What is Magnetism?

Human beings have known about magnets at least since ancient times. But they did not learn much about how magnets work or how to use them until several hundred years ago. The first known magnets were hard black stones called magnetite (an oxide of iron). No one knows when or by whom these stones were discovered, but the ancient Greeks knew of the lodestone’s power to attract iron. The Chinese discovered that a splinter of this rock, if hung by a thread, would always set itself in the north-south direction, they thus, used such an arrangement as a compass for guiding the ships. However, it is only recently that magnetism has become so widely applied and pervasive to many electrical devices that it looks really hard to imagine any electrical or electronic device without a magnet in its very constitution…

Properties of Magnets:

1.    Magnets attract strongly certain materials such as iron, steel, nickel and cobalt, which are called ferromagnetic materials.

2.    Magnetic poles are the places in a magnet to which magnetic materials, such as iron filings, are attracted. They occur in pairs of equal strength; in a bar magnet they are to be found near the ends.  

3.    If a magnet is supported so that it can swing in a horizontal plane it always zones to rest with its pole, the north-seeking or N pole pointing roughly towards the earth’s North Pole. A magnet can therefore, be used as a compass.

4.    If the N pole of a magnet is brought near the N pole of a suspended magnet, the two poles can be seen to repel each other. Two S poles also repeal each other. By contrast, N and S poles always attract each other. The law of magnetic poles thus states that: Like poles repel, unlike poles attract.

Repulsion is the only sure test for polarity. Attraction shows either those two poles are unlike or one piece is unmagnetised.

The first permanent magnets were made of steel (an alloy of iron). Modern magnets are however, much stronger which principally are of two types called as Alloy magnets & Ceramic magnets. Alloy magnets contain metals such as iron, nickel, copper, cobalt and aluminum. Ceramic magnets are otherwise, made from powders called ferrites which are compounds of iron oxide with other metal oxides. They are essentially brittle in nature.

Connecting concepts: Principal Applications of Magnetism!

The phenomenon of magnetism has been applied in our daily lives in variety of ways. For example, the electromagnet forms the basis of the electric motor and the transformer. Magnetic materials have made advancements possible in the area of computer technology. Parallel and antiparallel regions of magnetization in computer memories serve as the units of the binary number system used in computers. Magnetic materials are also used in tapes and disks on which data are stored. Besides, these tiny magnetic units used in computers, large, powerful magnets are employed in many other technologies. Strong magnets make it possible for the magnetic levitation trains to float above the track. These trains are being christened as MAGLEV-trains will soon overcome the principal limitation of conventional wheeled trains associated with the high cost of maintaining precise alignment of the tracks so as to avoid excessive vibration and rail deterioration at high speeds. Thus, Maglev trains can provide sustained speeds greater than 500 km/h yet, limited only by the cost of power required to overcome wind resistance. The world’s first Maglev train to be adopted into commercial service was at Birmingham, England started way back in the year 1986, but later shut down in 1997 after having operated for about 11 years although, a Sino-German Maglev is still operating currently over a stretch of around 30 kms in Shanghai, China. Due to this there is no friction between the vehicle and the tracks to slow the train down.

Doctors use powerful magnetic fields in nuclear magnetic resonance imaging (MRI) for effective diagnosis of the body’s anatomy such that it has given birth to a new field of medical diagnosis called as Biomagnetism. Magnetism has also found its principal application in the construction of so called SQUID that refers to (Superconducting Quantum Interfering Devices) which are provenly the most sensitive magnetic field detectors ever invented by man. Essentially, an ultra sensitive detector of magnetic flux (determined in the units of Tesla), SQUID makes the use of a superconducting ring being interrupted by one or two Josephson junctions. SQUID has found its most important application in Magnetoencephalography (MEG) by the virtue of which the body can be probed to certain depths without the need for the strong magnetic fields associated with a technique like MRI etc. Therefore, MEG serves as a non-invasive method for recording the minute magnetic fields that emanate from the brain by making use of a device that is now called as Neuromagnetometer that is virtually a helmet like device placed around the head of a patient during diagnosis. In fact, the only essential condition to work with SQUID is that it requires for its working an extremely low temperature that is as low as about 4.2 Kelvin. Of late, SQUIDs have also been used to measure the small and minute magnetic fields generated by a baby’s heart after placing the same around the mother’s abdomen and thus, allows one to diagnose the foetal heart conditions. Particle accelerators use super conducting magnets to keep the accelerated particles focused and moving in a curved path.

