Несколько текстов для зачёта (A little bit of physics)Посмотреть архив целиком
Resistor, component of an electric circuit that resists the flow of direct or alternating electric current. Resistors can limit or divide the current, reduce the voltage, protect an electric circuit, or provide large amounts of heat or light.
An electric current is the movement of charged particles called electrons from one region to another. The amount of resistance to the flow of current that a resistor causes depends on the material it is made of as well as its size and shape. Resistors are usually placed in electric circuits, which are devices formed when current moves through an electrical conductor (a material that allows the current to flow without much resistance, such as copper wire) and when the conductor makes a complete loop.
When a voltage, or electric potential, is applied to opposite ends of a circuit, it causes current to flow through the circuit. As the current flows, it encounters a certain amount of resistance from the conductor and any resistors in the circuit. Each material has a characteristic resistance. For example, wood is a bad conductor because it offers high resistance to the current; copper is a better conductor because it offers less resistance. In any electric circuit, the current in the entire circuit is equal to the voltage across that circuit divided by the resistance of the circuit. Resistors are often made to have a specific value of resistance so that the characteristics of the circuit can be accurately calculated.
Physicists sometimes explain the flow of current through a material, such as a resistor, by comparing it to water flowing through a pipe. A pressure difference maintained across two ends of the pipe by a pump is like the potential difference, or voltage, across a wire maintained by a battery. The rate of flow of water, analogous to the rate of flow of charge (current), depends on the type of pipe used. A long and thin water pipe offers more resistance than a short and thick one or a pipe that has obstructions. Similarly, the resistance of a conductor is dependent upon several factors, including its length, cross section, temperature, and a property called resistivity. Resistivity is an intrinsic characteristic of the material itself defined by the voltage divided by the density of current (current per unit cross section area) flowing across the material.
A material of high resistivity will require a higher electrical field to cause a given current density. If the resistivity of a material is known, as well as its dimensions, it can be used to calculate the resistance of a particular piece of material. The resistivity of a material is also dependent upon temperature. When a material resists the flow of current, it converts the electrical energy into other kinds of energy such as heat and light. This energy causes resistors to heat up and glow when enough current flows through them.
Resistors are designed to have a specific value of resistance. Most resistors used in electric circuits are cylindrical items a few millimeters long with wires at both ends to connect them to the circuit. Resistors are often color coded by three or four color bands that indicate the specific value of resistance. Some resistors obey Ohm’s law, which states that the current density is directly proportional to the electrical field when the temperature is constant. The resistance of a material that follows Ohm’s law is constant, or independent of voltage or current, and the relationship between current and voltage is linear. Modern electronic circuits depend on many devices that deviate from Ohm’s law. In devices such as diodes, the current does not increase linearly with voltage and is different for two directions of current.
Resistors can help divide voltages, and when combined with other elements can help convert voltages for a specific electrical design. Resistors can also be used to provide intense light or heat. For example, the heating element in a household cooking range is a resistor, as is the tungsten filament in a common incandescent lamp. Resistors with adjustable resistance are called rheostats or potentiometers. These types of resistors are used in appliances when the current needs to be adjusted or when the resistance needs to be varied, as with lights that dim or adjustable generators.
Electric Furnace, electrically heated device used industrially for melting metals or firing ceramics. It is also known as an electrothermic furnace.
The simplest type of electric furnace is the resistance furnace, in which heat is generated by passing a current through a resistance element surrounding the furnace or by utilizing the resistance of the material being heated. The heating element in an externally heated furnace may take the form of a coil of metal wire wound around a tube of refractory material or it may be a tube of metal or other resistive material such as carborundum. Resistance furnaces are particularly useful in applications in which a small furnace, with precisely controlled temperatures, is needed. Small resistance furnaces are widely used in laboratories and in shops for the heat treatment of tools. Larger furnaces are used for firing ceramics and melting brass. The highest temperature at which resistance furnaces are operated, for example, in the manufacture of graphite, is in the neighborhood of 4100° C (7366° F).
