Light Emitting Diode (LED)

When a p-n junction is forward biased, free electrons move through the junction and recombine with holes. The energy of a free electron (E₁) is greater than that of a bonded electron (E₂).

Thus it should be clear that during the recombining process, some energy is released. In a usual silicon diode, this energy is given out as heat.

However when we change the manufactured material, this energy in some occasions is converted or given out as light. When a free electron recombines with a hole in such materials, the light energy is released as photons.

The energy of such a photon is given by E₁ - E₂ = hc / λ. Here E₁ - energy of a free electron, E₂ - Energy of a bonded or recombined electron, h - Planck's constant, c- speed of light and λ - wave length.

(Photo credit: www.ucsusa.org)

Various colored light can be obtained by using various compounds to manufacture the semiconductor materials. The color of such LEDs depends upon the conductive substance used, the chemical composition of the substance and the doping concentration.

For example, Indium gallium nitride (InGaN) is used to make up the blue, white, true green, and UV types.

Aluminum gallium indium phosphide (AlGaInP or AlInGaP) is used to make the red, yellow, and orange types.

For easy recognition, the anode in any LED is produced to be longer than that of the cathode.

Usually a current of 10 mA and a forward biased voltage of 1.5v - 2.5v is necessary to light such a diode. If a LED is connected to a circuit that is provided with a higher voltage than the value specified above, a safety resistor (R_s) should be connected to the circuit.

(Photo credit : Wikipedia)

If the supplied/input voltage is V_s, the potential difference across the LED is V_d and the current that flows through the LED is I_s, then from Ohm's law

V_s - V_d = I_s R_s

Action of a transistor

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 these elements

Gold leaf electroscope

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 of

The Silver Mirror Test

What is the Silver Mirror Test? Silver Mirror Test is a test that is used to differentiate between aldose and ketose sugars using the Tollen's reagent. Tollen's reagent is an alkaline solution of ammoniacal silver nitrate and is used to test for aldehydes. *Ketones do not react with the Tollen's reagent. Silver ions in the presence of hydroxide ions form a brown precipitate of silver (I) oxide, Ag 2 O (s) . Ag 2 O This precipitate dissolves in aqueous ammonia, forming the diamminesilver(I) ion, [Ag(NH 3 ) 2 ] + . The reactions that take place in this experiment are as follows: 2Ag + (aq) + 2OH - (aq) → Ag 2 O (s) + H 2 O (l) Ag 2 O (s) + 4NH 3 (aq) + H 2 O (l) → 2[Ag(NH 3 ) 2 ] + (aq) + 2OH - (aq) What happens if Silver Nitrate is used without ammonia or why is ammonia used in this experiment? When ammonia is added to the silver nitrate solution, silver ions are less prone to reduction. Thus, silver is produce

Electromagnetic Induction

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

Flame test- By Aditya Abeysinghe

Introduction: Flame test is an experiment used to identify/distinguish metals using colors exposed to a flame. Since metallic halides easily evaporate, the test is usually done using the halide components of metals. Out of the halides, chlorides are frequently used because chlorides give a fast change of color when exposed to the flame as opposed to other halides. Materials needed: · A rod of platinum, nichrome, asbestos or graphite · Concentrated HCl solution · Bunsen burner · Metallic salt (a chloride of a metal) · A cobalt glass (if necessary) Method: Insert the rod of platinum, nichrome, asbestos or graphite into a concentrated solution of HCl. Hold the rod to a Bunsen flame. (to the colorless region) Continue this experiment until the top region of the flame turns colorless. Form a pulp by mixing the metallic salt with the concentrated HCl solution and apply

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