Devender

Before explaining how Light Emitting Diodes (LEDs) work, let me give a quick introduction on semiconductor. Semiconductors are not good conductors of electricity due to very few free electrons (about 4-6 orders less free electrons than metals). This has to do with chemical bonding in the semiconductors, check this post . But, semiconductors can be manipulated by doping them with foreign elements to increase the number of free electrons. Let us take Silicon (Si) for example, Si has 4 covalent bonds with 4 adjacent Si atoms. If we dope Si with Phosphorous or Arsenic (P and As), we will end up with one extra free electrons as P or As can form 5 bonds. Similarly if we dope Si with either Boron or Alumnium (B and Al), we will end up with a free hole (absence of electrons, these can conduct electricity too) as B and Al can only form 3 bonds. (Source of the image: LINK ) Doping is know as n-type if it results in an extra electron and p-type if an extra hole is created. An electron can

Introduction Solar cell convert solar energy directly into electric energy using a phenomenon known as photoelectric effect and that is why there are also known as Photovoltaics. Photoelectric effect in simple terms is release of electron due to absorption of a Photon. As I have already described in one of the podcast here that electrons reside in specifically defined states by rules of quantum mechanics. So, when a photon strikes an atom, one of the electrons can be knocked loose and can be captured into a circuit using electrode thereby converting solar energy into electric energy. (Source of the gif: http://www.bluffton.edu/courses/TLC/MontelA/Montel/Alternative_Energy_Website/jenaefinal.gif) Science behind Typically, number of solar cells are connected to each other and combination is known as Solar panels. Common solar cells are made from a junction which are engineered in a way that photons energy is greater or equal to the band gap. Band gap is an energy range in which n

First off, our eyes can only detect a small portion of electromagnetic spectrum known as visible region (400-700 nm in wavelength). Our retina has no sensitivity for radio, x-rays and other radiations outside of visible region. (Source of image: http://www.sun.org/encyclopedia/electromagnetic-spectrum) Color by reflection Color in a substance can be produced from various mechanisms, most common of which is the reflection. An object is red because it absorbs all the color except red which is reflected back and detected by retina in our eyes. Grass is green because it produces a pigment name Chlorophyll (used for photosynthesis). Chlorophyll is a stronger absorbed of red and blue light and reflects green color and hence the color of the grass. Color by scattering But reflection is not the only mechanism by which color is produced like color of sky can be explained by a scattering phenomenon knows as "Rayleigh Scattering". Rayleigh found that scattering power of partic

 Our Milky Way has more than 100 billion stars and there are billions of galaxies in the universe. So an obvious question is how it all started?   Postulation Humans have been trying to figure this out for centuries and ironically most accepted theory today was first proposed by a theologian named George Lemaitre in early 1927. It did not gain momentum until an astronomer named Edwin Hubble ( Hubble telescope named after him) found out that Universe is expanding. This led many to theorize that if universe is expanding so it must have been started as tiny and dense point. Hubble was mocked for years and in fact the name "Big Bang Theory" was coined sarcastically by astronomer Fred Hoyle.  Evidence? It was an accidental discovery in 1964 by astronomer Wilson and Penzias that led to second experimental evidence (first being expansion of the universe) for Big Bang Theory. They discovered persistent radio signal in their antennas which is attributed to energy phot

As we know from high school science class, atom consists of a nucleus comprised of neutrons and protons and nucleus is surrounded by electrons. But what are protons, neutrons and electrons made of? Proton is made of two types of elementary particles: one "down quark" and two "up quark". There are six types of quarks. Whereas, neutron is also made of quarks but from two "down quark" and one "up quark". Charge of "up quark" is positive 2/3rd of electron where for "down quark" it's negative 1/3rd of electron. And hence protons are positive and neutrons are neutral. Quarks can not exist as individual particles like electron does, and always found in combination known as hadrons. Protons and neutrons are hadrons. Quarks, are help together by strong interaction via exchange of gluons. Now hadrons can interact with each other by exchanging mesons. And due to this interaction between hadrons via mesons we have protons and neu