THE ELECTRICITY

Who Discovered Electricity?

The curious thing about electricity is that it has been studied for thousands of years – and we still don’t know exactly what it is! Today, all matter is thought to consist of tiny charged particles. Electricity, according to this theory, is simply a moving stream of electrons or other charged particles.

The word “electricity” comes from the Greek word electron. And do you know what this word meant? It was interestingly the Greek word for “amber”! You see, as far back as 600 BC the Greeks knew that when amber was rubbed, it became capable of attracting towards it some light bits of cork or paper.

Not much progress was made in the study of electricity until 1672. In that year, a man called Otto von Guericke produced a more powerful charge of electricity by holding his hand against a ball of spinning sulphur. In 1729, Stephen Gray found that some substances, such as metals, carried electricity from one location to another. These came to be called “conductors.” He found that others, such as glass, sulphur, amber and wax, did not carry electricity. These were called “insulators.”

The next important step took place in 1733 when a Frenchman called du Fay discovered positive and negative charges of electricity, although he thought these were two different kinds of electricity.

But it was Benjamin Franklin who tried to give an explanation of what electricity was. His idea was that all substances in nature contain “electrical fluid.” Friction between certain substances removed some of this “fluid” from one of the substance and placed an extra amount in the other. Today, we assuredly say that this “fluid” is composed of nothing, but the electrons which are negatively charged species of an atom revolving around the atomic nucleus in some discrete orbits.

Probably the most important developments in the science of electricity started with the invention of the first battery in 1800 by Alessandro Volta. This battery gave the world its very first continuous, reliable source of electric current and led to all the important discoveries of the use of electricity in the later part of the history…

While, today it has been conclusively proved that entire matter is made up of some tiny charged particles what we know now as electrons and it is the charge of these very electrons that what makes the matter charged and ensure the charging of the bodies that are made up of this charged matter. The charging of bodies is now also been understood in terms of the structure of the atoms composing the body. In its normal condition though, the atom is electrically neutral just because, the charge is balanced in an atom by the presence of an equal number of electrons and protons. However, it is usually the electrons that move from one place to another while the nuclei of the atoms remaining fixed in place. Therefore, how does a body become charged say, when a glass rod is rubbed with silk, some electrons from the rod attach themselves onto the silk, thus, making the glass positively charged – i.e. having lost electrons-

Connecting concepts: Electricity discharges in nature: “Thunder & Lightning”!

Lightning and thunder must have been among the first things about nature that mystified and frightened primitive man. When he saw the jagged tones of lightning in the sky and heard the claps and rumbles of thunder, he believed the gods were angry and that the lightning and thunder were a way of punishing man.

To understand what lightning and thunder actually are, we must recall a face we know about electricity. We know that things become electrically charged-either positively or negatively. A positive charge has a great attraction for a negative one.

As the charges become greater, this attraction becomes stronger. A point is finally reached where the strain of being kept apart becomes too great for the charges. Whatever resistance holds them apart, such as air, glass or other insulating substance, is overcome or “broken down”. A discharge takes place to relieve the strain and make the two bodies electrically equal.

This is just what happens in the case of lightning. A cloud containing countless drops of moisture may become oppositely charged with respect to another cloud on the earth. When the electrical pressure between the two becomes great enough to break down the insulation of air between them, a lightning flash occurs. The discharge follows the path which offers the least resistance. That’s why lightning often zigzags.

The ability of air to conduct electricity varies with its temperature, density and moisture. Dry air is a pretty good insulator, but very moist air is a fair conductor of electricity. That’s why lightning often stops when the rain begins falling. The moist air forms a conductor along which a charge of electricity may travel quietly and unseen.

What about thunder? When there is a discharge of electricity, it causes the air round it to expand rapidly and then to contract. Currents of air rush about as this expansion and contraction take place.  The violent collision of these currents of air is what that we hear as a thunder. The reason thunder rolls and rumbles when it is far away is that the sound waves are reflected back and forth from cloud to cloud.

Since light travels at about 186,284 miles (299,795 kilometers) per second and sound at about 335 meters per second through air, we always see the flash first and then hear the thunder later.

