Unit 6 ohm’s law

Georg Ohm (1787—1854) was a German physicist. His enunciation of the law in 1827 aroused such bitter antagonism that he lost his position. Years later, when his work was corroborated by other scientists, he was honored by a professorship in physics at the University of Munich. Ohm stated his law having no reliable voltmeters, ammeters or batteries. He employed thermocouples to generate currents.

What is an ohm? Every electrical conductor opposes the passage of electric charges through it. This opposition arises because of the moving charges colliding with the atomic nuclei and other particles of the conductor. In so doing, the moving charges give up energy, which appears as heat. According to Ohm's law, electrical resistance is the ratio of the potential difference to the current for a conductor at a given temperature.

The ohm, the practical unit of resistance, is defined in terms of the ampere and the volt, as follows:

One ohm is the resistance of a conductor through which the current is 1 ampere when the potential difference across the ends of the conductor is 1 volt.

One ohm equals 1 volt per ampere.

This is the well known and fundamental law in electricity which makes it possible to determine the current flowing through a circuit when the resistance in the circuit and the potential difference applied to it are known. What Ohm discovered was that the ratio of the potential difference between the ends of a metallic conductor and the current flowing through the metallic conductor is a constant. The proportionality constant is the electrical resistance.

applied to so many electrical phenomena. One of its simplest applications is using a dry cell directly connected by wires to a small light bulb. The battery maintains a potential difference of 1.5 volts across the lamp. The electron current flowing through the circuit being 0.5 ampere, the resistance of the circuit is

1. 5 vo Its

0. 5 arah.

Although the resistance as found here is assumed to be the resistance of the light bulb, it really includes the resistance of the connecting wires, as well as the resistance of the battery. In practice one usually uses wires of sufficiently low resistance that they can be neglected in most calculations. If they are not small, they cannot be neglected and must be added in as part of the R in Ohm's law.

Although electromotive force and potential difference are both measured in volts there is a real distinction between them. Electromotive force is defined as the work per unit charge done by the battery or generator on the charges in moving them around the circuit. Potential difference beetween two points is defined as the work per unit charge done by the charges in moving from one point to the other.

If any two of the three quantities: resistance, current and potential difference are known for a curcuit, the third can always be determined by substituting in Ohm's law. In other words, any one of the three factors may be

. . V

the unknown, and Ohm's law may be written in any one of three ways: / = -

v

. R =-. V=IR i EXERCISES I. Translate the following groups of words of the same stem:

science, scientist, scientific; to state, state, statement; to rely (upon), reliable, reliability, unreliable; to employ, employer, employee, unemployed, unemployment; to generate, generator, generation; to conduct, conductor, conductance, conductivity; to move, movement, motion; to collide, collision; to appear, appearance, to disappear, disappearance; to resist, resistor, resistance, resistivity, resistant; to differ, different, difference; to define, definition, definite, indefinite, indefinitely; equal, to equal, equally, equality, equation, to equalize, equalization; to assume, assumption; to measure, measure, measurement, measurable.

2. Translate the following sentences, paying attention to the dif­ferent meanings of the words in italics; give some examples:

1. In practice one usually uses wires of sufficiently low resistance that they can be neglected in most calculations. 2. Potential difference between two points is defined as the work per unit charge done by the charges moving from one point to the other. 3. The device in question is more efficient than the one used in the previous experiments. 4. One could hardly expect to measure voltage, say, 5 volts to even an average degree of accuracy with a voltmeter having 500 volts as its maximum. 5. Suppose one saws a piece of wood with a rapid motion. As a result, the saw begins to grow hot. 6. For most substances the permeability has a value of unity, nickel, iron and cobalt being the notable exceptions. 7. The coulomb is the quantity of electricity transported by a current of one ampere flowing for one second.8. In most electric apparatus depending on the magnetic field for its operation the field is produced by currents flowing in coils. 9Electrical generators convert mechanical energy of rotation into electricity directly by way of magnetism, for magnetism serves as an intermediate step between the mechanical energy to be applied to the generator and the electrical energy on the wires leading to it. 10. For the sake of clarity each armature winding is shown as a single loop. 11. Let us assume for the sake of simplicity that a vibrating tuning fork is placed in front of the microscope. 12. The nuclear p rays proper, which have a continuous spectrum, present the theoretical physicist with a puzzling problem. For, if particles of all possible energies leave the nucleus of the atom, the nucleus cannot remain as a unique structure of definite energy.

2. Analyse the Gerunds in the following sentences and translate the sentences:

1. Earthing is of vital importance in all electrical systems and for all apparatus. 2. It should be noted that earth does not mean merely touching the ground, strata or rocks, but metallic contact with the continuous earth system of a mine and the "earth" of the surface. 3.The method of connecting the armature and field winding of the series motor is exactly the same as for the corresponding generator. 4. Emitting a beta particle does not alter the mass number. 5. Screening cables is obtained by wrappings of flexible tinned copper. 6. On dipping a magnetized needle into iron filings it is seen that iron filings adhere most strongly at the ends of the needle. 7. A magnetic field equal to 10 oersteds is represented by drawing 10 lines of force per square centimeter. 8. Every substance capable of being magnetized is assumed to consist of a very large number of molecular magnets, probably no longer than the molecules out of which the substances are made. 9. If two rubber rods, electrified by being rubbed against fur are brought near each other, they will be found to repel each other. 10. Upon breaking the magnet into still shorter pieces we still get complete magnets. 11. Hammering, bending or twisting an iron rod when it is near a magnet, increases its magnetization. 12. Electromotive force is defined as the work per unit charge done by the battery or generator on the charges in moving them around the circuit. 13. Referring to the Einstein's equation, it is evident that the velocity of emission is just zero when the energy of one photon is just equal to the energy lost by the electron in being emitted. 14. Breaking the circuit causes sparking. 15. Many substances after being rubbed behave as amber does. 16. A number of methods have been devised for observing atomic collisions. 17. The electrons flow to the cathode because of its having lost electrons and being less negative than the negative side of the battery. 18. Even the best conductors of electricity offering some resistance to the current flow must be kept in mind while dealing with electrical phenomena. 19. In considering power in electrical circuits one should remember of all electrical devices having resistance.

2. Analyse and translate the following sentences:

1 .We know of silver and copper being very good conductors of electricity.

2. Only in the middle of XIX century the fact of the quantity of heat produced being proportional to the work done was definitely stated. 3. Mica is widely used as a dielectric in high grade condensers because of its having high voltage strength. 4. The p. d. across the terminals of a cell being less than the e. m. f. of the cell applies equally to a generator or, indeed, any source of e. m. f. 5. Marie Sklodowska's having made numerous experiments resulted in isolating two new elements. 6. When a bar of iron is thrust into a fire it becomes heated due to the atoms comprising the bar becoming agitated.7. Cliarfied particles are seldom found at rest in an electronic device. The mechanisms by which they are formed nearly always insure their having some residual velocity.

2. Define Ohm’s Law.

3. Learn to read the following formulas:

E = I X R is read: E equals (is equal to) I multiplied by R (or times

R);

I = E: R is read: I equals E divided by R; I X R is read: I

multiplied by R or I times R; y is read: E over I, or E

divided by I.

2. Note the plural of the following foreign nouns:

a datum — data; a phenomenon — phenomena; a formula formulae; an axis — axes; radius—radii;nucleus—nuclei;locus—loci.

2. Supplementary reading.