We have already discussed in the previous post that a hotter object has atoms with high kinetic energy. If this object is put in contact with a colder object, energetic atoms will start to transfer heat to atoms of colder object and this is phenomenon is known as heat conduction. But,  Sun is about 100 million miles away from the earth and yet we still feel it's heat. Space between earth and sun is vacuum  with no particles to conduct the heat. So, how is heat conducted 100 million miles away to earth without any heat carriers? This occur through the phenomenon known as "heat radiation".  Sun surface is very hot (about 6000 C) and so atoms of gases (hydrogen and helium) in Sun have very high kinetic energy. Electromagnetic radiation (heat) is emitted whenever an electric charge is accelerated.  These energetic atoms have positive charge and negative charge, and constantly bumping into other atoms and thus accelerating and decelerating. So, Sun emits heat in the form of

Let us talk today about how heat conduction occur and why metals are good heat conductors. Heat is energy that can be transferred from hotter to colder body and can be quantified from temperature. But then obvious question is how does this energy transfer takes place? In this discussion, we will only focus on transfer of heat only via conduction and do not take into consideration radiation phenomenon.  At microscopic level, we can quantify heat as motion of atoms. Hotter the body is, more kinetic energy the atoms have (they are shaking vigorously). Now, when you put a hotter body in contact with a colder body, atoms in hotter body which are moving vigorously start to shake the atoms of colder body in contact. This leads to transfer of kinetic energy (vibration of atoms in solids) from hotter body to colder body and thus transfer of heat.  For metals, you also have electrons which are free, so both electrons and atoms will acquire kinetic energy. In fact, both electrons and atom

In the last post , I discussed the difference in bonding between metals and ceramics, and how that is manifested in different properties exhibited by metals and ceramics. Now, let us try to understand why metals actually conduct electricity. The reason again ties down to bonding in metals. Like, I explained here , metals typically exhibit metallic bonding in which all the valence electrons are shared by all the atoms. There is a delocalization of electrons, unlike in covalent and ionic bonds which confine electrons only to neighboring atoms. Now, when an external electric field is applied to metals, these delocalized electrons are easy to move and thus conduct. This simple explanation is basis for Drude Model where Drude actually derives simple quantitative formula for conductivity. This explanation and thus theory is very elementary and is not be able to give correct quantitative numbers for many other properties (heat capacity and seebeck coefficient) of metals. A more detailed th

Recently, I have decided to change the blog to more science oriented. I have started a podcast ( link ) where I have decided to share my understanding of various topics in science. I will try to post 3-4 posts a week on topics related to science. So, for this post let's discuss why certain solids are metals and some ceramics. Solids, in general, can be distinguished as metals, ceramics and polymer. I am keeping the focus on difference between metals and ceramics and not going to discuss polymers in this post. If you checked my last post, matter exist in form of solid due "bonds" which keep atoms together in solid. I discussed three forms of bonds- metallic, covalent and ionic. Metals typically have metallic bonding, which is a delocalized bond where all the valence electrons are shared by each and every atoms. In contrast, covalent and ionic bonds are localized between neighborliness atoms. Atoms in ceramics typically are bound by covalent and ionic bonds. This is the

Why do matter exist as solids? Common forms of matter are solid, liquid and gas. Reason behind existence of matter as any form is governed by interactions between atoms. Electrons surrounding atoms' dictate this interaction. This interaction is what we call as a bond. So, why do atoms interact? Reason for any interaction has to do with decreasing overall energy of the system. There are three possible ways for electrons to interact with electrons of neighboring atoms: - Atom can share electrons with neighboring atom: known as covalent bond - One atom gives away its electron to neighboring atom: known as ionic bond - Atoms share electron with all the whole system, not just surrounding atom: known as metallic bond Like I said before, driving force for atoms interaction is to decrease their energy. Now, this interaction which is termed as bonds is what keep atoms together and forms solid. Solid will fell apart and form gas if there are no bonds. Now, these bonds determine how