Quantity of charge:

The same amount of charge is in fact carried by each and every electron in any given substance or material as such therefore, the more electrons an object loses or gains (by rubbing say, for example), the greater shall be its positive or negative charge whatsoever. Charge is measured in coulombs (C), a coulomb being the charge on about 6 million million million (6 x 10¹⁸) electrons.

Current Electricity

What is Current? The amount of charge passing per unit time through a conductor is called as current or in other words, Electric current is an electric charge in motion. In a solid conductor, such as a wire, the current consists of a swarm of moving electrons, while in certain liquids and in gases the carriers may include positively and negatively charged atoms as well. In addition, a beam of electrons or charged atoms for that matter may be made to travel in vacuum wherein, no conductor is being involved at all. Such a beam amounts to a current just as much as one in a wire or through a conductor. An electromotive force (e.m.f.) provided by a cell or a generator is essential to maintain a flow of current.

AC and DC currents: If current flow in a circuit is in the same direction it is called a direct current. On the other hand, if the electron’s flow is alternately backward and forward, it is then referred to as an alternating current.

The practical unit of current is ampere and one ampere is the rate of flow of one coulomb of charge per second which means a flow of 6.3 million electrons per second. Conventionally, the direction of electrical current is always opposite to the direction of flow of electrons.

Charge will have a tendency to move from one place to another if an electric potential difference (P.D.) exists between two places. Electrical charge will always flow from a higher potential to a lower potential. Potential difference between any two points is equal to the work done in moving unit positive charge from one point to another. Its unit is volt.

What is Electric Power? It is the rate of doing work and is measured in units called as watts. Every electrical equipment carries a label mentioning about the working voltage of the equipment or instrument as well as the power consumption by the same in watts. A 100 watts bulb for example, will give more light than a 40 watts bulb, but it will also use up more electrical power. Approximately, one unit of electricity is consumed by a 100 watt bulb in 10 hours (1 kilowatt hour = 1000 watts x 1 hour.)

Typical power consumptions of some of the common home appliances are:

Electric Current

The flow of electric current through a conductor produces several useful effects. They include (1) heat, (2) light, (3) magnetism, and (4) chemical effects.

Heat – When electricity flows through a conductor, the resistance of the conductor converts some of the electric energy into heat energy. Certain electric devices such as cookers, heaters, and toasters, generate heat by passing current through special heating units. These units are made of materials that have a fairly high resistance to current. In an electric cooker, for example, electricity travels through coils of special wire in the heating unit. The resistance of the coils causes them to become red hot. The coil of wire is wound on mica or fireclay frame which serve as insulators and are able to withstand high temperatures.

Light – The atoms of all substances contain energy. Ordinarily, an atom has a certain energy level. If an atom absorbs additional energy, it moves to a higher energy level. Such an atom is called an excited atom. After absorbing the additional energy, the atom soon drops back to a lower energy level. When the atom drops back, it gives off its excess energy in the form of light.

Connecting concepts: How does a filament lamp (bulb) glow or produce light?

 As the current flows through a conductor, it always generates some heat energy. The production of this heat energy in fact, causes a conductor to give off light. And this is how an incandescent light bulb glows or gives off light to light our homes. As the Current flows through the filament of a bulb, it makes the filament much hotter in consequence of which some of its atoms are excited to higher energy levels after being heated too much. When the said excited atoms drop back to their original lower energy levels, the filament glows up and thus, give off light.

In fact, the principle that higher the temperature of the filament to which it is heated, the greater shall be the proportion of electric energy being changed to light. Since the lamps are being noted for producing a good amount of glowing light, it is necessary that their filament should be heated up to a considerable level of temperature. This further calls for a need to have the filament made of such a material that carries a pretty high melting point.  And thus, this requirement is fulfilled by a material called as tungsten, a common material of which the filament is made of that is essentially a metal with a high melting point of (3400⁰C). Why do the filament lamps are gas-filled containing either nitrogen or argon instead of air?  Just because, the air being a mixture of gases should inevitably contain oxygen which could have easily caused the combustion and hence, of the filament no matter, the same is made of a material like tungsten. At the same time, the gases like nitrogen or argon reduce the evaporation of the tungsten which could have otherwise, condensed onto the bulb and have blackened it. Likewise, the compactness of the filament coil reduces cooling by convection currents being set in the gas. Unfortunately, a conventional filament lamp converts only 10% of the electrical energy being supplied to it. The rest 90% just gets wasted in the form of heat. This is the reason of their being less energy efficient and are increasingly being replaced by energy efficient fluorescent lamps…

