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| Condensers and Dielectric Materials
Nature of Electricity DIALOGUE A. When was the first recorded observation on electricity made? B. As much as I know it was made by the Greek philosopher Thales. A. What did he state, I wonder? B. Don't you know? He stated that a piece of amber rubbed with fur attracted light objects such as feathers and bits of straw. Did he make any experiments? A. No, as far as it is known Thales liked to speculate but he did not experiment systematically. Twenty-two centuries elapsed before there was any progress. B. Oh, it was just about the time that Galileo discovered the laws of the pendulum and accelerated bodies. So it was at the time when the study of magnetism and of electrical phenomena began. A. How was it found out that some substances could be "electrified"? B. It is a well-known fact that having been rubbed many substances behave like amber does. A. Can only similar substances become electrified or acquire electrical charges, being touched together and then separated? B. No. Later on it was discovered that any two dissimilar substances could be electrified. As a matter of fact rubbing is not essential. It merely forces the two substances into close contact. A. When was the modern concept of the nature of electricity arrived at? B. During the past century the idea of the nature of electricity was completely revolutionized. A. Yes, I know it quite well. Hitherto the atom has been regarded as the ultimate subdivision of matter. Today the atom is regarded as an electrical system. В. Oh, now I want to examine you a little. What do you know about the nucleus, the proton and the electron? A. I don't mind. In the electrical system there is a nucleus containing positively charged particles. These particles are called protons. The nucleus is surrounded by lighter negatively charged units — electrons. So, the most essential constituent of matter is made up of electrically charged particles. B. I see that you have an idea about this but you did not tell me when matter is neutral. A. But everybody knows that matter having equal amounts of both charges is neutral — that it produces no electrical effects. B. And what happens if the number of negative charges is unlike the number of positive ones? A. Well, then matter will produce electrical effects. Having lost some of its electrons, the atom has a positive charge; having an excess of electrons — it has a negative charge. B. So, as a matter of fact you do know the material. Electric Current DIALOGUE Demonstrator: When will electrons move? Student: If given a path, electrons dislodged from the parent atom will move. Demonstrator: Well, what do you know about the electric current? Student: The electric current is a quantity of electrons flowing in a circuit per second of time. Demonstrator: And what is the unit of measure for current? Student: The unit of measure for current is the ampere. One coulomb passing a point in a circuit per second, the current strength is I ampere. The ampere is therefore a rate unit. Demonstrator: Why do the electrons move along the circuit? Student: The electrons move along the circuit because the e. m. f. drives them. Demonstrator: When is the rate of electron flow doubled? Student: It is doubled, if the force is doubled. It means that other factors being constant, the current is directly proportional to the e. m. f. Demonstrator: What other factor determining the magnitude of the current do you know? Student: This is the ease with which the electrons are allowed to pass along the circuit. This ''ease" or conductivity may be defined as the number of amperes per volt in a circuit. Demonstrator: And when is a current proportional to the conductivity? Student: All other factors being constant, the current is directly proportional to the conductivity. If the conductivity is doubled, the current will be also doubled. Demonstrator: How is a magnetic field developed? Student: A stream of electrons in a circuit will develop a magnetic field around the conductor along which the electrons are moving. Demonstrator: What does the strength of the field depend upon? Student: The strength of the magnetic field depends upon the current strength along the conductor. Demonstrator: And what about the direction of the field? Student: The direction of the field is dependent upon the direction of the current flow. Demonstrator: When is the current called direct or alternating. Student: If the force causing the electron flow is unidirectional, the current is called direct. The force changing its direction of effort periodically, the current is known as alternating. Demonstrator: That will do! 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.
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 (see Fig. 2) 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. Electricity in Motion When an electric charge is at rest, it is spoken of as static electricity, but when it is in motion, it is referred to as an electric current. In most cases, an electric current is described as a flow of electric charges along a conductor. Such is the case, for example, in the experiment of charging an electroscope from a distant point by means of a long copper wire and a charged rubber rod. This experiment is explained by stating that electrons already in the wire are pushed along toward the electroscope by the repulsion of electrons from behind. No sooner does this current start,however, that the negative charge of the rod is dissipated and the current stops flowing. To make an electric-current flow continuously along a wire, a continuous supply of electrons must be available at one end and a continuous supply of positive charges at the other. This is like the flow of water through a pipe; to obtain a continuous flow a continuous supply of water must be provided at one end and an opening for its escape into some receptacle at the other. The continuous supply of positive charges at one end of a wire offers a means of escape for the electrons. If this is not provided for, electrons will accumulate at the end of the wire, their repulsion back along the wire stopping the current flow. 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. 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 ends, 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 which 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 of 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 circuit 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. 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.
