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Федеральное агентство по образованию

Хакасский технический институт – филиал

Красноярского государственного технического университета

АНГЛИЙСКОМ ЯЗЫК

ТЕХНИЧЕСКИЕ ТЕКСТЫ ДЛЯ ЧТЕНИЯ

НА ФАКУЛЬТАТИВНЫХ ЗАНЯТИЯХ

Методические указания

Абакан-2005

УДК

Английский язык. Технические тексты для чтения на факультативных занятиях: Методические указания для студентов специальности 100400 всех форм обучения / Сост. Л.Г. Михайлова. Красноярск: КГТУ, 2005. – 16 с.

Печатается по решению

редакционно-издательского совета университета

© КГТУ, 2005

Electric Circuits

The concepts of electric charge and potential are also essential in the study of electric currents. When an extended conductor has different potentials at its end, the free electrons of the conductor itself are caused to drift from one end to the other. In order for this flow to continue it is necessary that the potential difference be maintained by some electrical source such as an electrostatic generator, or, much more frequently, a battery or a direct-current generator. The wire and the electrical source together form an electric circuit, the electrons drifting around it as long as the conducting path is maintained. In effect such a flow of electrons constitutes an electric current.

Batteries and direct-current generators are sources of potential difference with urge the electrons around a circuit continually in one direction, producing a unidirectional current. For this reason such a source is said to have a fixed polarity, one terminal being called positive and the other negative. If it is desired to reverse the flow, then the terminals of the circuit must be reversed with respect to the source.

From the early days electrical science, current has been regarded as a flow of electricity from the positive terminal to the negative terminal in the external circuit connected to a source. Now we know a current through a conductor to be actually a movement of electrons, and since these have negative charges, they travel around the external circuits from the negative terminal to the positive terminal. The electron flow is, therefore, opposite to the conventional direction of current, making it necessary, in order to avoid confusion, to distinguish one from the other by name.

Lenz’s Law

Lenz’s law might have been predicted from the principle of the conservation of energy. When you move a magnet towards a coil and thus induce a current in its windings, the induced current heats the wire. In order to supply the energy to do this, you must do work in overcoming an opposing force. If the force did not oppose the motion, you would create energy. Thus the magnetic field of the induced current is seen to oppose the change.

Lenz’s law and the right hand rule can be used to determine the direction of an induced current. The north pole of a magnet being moved closer to a coil, the induced current causes a field which opposes the motion, a north pole being produced on the nearer face. To course this north pole, magnetic lines must emerge from this face of the coil. Now grasp the coil with your right hand, so that your fingers point in the direction of the induced magnetic field. Your thumb will point in the direction of the current, that is, counterclockwise.

The Induced Current Opposes the Change. A magnet pole being moved toward one face of a coil, the current induced in the coil produces a magnetic field. Moreover, this field always opposes the change of magnetic flux that is occurring. For example, move the north pole of a magnet closer to one face of a coil. The induced current will be counterclockwise and will oppose the change of flux through the coil. Remove the bar magnet, and the induced current in the coil will be clockwise, again opposing the change. The rule is expressed by Lenz’s law, as follows:

Whenever a current is induced, its magnetic field opposes the change of flux.