Advantage Fluorescent Lamps: (Question asked in GS-pre-2011)

They are three times as efficient as filament lamps and may even last for as much as over 3000 hours in terms of longevity as compared to a life span of hardly over 1000-hours of the conventional filament lamps. Although, fluorescent lamps cost more to install or in terms of their per unit price, but then, their running costs compensate this differential. Moreover, being the extended sources of light, fluorescent lamps also cause fewer problems of shadows.

Fluorescent lamps don’t carry any filament in them as against the conventional bulbs such that in these lamps, electricity is passed through vapors of different metals at very low pressures. The colour of the light produced by a given fluorescent lamp depends on the particular metallic vapor used say for example, mercury vapors will give green and sodium will give yellow light. They are thus, used for street lighting and named respectively as mercury or sodium lamps.

Similarly Neon lamps which gives bright orange-red coloured light, is used in advertising signs.

Noted that besides, giving out a coloured light source, a mercury vapour discharge tube also emits ultra-violet rays. When ultra-violet radiations which are essentially invisible rays, falls on certain minerals, they glow brilliantly with various colours. This phenomenon is called as fluorescence. Accordingly, the inside of a mercury discharge tube is coated with a mixture of various metallic powders which give out either a white or a tinted source of light. Some of these powders may contain beryllium compounds which however, are highly poisonous if they somehow enter a cut in the skin.

What is an electric Arc Lamp? The electric arc lamps are used in searchlights and as projection lamps, where an intensely concentrated source of light is required. It has metal electrodes surrounded by xenon gas. The temperature reached in the electric arc is in the region of 3700⁰C. This is well above the melting point of metals and therefore, the main applications of the arc lamps are in electric furnaces and welding equipments.

Electricity to Magnetism to Solenoids & the construction of Electromagnets & their use in Electrical Appliances:

A conductor carrying electricity is always surrounded by a magnetic field. For example, current flowing through a wire always sets up a magnetic field around the wire. If the said wire, carrying current and associated magnetic field along is wound into a coil, the magnetic field surrounding the wire is further strengthened. Such a coil is called a solenoid. If a soft iron rod is placed inside this solenoid, the current in the solenoid magnetizes the said iron too and consequently, even a much stronger magnetic field can be achieved. These are the discrete steps followed in the construction of electromagnets which have their application in almost every electrical equipment.

Most electromagnets in fact, consist of a solenoid wound around an iron core. The core, being magnetically soft, loses its magnetism when the current is switched off. An electromagnet is therefore, made to function as a temporary magnet such that its strength increases if either the current in the coil increases or the number of turns on the coil increases or for that matter, the poles are closer together.

There is also a small piece of soft iron used along with the electromagnet. This is called the armature that is attracted by the electromagnet when current flows through it.

Applications of Electromagnets: Electromagnets are used in as diverse applications as for lifting and transporting heavy steel items and for separating iron objects.

They are used in telephone receivers, electric motors, cyclotrons, etc.

And so far as the inherent magnetic effect of electricity is concerned, it finds its applications in constructing some measuring instruments such as the moving-coil galvanometer which has a high sensitivity and therefore, can be used to detect small currents of the order of even a hundred-millionth of an ampere.

The magnetic effect of the current and hence, the electromagnets find their one of the most pronounced uses in so called electric motors. An electric motor converts electrical energy into mechanical energy. It works on the principle that when an electric current is passed through a conductor kept in a magnetic field, a force acts on the conductor as a result of which the conductor begins to move (magnetic effect of current). Commercial motors convert about three-quarters of the electrical energy supplied to them into mechanical work. Electric motors are basic to the working of many gadgets.

 Similar magnetic effect of electricity and hence, electromagnets is found in the working of a Loudspeaker. A loudspeaker as such, converts electrical energy into sound energy. In this case, the electric vibrations obtained from the microphone are first amplified and then sent to the voice coil in a magnetic field through the terminals. In the loudspeaker eventually, the electrical energy is transformed into mechanical energy of vibrations in a cone and then ultimately to sound.

