106 Unit Twelve MAGNETISM READING PASSAGE Earth’s magnetic field Scientists still do not know exactly what causes the earth’s magnetic field. In the 16th century, it was believed that a mountain of magnetite (a magnetic mineral) was located at the North Pole and that this mountain caused the earth’s magnetic field. English physician William Gilbert was the first to propose, in 1600, that the earth itself was a massive magnet. Current theories hold that the earth’s magnetic field is created by currents within the liquid outer core of the earth, which is composed mostly of iron. This liquid core contains ions, or electrically charged atoms and molecules. The motion of these charged particles within the earth is thought to create the earth’s magnetic field. Scientists sometimes find it easier to model the earth’s magnetic field as if it were a perfectly symmetrical magnetic field coming from one large bar magnet. The poles of this hypothetical magnetic field, based on averaging the direction and strength of earth’s magnetic field, are called the geomagnetic poles. The north geomagnetic pole is located near Thule, Greenland, 1250 km (780 miles) from the geographical North Pole. The south geomagnetic pole is located near Vostok, Antarctica, 1250 km (780 miles) from the geographic South Pole. Paleomagnetism is the study of the earth’s magnetic field in ancient times. Scientists can study the earth’s ancient magnetic fields by measuring the magnetic orientation of certain rocks. When molten rock crystallizes, crystals of magnetic minerals align with the earth’s magnetic field. The rock thus records the direction of the earth’s magnetic field at the time of its crystallization. By measuring the magnetic orientation and determining the age of such rocks, scientists can measure the orientation of the earth’s magnetic field at different times in the earth’s history. Geologists can use systematic paleomagnetic measurements on rocks of different ages to map out the apparent movement of the earth’s magnetic field as a function of time. This map defines what is known as a pole path. Differences in the pole paths of the continents indicate the relative drift of these continents. Paleomagnetic measurements played a key role in the development of the theory of plate tectonics by providing evidence that the oceans grow from their centers outward. Strips of alternating magnetization found in the ocean floor on either side of the mid-ocean ridges were explained by assuming that oceanic crust continually splits and moves away from the mid- oceanic ridges. Additional crust is continually generated in the gap by the eruption of lava, and the solidifying lava records the orientation of the earth’s magnetic field at the time of 107 solidification. The continual creation of new crust produces a record of the direction of the earth’s magnetic field over time. Repeated reversal of the earth’s magnetic field produces strips of alternating magnetization in the ocean floor on either side of the mid-oceanic ridge. (From http://encarta.com ) READING COMPREHENSION Exercise 1: Answer the following questions by referring to the reading text 1. What was considered to cause the earth’s magnetic field? ………………………………………………………………………………………… ……………………………………………………………………………… 2. What is the latest assumption about the cause of the earth’s magnetic field? ………………………………………………………………………………………… ……………………………………………………………………………… 3. What does the writer mean by “this hypothetical magnetic field” in line 13? ………………………………………………………………………………………… ……………………………………………………………………………… 4. How can scientists measure the orientation of the earth’s magnetic field in the earth’s history? ………………………………………………………………………………………… ……………………………………………………………………………… 5. Why did the paleomagnetic measurements use to be important? ………………………………………………………………………………………… ……………………………………………………………………………… Exercise 2: Choose the best alternative 1. Scientists now a. have had enough evidence about the cause of the earth’s magnetic field b. is still working hard in finding the cause of the earth’s magnetic field. c. have not understood all about the cause of the earth’s magnetic field 2. William Gilbert a. was the first to find the cause of the earth’s magnetic field. b. was the first to make the assumption about the cause of the earth’s magnetic field. c. was the first to think that the earth itself was an enormous magnet. 3. The term geomagnetic poles refer to: a. the earth’s poles in its magnetic field b. the magnetic poles of the earth. c. the magnetic as well as the geographical poles of the earth. 