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4. Discuss the metalworking shop.

1) Equipment, 2) Metal proceccing operations, 3) Advantages and disadvantages of machine tools.

Electricity unit 15

1. Learn the notions and their definitions.

Electrical and Electronic Engineering The branch of engineering that deals with the effects and processes resulting from the behavior of tiny particles of matter called electrons.

Electric current a flow of free electrons through a conductor.

Conductance the property of a substance to carry out electricity.

Resistance the property of an elec­tric circuit to convert power from

electrical form to heat.

Electromotive force the work per unit charge done by the charges in moving from one point to the other.

Volt a unit of measuring electromotive force and electric potential.

Polarity concept of negative and positive.

Voltage difference between two points.

Transformer the device by which A. C. voltage is changed.

Battery a device by which chemical energy is transformed into electrical energy.

Generator a device by which mechanical energy is trans­formed into electrical energy.

Force of the electric current the work per unit charge done by the battery.

Text 1

2. Read, translate and retell the text. Electric Current

Electricity is a flow of an electric current now accepted as a slow drift of so-called “free electrons” through a conductor. When free electrons from one atom displace free electrons in a second atom, which in turn displace free electrons in another atom a continuous motion along a conductor takes place. This electron drift is known as current flow. When an electromotive force or voltage is impressed across a conductor, a field of force inside the conductor is created and electrons flow. This field gives direction to the random movement of the free electrons and causes their coordinated slow drift from one atom to another.

There are two general methods by which a continuous supply of electrical charge is obtained; one being by means of a battery, and the

other being by means of an electric generator.

The battery is known to be a device by which chemical energy is transformed into electrical energy and the generator as a device by which mechanical energy is trans­formed into electrical energy.

The electric circuit is a continuous path through which current flows, if an electromotive force (a battery), conductors (wires), and resistances (electric lamps of motors) are combined in a such a way as to have current flow an electric circuit has been made. Thus the curcuit is closed. One wire is for the current to flow to the electrical device from the power supply.The other wire is to allow the current to return to the source of energy. Sometimes a circuit is made completely by grounding it. When a circuit is broken so that a current is no longer able to pass, the circuit is said to be an open circuit. Electric devices in the circuit may be connected in series or in parallel.

A difference of potential determines the direction of electron flow. Electrons flow from the point which has an excess of electrons to a point that has a deficiency of electrons: that is electrons flow from negative (-) to positive (+). This concept of negative and positive is called polarity.

Regardlessof whether the current is DC, AC or pulsating DC, the direction of electron movement in a conductor is from negative terminal to positive. Wlthin a source of electromotive force such as a battery the electrons move from positive to negative. The device by which A. C. voltage is changed is called a transformer.