Electromagnetism in artificial pace-makers of the heart: The typical effect called as Electromagnetic induction is the real principle behind the working of artificial heart pace-makers and has enabled many people to lead active and useful lives through the medium of such heart pacemakers. By this effect, a tired heart may be made to beat regularly at a controlled rate to suit the user’s requirements in which a “tick-or-silent” switch gives assurance of correct functioning and if the supply plug becomes accidentally disconnected, the generator automatically gives a warning buzz to alert the patient.

The phenomenon of electromagnetic induction being induced in an induction coil also finds its most important application in some wireless transmitters as well as in internal combustion engines used in motor cars wherein, an induction coil plays its part in producing a high tension spark by which combustion of fuel is initiated in an engine.

The transformer, an appliance by which the amplitude of an alternating current, but not that of the DC current, can be increased or decreased, also works on the principle of electromagnetic induction.

“Step-up transformers” are used in power transmission at the generating station, in television and wireless sets for stabilizing the required voltage.

“Step-down transformers” are however, used in electric bells, in radio-sets for valve heaters and in power sub-stations so as to step down the voltage before its distribution to the consumers.

Last, but not the least, the appliances such as Generators or dynamos, meant to convert mechanical energy to electrical energy also works on the principle of electromagnetic induction.                     

Connecting concepts: The working of a wired telephone.

As we know that a telephone is a device by which speech (sound) can be conveyed from one place to another. Structurally, a telephone set is mainly made of a receiver, a transmitter (microphone); both are being connected by live wires. (Interestingly, today in a cell phone handset, we have both receiver & a transmitter combined in a single device and to call it a transceiver). A steady current is passed through the microphone by connecting it in series with a battery and the primary of a step-up transformer.

 Thus, during a talk or conversation, when the sound waves enter the microphone, the changes in resistance will cause the current in the primary circuit to vary, in consequence of which a high voltage AC is set up in the secondary of the transformer with the same frequency as that of the original sound waves entering the microphone. The AC so set up is then transmitted along lines to the receiver at the other end where the electric energy is converted back into sound energy.

Although, it is possible to transmit messages without using a transformer, but only over short distances. For long distance transmissions since, the line resistance remains to be so great and massive that even the small changes of resistance in the microphone would have almost a negligible effect on current change and the sound also would be too weak to be heard. May it be noted that in all forms of electrical sound-reproducing as well as sound recording devices say, as in the telephone, the first step in the process is always the conversion of sound vibrations into an electric current.

How does a tape recorder work?

In a tape-recorder, a plastic tape coated with magnetic oxide passes beneath the core of a coil that carries the varying ‘voice currents’ and so becomes permanently magnetized in the pattern of the original sound waves to reproduce the sound.

 The tape that has been magnetized and having an impression of the original sound impressed on it is now run past another coil and the magnetic pattern having been impressed on the same is then changed by electromagnetic induction into a variable current once more. This current is now amplified and fed into the loudspeakers so as to convert it back into a sound. In fact, the magnetic pattern that has been impressed on a tape above may be ‘erased’ by passing or subjecting the same in between the poles of a magnet and hence, the same tape is now ready for reuse…

Power Generation, its Transmission and Domestic Installations:

What kind of power is being supplied to homes? One of the main advantages of alternate current (AC) is that it can be easily and cheaply changed from one voltage to another by a transformer with a very little loss of energy. For this reason, the electric power is generally conveyed as AC current from power generating stations by means of high voltage overhead power lines what we call as GRID. From the grid, the said power is transmitted as AC to homes for domestic use just because; it can be transformed to very high voltage and transmitted over long distance with minimum power loss.

Electricity is generated in a typical power station at 11000 V and then stepped up to 132,000 V (132 (KV) by transformers located at the power station itself. It is then fed into the grid at this voltage (132 KV) and subsequently, stepped down in successive stages to about 33000 V and then to 6600 V at sub-stations in the neighborhood of towns and other areas where the energy is to be consumed.