108 4. Scientists in ancient time study the earth’s magnetic field by measuring the magnetic orientation and determining the age of a. the rocks of that time b. some kinds of rocks of the time c. a special kind of rock of that time 5. Geologists use the result of paleomanetic measurements in order to a. find out the pole paths of the earth’s magnetic field. b. to study about the world’s continents. c. to map out the magnetic field of the earth’s as a function of time Exercise 3: Match each word in column A with its definition in column B A B 1. mid-ocean ridge 2. strips 3. eruption 4. role 5. orientation 6. crystallization 7. symmetrical 8. magnetization 9. solidification a. the act of something coming out very suddenly and violently b. the process of turning into crystals c. a particular function d. having two halves which are exactly the same, except that one haft is the mirror image of the other. e. long narrow pieces of something f. the area in the middle of the ocean g. the direction an object faces or the direction of the line along which the body exists h. the process in which a liquid or semi-liquid turns into a solid i. the process of giving a substance the power to draw iron and other metals towards it. 109 GRAMMAR IN USE The gerund 1. Gerund forms A gerund is formed exactly like a present participle form of a verb (i.e. simply by adding –ing to the end of the verb) as follows: Active Passive Simple conducting being conducted Perfect having conducted having been conducted 2. Implications of the gerund a. A simple gerund refers to something happening at the same time as the action in the main clause Example : Conducting such an experiment is very dangerous. (In this case the action of conducting is at the same time with the being dangerous) b. A perfect gerund refers to something happening earlier than the action in the main clause Example : He made the report without having made enough observations. (the action of not making observation happens before the action of making report) It’s unnecessary to use a perfect if it is clear from the context that the time of the gerund is earlier than the time of the action in the main clause: The above example can be rewritten in this way: He made the report without making enough observation. (Everyone knows that to make a report, before that enough observations must be made for the sake of scientific accuracy). c. To form a negative we simply add the negative word ‘not’ before the gerund Example : He’s afraid of not being able to complete his thesis. 3. Gerund clause A gerund on its own or can be combined with other elements (with subject; object; or adverbial) to form a clause: Examples: i. conducting such an experiment ii. not being able to complete his thesis iii. having made enough observations 4. Functions of the gerund 110 A gerund can be treated as a noun/noun phrase, so it has a wide range of uses as a noun/noun phrase a. A gerund can be treated as a noun on its own or can be used to modify another noun Example: 1. Neighboring molecules beneath them are set into faster motion, and heat flows into the floor. 2. This is called the microwave background, and is the remnant of radiation from the ‘big bang’, the giant explosion that is believed to have occurred at the beginning of time in the Universe. 3. Perhaps nothing is so ingrained in our senses as the perpetual pulling of the earth on our surroundings. 4. The physical meaning of this new concept – mass- is related in the most intimate way to the identity in comparing weights which we have just noted. 5. It’s just that nothing would stop that fall, there would be no supporting force opposing the gravitational pull, so he would feel weightless. b. As a subject of a sentence Example: 1. Sitting and observing a substance to decay is a hard work. 2. Finding the cause of the earth’s field has taken much time of the scientists throughout the story. When functioning as the subject of a sentence, a gerund can be replaced by a to- infinitive, but there is a difference between them. While a gerund refers to something in general, a to-infinitive refers to something in a particular situation. c. As a complement after be Example : What he has to do now is quickly finishing his research. d. As a prepositional complement d.1. As a complement after a preposition following a noun Example : 1. The question of finding the cause of the earth’s magnetic field has drawn a great attention from physicists. 2. We can not see electric current in a wire, so we find different ways of explaining what is going on. The following nouns with their prepositions can be followed by a gerund 111 advantage of aim of/in amazement at anger about/at annoyance about/at anxiety about apology for awareness of belief in boredom with danger of/in difficulty (in) effect of excitement about/at expense of/in fear of insistence on interest in job of matter of objection to pleasure in/of point of/in possibility of problem in/of worry about prospect of purpose of/in question about/of reason for satisfaction with success in surprise at task of work of d.2. As a complement after a preposition following an adjective Example: 1. This internal energy spreads out, making it much less effective at producing work than, say, the organized kinetic energy of a moving object. 2. Energy has been called the “common denominator” of the natural science because its conservation law makes it so useful in understanding any physical process. The following are the adjectives with their prepositions that can take a gerund afraid of amazed at angry about/ at annoyed about/at anxious abut ashamed of aware of bad at bored with capable of content with dependent on different from/to excited about famous for fed up with fond of good at grateful for guilty of happy about/with interested in keen on nervous of pleased about/with ready for responsible for satisfied with sorry about/for successful in surprised at worried about wrong with Other prepositions that are followed by a gerund after against as a result as well as because of besides by by means of despite for in in addition to in favor of in spite of instead of on account of since through what about with 112 before how about on without Example: 1. Absorption spectra of gases may be obtained by passing white light through a sample of gas before the light enters the prism. 