I dedicated my PhD thesis to these questions, namely, the question of the responsibility of the State in the reparation of historical damage. In addition, my research mainly deals with the protection of rights and freedoms, mechanisms of justice and reparation in post-conflict situations, typically, Truth and Reconciliation Commissions, and the different forms of reparation considered in such contexts, which somewhat fall outside the typical legal framework. I also had t...

[MUSIC PLAYING] MIKE TEODORESCU: Hello, and welcome to this module on protected attributes and fairness through unawareness. My name is Mike Teodorescu. I'm an assistant professor of information systems at Boston College, as well as a visiting scholar at MIT D-Lab. What this module will cover will be examples of laws that codify protected attributes, as well as the base case scenario for fairness in machine learning, which is called fairness through unawareness. The use of machine learning presents both risks and opportunities. Machine learning can reduce costs by automating repetitive tasks, but could also increase biases. Certain individual attributes are commonly labeled as protected attributes, as they can be sources of social bias. These are race, religion, national origin, gender, marital status, age, and socioeconomic status. In the United States, discrimination based on these protected attributes in housing, lending, and employment is illegal. Some of the laws...

PROFESSOR: Hello and welcome to the help session on pedigrees. Today, we will be working out a problem together. If you have not yet had a chance to work it on your own, please do so now, and return to this video when you are done. Now that you've had a chance to look at this problem, let's work it out together. The first part of this question asks, what is the mode of inheritance that is observed in this pedigree? So, we know that there are two main types of modes of inheritance. It can either be dominant or recessive. And from there it can either be autosomal or X-linked. If a disease follows a dominant inheritance pattern, generally, it must be present in every generation. So here we notice that the disease is present in the first generation, but it's not present in anyone in the second generation. However, then it reappears in the third generation. This suggests that the disease is recessive. So now, do we think this disease is autosomal or is it X-linked?...