What is a grid? A Grid is a system of overhead wires connecting large power stations in the country and feeding their power to any part of the country through the agency of power sub-stations. The main sub-stations are inter-connected to several generators in the country thereby, forming a complex system of interconnections what we call a grid system. The great advantage of grid system is that if there is a failure of power in one station, the power can be tapped or accessed from some other station in the said grid system. Big consumers like factories, etc. get their power at high voltage of 6600 V. For domestic consumers on the other hand, the voltage is stepped down to 220 V which is its effective value; the peak voltage value is however, 311 V.

What constitutes a Household Circuit?

In our homes, we receive supply of electric power through mains, either supported overhead on electric poles or by underground cables. One of the wires in this supply, usually with red or brown insulation on it, is called a live wire (or positive). Another wire among them with black or blue insulation on it is called a neutral wire or (negative wire). Potential difference between them is 220 volts (in India).

The so called neutral wire is earthed at the local substation and although, current does pass through it, but one may not get a shock if he/she happens to touch it accidentally anyway just because, the PD (potential difference) between it and the earth is zero.

 At the meter-board in the houses, these wires go into the watt-hour meters through a main fuse – i.e. a double fuse, one each in both the wires. Then through a main switch they are connected to live wires-

 Connecting concepts: What is a fuse? A fuse is a short length of wire made of some material with a low melting-point (often tinned copper) which melts and breaks the circuit immediately when the current through it exceeds a certain value. However, there could be two possible reasons for the flow of an excessive current through a circuit such as ‘short-circuits’ due to worn out of an insulation on connection wires and secondly, an overloaded circuit. A short circuit occurs when two wires, a negative and a positive, come into contact with each other. If it happens then the resistance of the circuit decreases considerably and consequently, the flow of the current increases enormously that amounts to heating up of the live wires to produce an electric arc. In the absence of a fuse, these would cause the wiring to become hot with the consequent risk of a grave fire.   

in the homes. The main switch is also a double switch, one in each wire. When it is in “off” position both live wires are disconnected from the mains and any repair, etc. can be done easily. A second fuse of lower capacity than that at the main fuse in also connected in the live wire so that if there is any short circuit in this line, this fuse melts away rather than the main fuse. Various appliances in the house are then connected to these lives wires, each with its own independent switch. If switches were connected to the neutral wire, the sockets would remain alive even if the switches were in ‘off’ position. In order that each appliance gets an equal supply of voltage, they are connected in parallel with each other. This also ensures that when one is switched ‘on’ or ‘off’, others are not affected. The main line also carries a third wire (the earth wire) connected to the earth terminal. This is for safety purposes. In a big house, several pairs of lines start from the main switch and go into different portions of the house. Each pair of live wires carries its own fuse so that in case of a short circuit only the current of that portion of the house is cut off and other lines are not affected.

Connecting concepts: What do a “Plug” and its “wall Socket” represent in a common household circuit?  

A three pin plug is usually used in the sockets placed in the complete ring circuit of the house as shown in the picture below. The earth pin goes to the top connections on all power sockets and is longer so that the appliance is earthed before being connected to the live wire. The earth pin is connected to the metal case of the appliance which is thus joined to earth by a path of almost zero resistance. If, for example, the element of an electric fire breaks or sags and touches the case, a large current flows to earth and ‘blows’ up the fuse. Otherwise, the so called, metal case would become ‘live’ and anyone touching it would receive a shock which might be too fatal especially, if they were ‘earthed’ say, while standing on a concrete floor or otherwise, holding a water tap.

How to calculate the Cost of Electricity (in money terms) consumed by you?

Electrical energy used or consumed anywhere, is reckoned in kilowatt-hours (Kwh) as the units of consumption. The unit “watt” that remains written on electric lamps and on almost every other electrical appliance, is essentially an indicative of the unit of power or the rate at which electrical energy is being consumed by a particular appliance per unit time.

If we multiply the number of watts by the number of hours and divide the same by 1000 so as to convert the watts to kilowatts, then, what we have actually calculated is the electrical energy used up by us over a particular time period of course, in hours. For instance, a 60-watt lamp used for 100 hours, consumes an energy of 60 x 100/1000 = 6 kilowatt-hours, or 6 units (1-KWh= 1 unit of electricity consumption). The electricity bill is thus, generated in two parts: (a) a fixed amount which you have to pay monthly whether or not you have used any electricity during a particular month, for you have been away to vacations for two months and (b) the actual consumption of energy to be reckoned as a certain amount of bill on per unit basis of your energy consumption in a month or so…..