2. By measuring the magnetic orientation and determining the age of such rocks, scientists can measure the orientation of the earth’s magnetic field at different times in the earth’s history. 3. Using springs of various thickness, one can make scales for measuring very large and also very small weights. e. As a complement after adjectives expressing degrees of difficulty Example : 1. It’s difficult detecting an element without using a spectroscope. 2. It’s impossible keeping a car in steady speed. Note: Refer to the Grammar in use part in UNIT ELEVEN for these adjectives. However, It’ is more common to use a to-infinitive instead of a gerund. f. As a direct object after some verbs These verbs include some one - word verbs and some prepositional verbs Example : 1. We can live quite happily without thinking about why this is so. Once we start thinking about the force of gravity , which makes things fall, we may come up with some odd ideas. 2. He’s thinking of taking a course of physics. 3. Some energy goes into raising the temperature of the cylinder walls and the piston, and that part spreads outward, doing no useful work The following verbs take gerund as a direct object 113 admit advise allow anticipate appreciate avoid can’t help confess consider delay deny detest dislike enjoy escape excuse face finish give up imagine involve justify keep (on) leave off mention mind miss permit postpone practice put off quit recommend resist resume risk save suggest tolerate The following prepositional verbs can take a gerund admit to (dis)agree with aim at apologize for (dis)approve of believe in benefit from care for confess to count on depend on feel like get on with insist on object to pay for put up with rely on resort to succeed in think of vote for Note : Of all the adjectives, nouns, and verbs listed, many may not be used frequently in a document of purely scientific matter (with technical sense only). However, if you have a chance to get access to wider range of reading materials (especially those about our universe and human beings, which are viewed in many aspects of physics), you can find them with more frequent use. PRACTICE Exercise 1 : Complete the sentences with the correct form of the verbs given: to-infinitive or gerund or present participle. State each case of a gerundthat is used. 1. You can use your knowledge of how charged particles and electric currents are affected by fields (interpret) ……………… diagrams of (move) ……………… particles. 2. You can use such an arrangement (observe) ……………… the effect of (change) ……………… the strength and direction of the field, and the effect of (reverse)………………the field. Note that you can seriously damage a television set by (bring) ……………… a magnet close to the screen. 114 3. You can make a field in two ways: (use) ……………… a permanent magnet, or (use) ……………… an electric current. There is really no fundamental difference between these two ways of (create) ……………… magnetic fields. You should be familiar with the magnetic field patterns of bar magnets. These can be shown up (use) ……………… iron bar fillings or (plot) ……………… compass. We represent magnetic fields, like gravitational and electric fields, by (draw) ……………… lines of force. 4. In a solenoid, (reverse) ……………… the current reverses the direction of the field. 5. Here are some useful rules for (remember) ……………… the direction of the magnetic field produced by a current: • The right - hand grip rule gives direction of field lines in an electromagnet. Imagine (grip) ……………… the coil, so that your fingers go around it (follow) ……………… the direction of the current. Your thumb now points in the direction of the field lines inside the coil, i.e. points towards the electromagnet’s north pole. • The corkscrew rule is a way of (remember) ……………… the direction of the field lines around a current - carrying wire. Imagine (push) ……………… a corkscrew into a cork, and (turn) ……………… it. The direction in which you push is the direction of the current, and the field lines go round the direction in which you are turning the corkscrew. 6. The magnet creates a fairly uniform magnetic field. The rod has a current (flow) ……………… through it. As soon as the current is switched on, the rod start (roll) ………………, (show) ……………… that a force is acting on it. We use Fleming’s left-hand rule (predict) ……………… the direction of the force. There are three things here, all of which are mutually at right-angles to each other – the magnetic field, the current in the rod and the force on the rod. These can be represented by (hold) ……………… the thumb and first two fingers of your left hand so that they are mutually at right-angles. Your fingers then represent: thuMb-Motion; First finger- Field; seCond finger-Current. You should practice (use) ……………… your left hand (check) ……………… that the rule correctly predicts these directions. 