Aditya Abeysinghe Presentations

In this note I will present details only about the npn transistor. For those who are interested only on the pnp transistor, you can switch the action of the npn transistor and the behavior of current to adjust with the characteristics of the pnp transistor. (Photo credit: mahasona10000.blogspot.com ) As in the case of a p-n junction,the two n parts of the npn transistor contain an excess of free electrons. In contrast, the p part contains excess holes. As in the case of the p-n junction, in the npn transistor depletion regions develop and junction barriers occur. (Photo credit: daenotes.com) For the correct functioning of the transistor, the first p-n junction is forward biased and the second p-n junction is reverse biased. This results for the first p-n junction to be of low resistance and the second p-n junction to be of high resistance. (Photo credit: www.nzart.org.nz ) The letters of t...

What is a gold leaf electroscope? Gold leaf electroscope is an instrument for detecting and measuring static electricity or voltage. The following diagram shows a modern gold leaf electroscope: Why is gold used to make such an electroscope? Gold is popularly used to make such electroscopes since gold is even sensitive to very minute charges (This is achieved by the malleability of gold - hammering into sheet form and making the mass thin). When we introduce a charged object to the disk of the electroscope ( explained below) we practically do not know the magnitude of the charge. Thus a simple conductor may not be the best option to show a reasonable deflection or response even to minute charges. Thus gold is assumed to be sensitive to charges of any magnitude which makes it more suitable as the indicator.  Besides gold is also a non corrosive metal. We practically do not use anhydrous materials inside the glass case. The penetration...

What is Lenz's law? An electric current induced by a changing magnetic field will flow such that it will create its own magnetic field that opposes the magnetic field that created it. Lenz's law is sometimes similar to the Faraday's law in that both laws discuss about induction. However, Lenz's law clearly demonstrates the direction of the induced current as opposed to the theoretical observation by the Faraday's law. To understand Lenz's law, consider the system below: (Photo credit: www.pinkmonkey.com) Before explaining this system using Lenz's law, let us remind about the direction of the field lines around the magnet. Now let us consider the above system in detail. Consider Fig 1(a): When the north pole is brought close to the magnet, the field lines will be as follows:  As Lenz's law states ' an electric current induced by a changing magnetic field will fl...

What is electromagnetic induction? When an alternative current is allowed to flow through a certain circuit and that circuit is kept near a neutral circuit, the former circuit induces charge distributions in the latter circuit resulting in an induced current in the latter circuit. This phenomenon is called the electromagnetic induction. Thus a potential difference arises across the circuit when exposed to a varying magnetic field. Faraday's law- Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil.  (Photo credit: gic-edu.com) Thus, fluctuating magnetic fields cause currents to flow in conductors placed within them. This is called induction because there is no physical connection between the conductor and the magnet. The current is said to be induced in the conductor by the magnetic field. In order to produce the maximum force needed for induction, usually the cond...

What is electromagnetic shielding? Electromagnetic induction is the process of reducing the electromagnetic field of space. We know that electromagnetic waves are a combination of any two electric and magnetic waves held perpendicularly. Similarly, electromagnetic radiation is a combination of electric and magnetic fields.   (Photo credit: www2.astro.psu.edu) Let's study these two systems separately. Electric fields produces forces on the charge carriers within the conductor. When an electric field is applied to a certain conductor, it produces a current that causes the charges to move inside the conductor. (Photo credit: www.learnemc.com ) Magnetic fields produces eddy currents. These eddy currents behave so as to cancel the applied magnetic field. (Photo credit: www.mdpi.com) All these result in the reflectance of the electromagnetic radiation from the surface of the conductor. Because there are...

Lucas reagent is a solution of anhydrous zinc chloride in concentrated hydrochloric acid and is used primarily to classify alcohols of low molecular weight. The result is observed by any change in the turbulence of the medium. This turbulence is due to the presence of a chloroalkane in the solution. Alcohols are mainly of three types: 1. Primary alcohols 2. Secondary alcohols 3. Tertiary alcohols Furthermore any alcohol can be distinguished from other organic compounds from the molecular formula and the presence of a hydroxyl (OH) group. Primary alcohols- Primary alcohols contain the functional group -CH 2 OH. Examples of primary alcohols are ethanol and butanol.         Ethanol Secondary alcohols- Secondary alcohols contain the functional group -CHROH.  Here R = CH 3 . Examples of secondary alcohols are Propan - 2 - ol and Butan -2-ol.   Propan - 2- ol Tertiary alcohols -  Tertiary ...

What is Brady's reagent? A solution of 2,4-dinitrophenylhydrazine in a mixture of methanol and sulphuric acid is known as Brady's reagent. Brady's reagent is used to test for the presence of aldehydes and ketones in a given sample. 2,4-dinitrophenylhydrazine is the following molecule: (Photo credit: coleparmer.com) Experiment: Add a few drops of the selected sample of aldehyde or ketone to the Brady's solution. After some time a bright orange or yellow precipitate should be formed signifying the presence of the Carbon Oxygen double bond. Reaction: (Photo credit: wwwchem.uwimona.edu.jm) As shown in the above diagram, the carbon- oxygen double bond of the aldehyde or the ketone breaks releasing the oxygen atom. Similarly, two hydrogen atoms from the 2,4 DNP breaks down and the remaining molecule from the aldehyde and the remaining portion of the 2,4 DNP forms the final compound. The remaining oxygen and the hydrogen at...

In 1879, Edwin Hall found that an e.m.f. (electro motive force) or voltage is created transversely or across a current -carrying conductor when a perpendicular magnetic field is applied. This is popularly called the Hall effect. What happens during hall effect? The Hall effect is due to the nature of the current in a conductor. Current consists of the movement of many small charge carriers, typically electrons, holes, ions. To understand Hall effect let us take a slab of metal carrying a current. The flow of electrons is always in a direction so as to oppose the convectional current flow.  Now we will apply a magnetic field B such that B is perpendicular to the face PQRS. When an electric field is applied perpendicular to the magnetic field, then according to the right hand rule, a force acts on each electron in the direction SR - PQ. Thus electrons collect along the side PQ and this makes PQ more negat...