Symbolically, Ohm's law is often written
Using Ohm's law is of great importance because of its being generally 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
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 between 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 circuit, the third can always be determined by substituting in Ohm's law. In other words, any one of the three factors may be the unknown, and Ohm's law may be written in any one of three ways: Charged Body DIALOGUE Demonstrator: What body is considered to be negatively charged and what body is considered to be a positively charged one? Student: From the electric viewpoint a negatively charged body is one having more than its normal number of electrons and a positively charged body is one having less than its normal number of electrons. Demonstrator: Is the normal atom charged or not? Student: The normal atom is not charged. This means that it does not exert any attractive or repulsive force on the other atoms. Demonstrator: What do you know about the structure of the atom? Student: The structure of the atom is as follows. In the centre part of the atom are grouped some positive and negative particles, catted protons and neutrons respectively. The nucleus is surrounded with a cloud of' electrons. So an atom having an equal number of positive and negative particles shows no electrical charge, the negatives and the positives just neutralizing each other. Demonstrator: Why is the nucleus said to be positively charged? Student: It is said to be positively charged because the center part of the atom always consists of protons and neutrons only. Demonstrator: What determines the nature of he atom? Student: The number and arrangement of the outer electrons as well as protons and neutrons determine what the atom is whether hydrogen, oxygen, copper, gold, etc. Demonstrator: How do different elements differ? Student: All elements we know differ only in the number and arrangement of protons and electrons. Demonstrator: What do you know about tungsten? Student: Tungsten has very many electrons placed in certain well-known complex arrangements. Demonstrator: What happens if one electron is removed from the atom by some means or other? Student: In this case the balance between positive and negative charges is destroyed; an excess of positive charge exists on the atom, the atom is positively charged. The particle formed after one electron had been removed from the atom is called an ion. Demonstrator: Thai's right. Induced Electric Currents The discovery of induced electric currents goes back more than one hundred years to 1831 and the experiment of Michael Faraday. A straight bar magnet plunged into a coil of wire was found to produce an electric current. The N pole of the magnet being plunged into the coil, a galvanometer needle deflects to the right. It being withdrawn, the needle deflects to the left, indicating a current in the opposite direction. If the S pole be moved down into the coil, the needle deflects to the left and as it is withdrawn, the deflection is to the right. The relative motion of the coil and magnet is what produces the current and it makes no difference whether the coil alone moves15, whether the magnet alone moves, or whether they both move. In either case, if the relative motion ceased, the current would stop. A "somewhat old-fashioned" way of describing the action is to say that only when a wire is cutting the line of force is there an induced e. m. f. A somewhat more acceptable statement at the present time is, in effect, that only when the total magnetic flux linking a closed electrical circuit is changing is there an induced e. m. f. A flexible wire connected to an ammeter, and held in the hands, is moved in various ways across the pole of a magnet. In case a straight section of the wire were held over the N pole and moved to the right, an electron current would flow in the one direction. Were the wire moved in the opposite direction, the induced e. m. f. and current would reverse direction. Should the wire be moved vertically upward or downward, parallel to the magnetic induction, no current will flow. In other words, there is an induced e. m. f. only when the total number of lines of induction through the closed circuit is changing. A current being produced means that electrical energy has been created. It has been created at the expense of mechanical work, for in moving the wire across the field, a force F had to' be exerted for a distance S. The faster the wire moves, and the stronger the field through which it moves, the greater is the required force and the greater is the induced e. m. f. and the resultant electron current. Provided the wire stops moving in mid-field the e. m. f. drops to zero. These are the essential principles of the electric generator. Lenz's Law Lenz's law might have been predicted from the principle of the conservation of energy. When you move a magnet toward 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 cause 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. This rule is expressed by Lenz's law, as follows: Whenever a current is induced, its magnetic field opposes the change of flux. SELF-INDUCTION An e.m.f can be induced by varying the number of magnetic lines threading through a circuit, the induced current always opposing the change that is occurring, no matter what causes the change of magnetic flux. It may be due to the motion of a magnet or to the change of current in a nearby electrical circuit as in the transformer. The change of magnetic flux may also be due to a change of current in the coil itself, this effect being known as self-induction. Suppose several hundred feet of wire, in a single loop, to be connected in series with an incandescent lamp, a 115-volt direct-current source, and a switch. The switch being closed, the current in the