The electric-arc furnace is the most widely used type of electric furnace for the production of quality alloy steels and range in capacity from 227 kg (500 lb) to 181 metric tons. In these furnaces the heat is generated by an arc struck between the metal being heated and one or more electrodes suspended above the metal. A typical form of arc furnace has three electrodes, fed by a three-phase power supply, giving three heating arcs. The electrodes are made either of graphite or of carbon.
A more recently developed type of electric furnace is the induction furnace, consisting of a crucible in which a metallic charge is heated by eddy currents induced magnetically. Around the crucible is wound a coil through which high-frequency alternating currents are passed. The magnetic field of this coil sets up eddy currents in the metal in the crucible. Induction furnaces have a number of advantages, chief among them being the speed at which metal can be melted. At comparatively low frequencies the induced eddy currents exert a stirring action on the molten metal. Because the higher frequencies are the most effective for heating, some induction furnaces have two coils, one for high-frequency current and one for low-frequency. The earlier types of induction furnaces operated at frequencies between 60 and 60,000 cycles per second, but some modern furnaces are designed to use frequencies of 1 million cycles or more per second.
A special type of furnace, called an electrolytic furnace, is used in the production of aluminum, magnesium, and sodium. In the electrolytic furnace, a salt is fused by the heat generated by the passage of a large electric current and is at the same time electrolyzed so that the pure metal is deposited at one electrode.
Insulation, any material that is a poor conductor of heat or electricity, and that is used to suppress the flow of heat or electricity.
The perfect insulator for electrical applications would be a material that is absolutely nonconducting; such a material does not exist. The materials used as insulators, although they do conduct some electricity, have a resistance to the flow of electric current as much as 2.5 × 1024 greater than that of good electrical conductors such as silver and copper. Materials that are good conductors have a large number of free electrons (electrons not tightly bound to atoms) available to carry the current; good insulators have few such electrons. Some materials such as silicon and germanium, which have a limited number of free electrons, are semiconductors and form the basic material of transistors.
In ordinary electric wiring, plastics are commonly used as insulating sheathing for the wire itself. Very fine wire, such as that used for the winding of coils and transformers, may be insulated with a thin coat of enamel. The internal insulation of electric equipment may be made of mica or glass fibers with a plastic binder. Electronic equipment and transformers may also use a special electrical grade of paper. High-voltage power lines are insulated with units made of porcelain or other ceramic, or of glass.
The specific choice of an insulation material is usually determined by its application. Polyethylene and polystyrene are used in high-frequency applications, and mylar is used for electrical capacitors. Insulators must also be selected according to the maximum temperature they will encounter. Teflon is used in the high-temperature range of 175° to 230° C (350° to 450° F). Adverse mechanical or chemical conditions may call for other materials. Nylon has excellent abrasion resistance, and neoprene, silicone rubber, epoxy polyesters, and polyurethanes can provide protection against chemicals and moisture.
Thermal insulating materials are used to reduce the flow of heat between hot and cold regions. The sheathing often placed around steam and hot-water pipes, for instance, reduces heat loss to the surroundings, and insulation placed in the walls of a refrigerator reduces heat flow into the unit and permits it to stay cold.
Thermal insulation may have to fulfill one or more of three functions: to reduce thermal conduction in the material where heat is transferred by molecular or electronic action; to reduce thermal convection currents, which can be set up in air or liquid spaces; and to reduce radiation heat transfer where thermal energy is transported by electromagnetic waves. Conduction and convection can be suppressed in a vacuum, where radiation becomes the only method of transferring heat. If the surfaces are made highly reflective, radiation can also be reduced. Thus, thin aluminum foil can be used in building walls, and reflecting metal on roofs minimizes the heating effect of the sun. Thermos bottles or Dewar flasks (see Cryogenics) provide insulation through an evacuated double-wall arrangement in which the walls have reflective silver or aluminum coatings. See also Heat Transfer.