Define brifly the following terms killogram, Kelvin,mole,candela, Ampere
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Answer:
The kilogram (kg) — the fundamental unit of mass in the International System of Units (SI) — was defined as exactly equal to the mass of a small polished cylinder, cast in 1879 of platinum and iridium. Kept in a triple-locked vault on the outskirts of Paris, the platinum-iridium cylinder was officially called the International Prototype of the Kilogram (IPK). It even had a nickname: Le Grand K (The Big K). The accuracy of every measurement of mass or weight worldwide, whether in pounds and ounces or milligrams and metric tons, depended on how closely the reference masses used in those measurements could be linked to the mass of the IPK. That situation has changed radically. In November 2018, the international scientific community voted to redefine the kilogram, freeing it from its embodiment in one golf-ball-sized artifact, and basing it instead on a constant of nature. That transformation was as profound as any in the history of measurement.
The kelvin (K) — the SI unit of temperature — now has a radically new definition. The Kelvin temperature scale — named for the celebrated British physicist Lord Kelvin (1824–1907) — rarely makes an appearance. People are more familiar with the Fahrenheit and Celsius scales, which are used for most practical temperature measurements, such as in weather forecasts, food preparation, manufacturing, etc. Historically, both scales center around defined points such as the melting point of ice, the temperature of the human body or the boiling point of water. The kelvin unit is not expressed in degrees like Celsius or Fahrenheit are. It is used by itself to describe temperature. For example, “mercury loses all electrical resistance at a temperature of 4.2 kelvins.” A change of one kelvin is the same amount of temperature change as one degree Celsius, but the Kelvin scale is “absolute” in the sense that it starts at absolute zero, or what Kelvin and other scientists called “infinite cold.” (0 K = -273.15 degrees C = -459.67 degrees F. Room temperature is about 70 degrees F, 21 degrees C or 294 K.)
The mole (mol) is an amount unit similar to familiar units like pair, dozen, gross, etc. It provides a specific measure of the number of atoms or molecules in a bulk sample of matter. A mole is defined as the amount of substance containing the same number of discrete entities (atoms, molecules, ions, etc.) as the number of atoms in a sample of pure 12C weighing exactly 12 g. One Latin connotation for the word “mole” is “large mass” or “bulk,” which is consistent with its use as the name for this unit. The mole provides a link between an easily measured macroscopic property, bulk mass, and an extremely important fundamental property, number of atoms, molecules, and so forth. The number of entities composing a mole has been experimentally determined to be 6.02214179
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, a fundamental constant named Avogadro’s number (NA) or the Avogadro constant in honor of Italian scientist Amedeo Avogadro. This constant is properly reported with an explicit unit of “per mole,” a conveniently rounded version being 6.022
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/mol.
The Candela (cd), unit of luminous intensity in the International System of Units (SI), defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 1012 hertz and has a radiant intensity in that same direction of 1/683 watt per steradian (unit solid angle). The candela has replaced the standard candle or lamp as a unit of luminous intensity in calculations involving artificial lighting and is sometimes called the “new candle.”
The ampere (A), the SI base unit of electric current, is a familiar and indispensable quantity in everyday life. It is used to specify the flow of electricity in hair dryers (15 amps for an 1,800-watt model), extension cords (typically 1 to 20 amps), home circuit breakers (15 to 20 amps for a single line), arc welding (up to around 200 amps) and more. In November 2018, however, the redefinition of the ampere ― along with three other SI base units: the kilogram (mass), kelvin (temperature) and mole (amount of substance) ― was approved. Starting on May 20, 2019, the ampere is based on a fundamental physical constant: the elementary charge (e), which is the amount of electric charge in a single electron (negative) or proton (positive). The ampere is a measure of the amount of electric charge in motion per unit time ― that is, electric current. But the quantity of electric charge by itself, whether in motion or not, is expressed by another SI unit, the coulomb (C). One coulomb is equal to about 6.241 x 1018 electric charges (e). One ampere is the current in which one coulomb of charge travels across a given point in 1 second.