Материал: Андрианова English Thesaurus of Student-Physicist 2013

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Generators

Generators and motors are important practical devices that operate on the principle of electromagnetic induction. The alternating-current (AC) generator is a device that converts mechanical energy to electrical energy. In its simplest form, the AC generator consists of a wire loop rotated in a magnetic field by some external means. In commercial power plants, the energy required to rotate the loop can be derived from a variety of sources. For example, in a hydroelectric plant, falling water directed against the blades of a turbine produces the rotary motion; in a coal-fired plant, heat produced by burning coal is used to convert water to steam, and this steam is directed against the turbine blades. As the loop rotates, the magnetic flux through it changes with time, inducing an emf and a current in an external circuit. The ends of the loop are connected to slip rings that rotate with the loop.

The components of the direct current (DC) generator are essentially the same as those of the AC generator, except that the contacts to the rotating loop are made by a split ring, or commutator. A pulsating DC current is not suitable for most applications. To produce a steady DC current, commercial DC generators use many loops and commutators distributed around the axis of rotation so that the sinusoidal pulses from the loops overlap in phase. When these pulses are superimposed, the DC output is almost free of fluctuations.

Self-induction

When the switch is closed, the current doesn't immediately change from zero to its maximum value. The law of electromagnetic induction - Faraday's law - prevents this. What happens instead is the following: as the current increases with time, the magnetic flux through the loop due to this current also increases. The increasing flux induces an emf in the circuit that opposes the change in magnetic flux. The net potential difference across the resistor is the emf of the battery minus the opposing induced emf As the magnitude of the current increases, the rate of increase lessens and hence the induced emf decreases. This opposing emf results in a gradual increase in the current. For the same reason, when the switch is opened, the current doesn't immediately fall to zero. This effect is called self-induction because the changing flux through the circuit arises from the circuit itself. The emf that is set up in the circuit is called a self-induced emf.

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The inductance of a coil depends on the cross-sectional area of the coil and other quantities, all of which can be grouped under the general heading of geometric factors. The SI unit of inductance is the henry (H).

A circuit element that has a large inductance, such as a closely wrapped coil of many turns, is called an inductor. The circuit symbol for an inductor is RL. The emf induced by an inductor prevents a battery from establishing an instantaneous current in a circuit. The battery has to do work to produce a current. We can think of this needed work as energy stored in the inductor in its magnetic field.

1.What did Hans Christian Oersted discovered in 1819?

2.What did Faraday concluded from his obswervations?

3.Describe the experiment demonstrating the main idea of electromagnetic induction.

4.What components is alternating-current generator consist of?

5.Could you explain the phenomenon of self-induction?

Exercises

1. Rearrange the letters in the anagrams to form equivalents for the Russian words:

2. Match the words in A with the words in B to form word combinations:

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3.Fill in the gaps with the missing words from the list: commercial generators, induced emf, magnetic flux, self-induced

emf, Lenz's law

1.An _____ is produced in the secondary circuit by the changing magnetic field.

2.The value of the _____ is proportional to the total number of lines passing through the loop.

3._____ says that if the magnetic flux through a loop is becoming more positive, say, then the induced emf creates a current and associated magnetic field that produces negative magnetic flux.

4.In the United States and Canada the frequency of rotation for

_____ is 60 Hz, whereas in some European countries 50 Hz is used.

5.The _____ must be proportional to the rate of change of the current with time.

UNIT 21

ALTERNATING CURRENT CIRCUITS AND

ELECTROMAGNETIC WAVES

Read the following texts. Study the thesaurus on fig. 21, answer the questions after the texts

Every time we turn on a television set, a stereo system, or any of a multitude of other electric appliances, we call on alternating currents (AC) to provide the power to operate them. We begin our study of AC circuits by examining the characteristics of a circuit containing a source of emf and one other circuit element: a resistor, a capacitor, or an inductor. Then we examine what happens when these elements are connected in combination with each other. Our discussion is limited to simple series configurations of the three kinds of elements.

We conclude this Unit with a discussion of electromagnetic waves, which are composed of fluctuating electric and magnetic fields. Electromagnetic waves in the form of visible light enable us to view the world around us; infrared waves warm our environment; radio-frequency waves carry our television and radio programs, as well as information about processes in the core of our galaxy. X-rays allow us to perceive structures hidden inside our bodies, and study properties of distant, collapsed stars. Light is key to our understanding of the universe.

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Resistors in an AC circuit

An AC circuit consists of combinations of circuit elements and an AC generator or an AC source, which provides the alternating current. To explain the concept of alternating current, we begin by discussing the current-versus-time curve. The current and voltage are in step with each other because they vary identically with time. Because the current and the voltage reach their maximum values at the same time, they are said to be in phase. Notice that the average value of the current over one cycle is zero. This is because the current is maintained in one direction (the positive direction) for the same amount of time and at the same magnitude as it is in the opposite direction (the negative direction). However, the direction of the current has no effect on the behavior of the resistor in the circuit: the collisions between electrons and the fixed atoms of the resistor result in an increase in the resistor's temperature regardless of the direction of the current.

The important quantity in an AC circuit is a special kind of average value of current, called the rms current - the direct current that dissipates the same amount of energy in a resistor that is dissipated by the actual alternating current. To find the rms current, we first square the current, then find its average value, and finally take the square root of this average value. Hence, the rms current is the square root of the average (mean) of the square of the current. In this Unit we use rms values when discussing alternating currents and voltages. One reason is that AC ammeters and voltmeters are designed to read rms values. Further, if we use rms values, many of the equations for alternating current will have the same form as those used in the study of direct-current (DC) circuits.

Capacitors in an AC circuit

To understand the effect of a capacitor on the behavior of a circuit containing an AC voltage source, we first review what happens when a capacitor is placed in a circuit containing a DC source, such as a battery. When the switch is closed in a series circuit containing a battery, a resistor, and a capacitor, the initial charge on the plates of the capacitor is zero. The motion of charge through the circuit is therefore relatively free, and there is a large current in the circuit. As more charge accumulates on the capacitor, the voltage across it increases, opposing the current. After some time interval, which depends on the time constant RC, the current approaches zero. Consequently, a capacitor in a DC circuit limits or impedes the current so that it approaches zero after a brief time.

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