TOPICS ON ELECTRICITY AND MAGNETISM

Electric Units

The practical units are based upon the electromagnetic system of electrical units.

The most important units are: Resistance, Potential Difference and Current Strength.

Resistance is that property of a substance that opposes the flow of an electric current through it. The practical unit of resistance (R.) is the ohm (Ω) .

Potential Difference is a state of electrification in which one point is at a higher potential than another, causing a flow of electricity from the higher to the lower potential. The volt is the practical unit of potential difference (P.D.). One volt is required to overcome a resistance of one ohm in order that one ampere can flow. The volt may be defined as the electromotive force (E.M.F.) induced in a conductor. It is also called the unit of pressure.

Current Strength is the rate of flow of electricity along a conductor due to one end of the conductor being at a higher potential than the other end. The practical unit of current is the ampere (A).

Coulomb is the unit of quantity. It is the quantity of electricity that passes a cross-section of the conductor in one second when the rate of flow is one ampere. In practice it is more usual to use the ampere-hour than the smaller unit, the coulomb.

The practical unit of energy is the joule (J). It represents the energy expended in one second by an electric current of one ampere in a resistance of one ohm. Really, the unit of energy is the erg, but it is so small that the joule is used as a practical unit.

The practical unit of power is the watt (W.). It is equal to the work done at the rate of one joule a second or to the work represented by the current of one ampere under the pressure of one volt.

The practical unit of inductance is the henry (H.) also called coefficient of self-induction. An electric current has an inductance of one henry when a rate of charge of one ampere per second induces an electromotive force of one volt.

The unit of electric capacity is the farad (F.) which is the capacity of a condenser that, charged with one coulomb, gives a difference of potential to one volt. This unit is so large that, for practical purposes, the micro­farad (one millionth of farad) is generally used.

Faraday's discovery of the principles of induction had a far reaching effect in the electrical world.

We depend on it for our whole system of electric distribution.

The transformer is based on this discovery. Changes in the flow of current in one coil induce current in another coil set close to the first and these induced currents are much stronger if the two coils are wound upon an iron frame. If the second coil has twice as many loops in it as the first has, then the voltage of the induced currents will double that in the first. If the volts are doubled the current amperes are halved. On the other hand, if the pulsing cur­rent is put into the second coil, then the current in the first will have half the volts and twice the amperes.

By step-up transformer, we mean one that increases the voltage and diminishes the current. It consists in principle, of an iron core on which is wound a primary coil of a small number of turns of thick insulated wire and, forming a separate circuit, a secondary coil of a larger number of turns of thin insulated wire. When the low-voltage current is passed through the primary coil, it induces a current in the secondary by producing an alternating magnetic field in the iron core. The ratio of the voltage in the primary to that in the secondary is very nearly equal to the ratio of the number of turns in the primary to that in the secondary.

The step-down transformer works on the same principle with the coils reversed.

Electrical Standards

Electric standards are the fundamental electric units to which all electric measurements are referred.

As the accurate derivation of the common electrical units from these basic standards is generally expensive, primary physical standards of voltage, resistance, inductance, capacitance and frequency are maintained. Secondary standards are of a lesser degree of accuracy, but need not be maintained so carefully as the former.

The standard of resistance is a strip of wire of a special metal alloy of a very low temperature coefficient of resistance. It is carefully supported mechanically so that changes in temperature will not cause the supports to change the length or cross section of the wire. Each end of the strip is fastened to a block of conducting material and each block is fitted with a current terminal and a potential terminal.

A voltage standard is maintained by standard cells. These are specially made with batteries having a mercury-cadmium electrode system, with a cadmium-sulphate solution as an electrolite.

A standard of induction involves a wire coil wound in a form designed to keep the geometrical arrangement of the wire from changing. Primary standards are wound as a single-layer coils with the wires laid in grooves accurately cut in the surface of a fused quartz or marble cylinder. Secondary standards are usually multi-layer coils wound so as to make the geometric arrangement of the coils remain fairly constant with changes in temperature.