circuit will increase, in a few millionths of a second, to a steady value determined by Ohm's Jaw. Now let this wire be wound onto an iron rod to form a coil. When the switch is again closed, the current will increase to the same final value as before, but the time required will be several hundredths of a second. In the coil there are hundreds of turns of wire, side by side. The current in each turn causes magnetic lines that thread through the other turns. An increase of current in any loop varies the flux through all the others, the change of flux of magnetic lines generating an e.m.f. This induced e.m.f opposes the change of current. Self-induction is known to oppose not only the increase of current in a coil but the decrease also. The circuit being opened, the current will not stop instantly. The forward induced e.m.f will cause a spark to appear at the switch. In order to demonstrate self-induction, connect a large electromagnet and an incandescent lamp, in parallel with each other, through a resistor to a direct-current source. When you close the switch, at first the increasing current through the coils of the electromagnet increases the flux, thus generating an opposing e.m.f. Self-induction impedes the current through the coil. Most of the current flows through the lamp, which glows brightly. The current having become constant, most of the current flows through the coils, and the lamp becomes dim. The switch being opened, the flux through the coils will decrease rapidly. The induced e.m.f will make the lamp glow brightly for an instant. Condensers and Dielectric Materials The dielectric of a condenser is one of the three essential parts. It may be found in solid, liquid, or gaseous form or in combinations of these forms in a given condenser. The simplest form of a condenser consists of two electrodes or plates separated by air, this representing a condenser having a gaseous dielectric. If this imaginary condenser had the air between the plates replaced by a non-conducting liquid, such as transformer oil, and if the distance between the plates were the same as in the first case, the capacitance would be found to have increased several times on account of the oil having a higher value of dielectric constant than air which is usually taken as 1. The space between the plates being occupied by a solid insulator, a condenser would result, which would be practical, as far as the possibility of constructing it is concerned. It would be found, in this case too, that the capacitance of the condenser was several times larger than when air was the dielectric. The mechanical construction of either air or liquid dielectric condensers requires the use of a certain amount of solid dielectric for holding the two sets of plates. There are a great many dielectric or insulating materials available from which one may choose. A material which is very good from the electric standpoint is often found to be poor mechanically or vice versa, air being the gas generally used as a dielectric. Compressed air has been used in some high-voltage condensers, compressed nitrogen and carbon oxide being also in use. Several kinds of oil have been used in condensers, such as castor oil, cottonseed oil, and transformer oil. More recently electrolytic condensers have come into use in radio equipment for use as filters and by-pass condensers where a large capacitance is required and either a d. c. or pulsating d. c. is applied. Among the solids to meet the requirements as the condenser dielectric are mica, ceramic materials, and paper. Solid insulators used as mechanical supports in condensers include quartz, glass, porcelain, bakelite, mica, amber, hard rubber, etc. Some Facts about Magnets Being heated a magnet loses some or all of its magnetism. A magnet being broken in two, each piece becomes a magnet with its own pair of poles.1 This subdivision could be carried on until we were down to the smallest particle of iron, a molecule. Conversely, two identical bar magnets being brought end to end with opposite poles in close contact, the poles touching seem to disappear and we have but two poles at the extreme ends. A tube of iron filings may be magnetized by stroking it with a magnet in the usual way. The filings being shaken, the magnetism disappears. These facts give rise to the very plausible theory of magnetism generally accepted. We know iron molecules to be magnets at all times. When they are arranged in a bar of steel or iron so that the fields of force of all or most of the molecules are in the same direction, their fields are added to one another and the bar is a magnet. The little magnet molecules form chains, their poles disappearing except at the ends of the chain. This condition is not an equilibrium condition because of the like poles in adjacent chains repelling each other. The rigidity of steel holds the molecules in this position. In soft iron, however, as soon as we take the bar out of the magnetizing field, the molecules adjust themselves on account of the repulsions of like poles of molecules in adjacent chains, leaving the iron unmagnetized. We can see why hard steel makes permanent magnets and soft iron does not. We can also explain why soft iron has a higher permeability than steel. When placed in a magnetic field, the molecules of steel do not readily turn around in the direction of the lines of force. But this alignment is necessary if the body is to absorb lines of force. Heating which increases molecular motion, or jarring causes a magnet to be demagnetized due to its permitting the molecules to adjust themselves to the equilibrium position.
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Scientists proved electrical energy to be released from matter by chemical reaction (batteries), heat (thermocouples), electromagnetic induction (generators), and friction (static generators).
