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بمب اتمی - Atomic Bomb

Atomic Bomb

Atomic Bomb, powerful explosive nuclear weapon fueled by the splitting, or fission, of the nuclei of specific isotopes of uranium or plutonium in a chain reaction. The strength of the explosion created by an atomic bomb is on the order of the strength of the explosion that would be created by thousands of tons of TNT (see Trinitrotoluene).

An atomic bomb must provide enough mass of plutonium or uranium to reach critical mass, the mass at which the nuclear reactions going on inside the material can make up for the neutrons leaving the material through its outside surface. Usually the plutonium or uranium in a bomb is separated into parts so that critical mass is not reached until the bomb is set to explode. At that point, a set of chemical explosives or some other mechanism drives all the different pieces of uranium or plutonium together to produce a critical mass. After this occurs, there are enough neutrons bouncing around in the material to create a chain reaction of fissions. In the fission reactions, collisions between neutrons and uranium or plutonium atoms cause the atoms to split into pairs of nuclear fragments, releasing energy and more neutrons. Once the reactions begin, the neutrons released by each reaction hit other atoms and create more fission reactions until all the fissile material is exhausted or scattered

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فر مایکروویو - Microwave

Microwave Oven

Microwave Oven, appliance that uses electromagnetic energy to heat and cook foods. A microwave oven uses microwaves, very short radio waves commonly employed in radar and satellite communications. When concentrated within a small space, these waves efficiently heat water and other substances within foods.

In a microwave oven, an electronic vacuum tube known as a magnetron produces an oscillating beam of microwaves. Before passing into the cooking space, the microwaves are sent through a fanlike set of spinning metal blades called a stirrer. The stirrer scatters the microwaves, dispersing them evenly within the oven, where they are absorbed by the food. Within the food the microwaves orient molecules, particularly water molecules, in a specific direction. The oscillating effect produced by the magnetron changes the orientation of the microwaves millions of times per second. The water molecules begin to vibrate as they undergo equally rapid changes in direction. This vibration produces heat, which in turn cooks the food.

Microwaves cook food rapidly and efficiently because, unlike conventional ovens, they heat only the food and not the air or the oven walls. The heat spreads within food by conduction (see Heat Transfer). Microwave ovens tend to cook moist food more quickly than dry foods, because there is more water to absorb the microwaves. However, microwaves cannot penetrate deeply into foods, sometimes making it difficult to cook thicker foods.

Microwaves pass through many types of glass, paper, ceramics, and plastics, making many containers composed of these materials good for holding food; microwave instructions detail exactly which containers are safe for microwave use. Metal containers are particularly unsuitable because they reflect microwaves and prevent food from cooking. Metal objects may also reflect microwaves back into the magnetron and cause damage. The door of the oven should always be securely closed and properly sealed to prevent escape of microwaves. Leakage of microwaves affects cooking efficiency and can pose a health hazard to anyone near the oven.

The discovery that microwaves could cook food was accidental. In 1945 Percy L. Spencer, a technician at the Raytheon Company, was experimenting with a magnetron designed to produce short radio waves for a radar system. Standing close to the magnetron, he noticed that a candy bar in his pocket melted even though he felt no heat. Raytheon developed this food-heating capacity and introduced the first microwave oven, then called a radar range, in the early 1950s. Although it was slow to catch on at first, the microwave oven has since grown steadily in popularity to its current status as a common household appliance.

Microsoft ® Encarta ® 2008. © 1993-2007 Microsoft Corporation. All rights reserved.

نوترون - Neutron

Neutron

I

INTRODUCTION

Neutron, electrically neutral elementary particle that is part of the nucleus of the atom. Elementary particles are the smallest parts of matter that scientists can isolate. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. See also Atom

Neutrons and protons bind tightly together to create atomic nuclei. The number of protons an atom contains determines which chemical element it is, ranging from 1 proton for hydrogen to 92 for uranium, the largest naturally occurring element. Each atom usually contains about as many neutrons as protons, but different atoms of the same element may have different numbers of neutrons.

