Unit 3. Electromotive force

When free electrons are dislodged from atoms, electrical energy is released and made available to do work. Chemical reaction, friction, heat and electromagnetic induction will cause electrons to move from one atom to another. Scientists proved electrical energy to be released from matter by chemical reaction (batteries), heat (thermocouples), electromagnetic induction (generators), and friction (static generators). Whenever energy in any form is released, a force is developed. Electrical energy being released, a force called electromotive force (e. m. f.) is developed. An e. m. f. is present, whenever free electrons are moved from atoms, any of the above named methods being used to produce such electron motion.If the force exerts its effort always in one direction, it is called direct; the force changing its direction of exertion periodically is referred to as alternating.

The chemical reaction in a dry cell produces a negative charge or potential on the zinc. This charge being always negative, the e. m. f. is unidirectional (one way). Heat and friction, too, are sources of a unidirectional force. Electromagnetic induction, however, is certain to produce an alternating force.

If the south end of a bar magnet is passed into a coil of wire connected to a force-measuring instrument (voltmeter), the meter needle will move in one direction. If the south pole of the magnet is withdrawn from the coil, the needle will move toward the opposite side of the meter, thus showing the force to be alternating. The direction of force effort is seen to be dependent upon the direction in which the field is cut. The magnitude of the electrical force depends on the conditions at the source, such as the number of magnetic lines of force per unit of time. In the battery, the determining factors are kinds of electrolytes and the kind of the metals to be used for the plates. The common dry cell is found to develop 1.5 volts of electrical force regardless of the size of the cell. Large amounts of force can be obtained only by putting many cells in series.

The force developed by the generator depends on the number of coils in the armature, on the speed of the armature, and on the strength of the magnetic field from the field magnets, i. е., the number of lines of magnetic force cut by a coil per second. The volt is known to be the unit of measure for electrical force.

Wherever an e. m. f. is developed, there is also a field of energy called an electrostatic field. This field can be detected by an electroscope, the strength being measured by an electrometer.

EXERCISES I. Give words of the same stem and translate them:

to measure, to move, to induce, to determine, to develop, to exert, to direct, to depend, to unite, to cause.

2. Find in the text synonyms for the following words and give some examples of their use:

to make, substance, to name, force, to indicate, movement, to receive, amount, since, usual, to apply, velocity, dimension, to define.

2. Find the Infinitive constructions in the following sentences and translate the sentences:

1. We know the ampere to represent the rate of electricity flow through the circuit. 2. Electric pressure or e. m. f., which is measured in volts, causes electricity to flow. 3. The two irons in question lie parallel to the axis of the coil, and assuming their permeabilities to be constant, the pole strength will be proportional to the current. 4. The only force which can make an electron move is that due to the fields of other electrons. 5. Above about 500 kilocycles it is difficult to make an arc develop appreciable negative resistance. 6. To make an electric current flow continuously along a wire, a continuous supply of electrons must be available at one and a continuous supply of positive charges at the other. 7. The passage of current through a conductor of wire causes heat to be generated in it. 8. Heating causes a magnet to be demagnetized. 9. To avoid possible breakdowns of the insulation, the practice is to put extra insulation on the end turns to enable them to withstand the extra stress to which they are subjected at the moment of switching on. 10. A compensator allows the motor to take an excess current without putting a heavy overload on the mains. 11. Some tests enable the voltage absorbed by the impedance of the winding to be determined in addition to regulation. 12. If a current is passed through a coil of wire wound around a piece of iron, the latter will be found to be magnetized in a definite direction. 13. The magnetic field in the above case may be considered to be radial across the air gap. 14. Pure annealed copper is said to have 100 per cent conductivity. 15. Faster moving molecules meet oftener and being of higher energy content are more likely to react on meeting. 16. If the amounts of inductance and capacity are so small that their influence upon the current is negligible compared to the influence of the resistance the circuit may be considered to have nothing but resistance opposing the flow of current. 17. Copper and silver electrodes are certain to give a higher resistance gap than such metals as zinc, magnesium and so on. 18. The liquid particles seem to have a greater attraction for the solid than for their own molecules, this force of attraction between molecules of different kinds being called adhesion. 19. Having been rubbed many substances were proved to behave as amber does. 20. The beginner has probably noticed that if he hears the word "current" he is likely to hear the word "voltage" or "resistance". 21. A generator appears to be a device by which mechanical energy is transformed into electrical energy. 22. Incandescent lamps are seen to be examples of the application of the heating effect of an electric current, a conversion of energy taking place from an electrical form to that of heat. 23. In many circuits the potential distribu-tion changes with frequency and as the electrostatic energy (hence capacity) depends upon the potential distribution, ' the capacity may be expected to change with frequency. 24. In the majority of elementary calculations the inductance or the coefficient of self-inductance is assumed to be a constant for a given coil. 25. When the two conducting bodies are separated by a dielectric they are said to possess capacity and the combination is called a condenser.26. If, as in the above case, the voltage happens to be switched on at the instant when it passes through zero certain abnormal conditions are momentarily set up.27. If the magnetic force is reversed the induction density appears to lag behind the magnetic force.

2. Be ready to speak about:

1. Different methods of releasing electrical energy from matter.

2. The development of the electromotive force.

3. The direction of the e. m. f.

4. Batteries as a source of electrical energy.

5. The force developed by the generator.

6. The electrostatic field.