A capacitance standard consists of a couple of electrode structures arranged in such a way that the geometrical relation of one structure to the other does not change noticeably when the temperature is changed. Some structures are concentric spheres, coaxial cylinders and interleaved plates. Spheres and cylinders are well suited to serve as primary standards whereas cylinders and interleaved plates are suitable as secondary standards. Air is used as the dielectric, and a high quality material, such as quartz is employed as the solid support in one of the electrode structures.

Standard frequency is maintained by the oscillations of quartz crystals incorporated into an electron tube oscillator circuit. These crystals possess a high degree of stability.

Galvanometer , ammeter , voltmeter, Wheatstone bridge .

A galvanoscope is an instrument or apparatus, such. as a magnetic needle, for detecting the presence and direction of electric currents, specially those of feeble intensity.

A galvanometer is an instrument for measuring a small electric current or for detecting its presence or direction by means of a magnetic needle or of a coil in a magnetic field. The most sensible galvanometers are of the needle type but they are affected by external magnetic forces and are less reliable than those that contain moving coils, the latter are not so easily disturbed and have a constant sensitivity.

An estatic galvanometer is one having two needles with their poles in opposite directions so as to reduce the effect of the magnetism of the earth.

The electrical measuring devices which merely indicate, such as ammeters and voltmeters, are called instruments, while the devices which totalize with time, such as watt-hour meter, amperehour meter, etc. are called meters.

An ammeter is an instrument designed to measure electric current; it usally uses the magnetic or thermal effects to move a pointer across a scale.

A voltmeter is an instrument for measuring in volts the differences of potential between different points of an electric circuit.

The Wheatstone bridge is a device for the measurement of resistances; it is so called because the balance between the resistances to be measured is indicated by the absence of a current in a certain wire forming a bridge of connection between two branches of the circuit.

Electric Current .

When an electric force, as that supplied by a battery, is connected across a conductor, the free electrons, that first had been in a state of chaotic motion in every direction, are guided in an orderly fashion, atom to atom, from the negative terminal of the battery, through the. wire, to the positive one. Then, we can say that an electric current is the orderly motion of free electrons under the application of an electric force. In spite of the motion being rather slow, the impulse is transmitted almost at the speed of light.

Electric current may be either alternating (AC) or direct (DC), the former being pulsing and the latter continuous.

Alternating current is an electric flow which, after reaching a maximum in one direction, decreases, reverses and finally reaches a maximum in the opposite direction, the cycle being repeated continuously. The number of such cycles per second is called frequency, which is marked on the front of our electricity supply-meter. It is necessary to know it before buying certain types of household apparatuses.

Alternating current as well as direct current can perform many things, but AC does many things essential to our industrial life that DC does not. Indeed, AC helps to get an abundant supply of cheap current generated in the most economical way in large power stations.

As to DC, it is substantially constant in value and it always flows in the same direction.

The simplest form of a DC electric circuit is a battery with a resistance connected to its terminals. The resistance represents an external device or load connected to the battery.

A complete DC circuit should have an unbroken path for the current flow from the battery, through the load, and back into the battery. The amount of current flow through the circuit, the applied voltage, and the resistance are submitted to a relationship referred to as Ohm's law: "The current flowing in an electrical circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit".

Electric current generates heat in overcoming the resistance of a conductor. This heat leaves the conductor by convection and by radiation. The energy used by the current to heat the conductor is lost..

Heat produced by electric current makes an electric light bulb incandesce. The filament in a.bulb is only a thin piece of wire which becomes white hot when current flows through it.

Copper is used as an electrical conductor because it offers less resistance to the flow of current than other materials. However, there is some resistance even in copper and it cannot be eliminated.

The amount of resistance offered by a conductor depends on the material, length and diameter. A short conductor of large diameter offers less resistance than a long one with small diameter. As all the resistance cannot be eliminated from copper and changing the length of the wiring is impractical, the only means to reduce resistance and carry the necessary load is to increase the size of the wire.

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