Atoms that differ only in the number of neutrons are called isotopes. For example, most atoms of the simplest element, hydrogen, have a nucleus containing only a single proton. In natural hydrogen, however, 0.015 percent of the atoms have a neutron in addition to the proton. This isotope is called heavy hydrogen or deuterium. An element usually has several isotopes, all nearly identical in the way they react chemically with other elements and each other. Scientists can distinguish different isotopes of an element by examining properties of the element’s nuclei, such as the mass of the nucleus.

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پروتون - Proton

Proton

I

INTRODUCTION

Proton, elementary particle that carries a positive electric charge and, along with the electron and the neutron, is one of the building blocks of all atoms. Elementary particles are the smallest parts of matter that scientists can isolate. The proton is one of the few elementary particles that is stable—that is, it can exist by itself for a long period of time. Protons and neutrons are the building blocks of the atomic nucleus, the center of the atom. Electrons form the outer part of the atom. Protons have a positive electrical charge of 1.602 x 10-19 coulomb. This charge is equal but opposite to the negative charge of the electron. Neutrons have no electrical charge. Protons have a mass of 1.67 x 10-27 kg and, along with neutrons, they account for most of the mass in atoms. Atoms contain an equal number of protons and electrons so that every atom has an overall charge of zero.(See also Atom and Electricity)

The number of protons in the nucleus of an atom determines what kind of chemical element it is. All substances in nature are made up of combinations of the 92 different chemical elements, substances that cannot be broken into simpler substances by chemical processes. The atom is the smallest part of a chemical element that still retains the properties of the element. The number of protons in each atom can range from one in the hydrogen atom to 92 in the uranium atom, the heaviest naturally occurring element. (In the laboratory, scientists have created elements with as many as 116 protons in each nucleus.) The atomic number of an element is equal to the number of protons in each atom’s nucleus. The number of electrons in an uncharged atom must be equal to the number of protons, and the arrangement of these electrons determines the chemical properties of the atom.

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الکترون - Electron

Electron

I

INTRODUCTION

Electron, negatively charged particle found in an atom. Electrons, along with neutrons and protons, comprise the basic building blocks of all atoms. The electrons form the outer layer or layers of an atom, while the neutrons and protons make up the nucleus, or core, of the atom. Electrons, neutrons, and protons are elementary particles—that is, they are among the smallest parts of matter that scientists can isolate. The electron carries a negative electric charge of –1.602 x 10-19 coulomb and has a mass of 9.109 x 10-31 kg. See also Atom.

Electrons are responsible for many important physical phenomena, such as electricity and light, and for physical and chemical properties of matter. Electrons form electric currents by flowing in a stream and carrying their negative charge with them. All electrical devices, from flashlights to computers, depend on the movement of electrons. Electrons also are involved in creating light. The electrons in the outer layers of the atom sometimes lose energy, emitting the energy in the form of light. Because electrons form the outer layers of atoms, they are also responsible for many of the physical and chemical properties of the chemical elements. Electrons help determine how atoms of an element behave with respect to each other and how they react with atoms of other elements. See also Chemistry.

II

ELECTRONS AS ELEMENTARY PARTICLES

The electron is one of the most fundamental and most important of elementary particles. The electron is also one of the few elementary particles that is stable, meaning it can exist by itself for a long period of time. Most other elementary particles can exist independently for only a fraction of a second.

Electrons are among the smallest of all elementary particles and have no detectable shape or structure. At the same time, they do have a property that scientists can measure called spin, or intrinsic angular momentum. An electron’s spin makes it act as a tiny magnet. Electrons can spin clockwise or counterclockwise.

The electron is affected by three of the four fundamental forces that define the nature and interaction of everything in the universe: gravitation, the electromagnetic force, and the weak nuclear force. Gravitation is the attractive force between every object in the universe that has mass. Gravitation affects the electron because the electron has mass. The electromagnetic force affects objects with an electric charge, so the electron’s negative electric charge subjects it to the forces of electromagnetism. The electron attracts positively charged particles, such as protons, and repels negatively charged particles, such as other electrons. The electron is also sensitive to the weak nuclear force, a very feeble force that affects certain types of elementary particles and is only important over very short distances. The one fundamental force that does not affect the electron is the strong nuclear force, which is the force that binds protons and neutrons in the atom’s nucleus

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