7. Scientists have put considerable effort into (research) ……………… for particles that have just one magnetic pole (magnetic monopoles). 8. We can generate electricity by (spin) ……………… a coil in a magnetic field. This is equivalent to (use) ……………… an electric motor backwards. 9. Another use of electromagnetic induction is in transformers. An (alternate) ……………… current in the primary coil produces a (vary) ……………… magnetic field in the core. The secondary coil is also wound round this core, so the flux (link) ……………… the secondary coil is constantly changing. Hence a (vary) ……………… e.m.f. is induced across the secondary. 115 10. Ampere’s (find) ……………… revealed that when a charged particle crosses magnetic lines, it gets pushed to one side. 11. The tendency of a compass needle (dip) ……………… is a nuisance for compass users. (eliminate) ……………… this motion in a compass made for use in North America, the needle is suspended off center, or even counterweighted on the southern end, so that it moves only in the horizontal plane of the compass. 12. Electromagnets are the (work) ……………… parts of some of the instruments used (measure) ……………… currents and voltages. 13. In 1681, an English ship (sail) ……………… to Boston was struck by lighting. After the storm had passed, the sailors noticed that the ship’s compass no longer pointed north. Somehow, the lighting had reserved the magnetic poles. Nevertheless, (use)……………… the wrong end of the compass for orientation, they came safely into Boston Harbor. 14. A person moves by (push) ……………… off from the Earth; a boat sails because the rowers push against the water with their oars; Thus, (push)………. off from a support seems (be) ……………… a necessary condition for motion; even an airplane moves by (push) ……………… the air with its propeller. But is it really? Might there not be some intricate means of moving without (push) ……………… off from anything. 15. If you rub a strip of plastic so that it becomes charged, and then hold it close to your hair, you feel your hair (pull)……………… upwards. PROBLEM-SOLVING Paragraph building Task one From the prompts given, build up sentences with the addition of the supplementary material above each set. Delete the words /phrases in Italic 1. MAGNETIZED/ SO THAT/ UPRIGHT/ WITH/ PROTRUDING/, AND/ HORIZONTALLY Place a knitting needle in a cork it will float in a trough of water its north pole will just protrude out of the cork support a bar magnet above the water ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………… 2. THAT/ ;/ (1791-1867)/ TO/ OF/ WHAT/ SURROUNDING it is important to realize this lines of force have no objective existence [...]... calculation of the fields everywhere and any time from knowledge of the charges and currents An unexpected result arising from the solution of these equations was the prediction of a new kind of electromagnetic field, one that was produced by accelerating charges, that was propagated through space with the speed of light in the form of an electromagnetic wave, and that decreased with the inverse square of the... universally applicable law of physics, but rather describes the behavior of a limited class of solid materials The historical concepts of magnetism, based on the existence of pairs of oppositely charged poles, had started in the 17th century and owe much to the work of Coulomb The first connection between magnetism and electricity, however, was made through the pioneering experiments of the Danish physicist... force the particles toward the negative terminal, overcoming the opposing forces of Coulomb's law The German physicist George Simon Ohm first discovered the existence of 123 a simple proportionality constant between the current flowing and the electromotive force supplied by a battery, known as the resistance of the circuit Ohm's law, which states that the resistance is equal to the electromotive force,... the case of the electromagnetic field one can think of the electric-field strength as taking the place of the upand-down motion of each piece of the string, with the magnetic field acting similarly at a direction at right angles to that of the electric field The electromagnetic-wave velocity away from the source is the speed of light (From http://encata.com) Order in matter Matter is composed of atoms... ………………………………………………………………………………………… ……………………………………………………………………………… 8 AS/ AN INDEPENDENT/ WHEN/ IT/ FREE TO MOVE we can define a line of magnetic force a line of magnetic force is the path of a needle the path is traced out by a north pole the north pole is under the influence of a magnet ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………… 9 THE/ ROUND A MAGNET/... detection of neutrino oscillation (one of the top physics stories of 1998) stands on a firm understanding of the complex chain of events whereby a cosmic ray in outer space leads to a burst of light in a cavern beneath Japan 2 Mar’s magnetism The Mars Global Surveyor spacecraft has discovered patterns of magnetized surface rock—broad stripes of magnetic material pointing in one direction alternating with... Rudolf Hertz succeeded in actually generating such waves by electrical means, thereby laying the foundations for radio, radar, television, and other forms of telecommunications The behavior of electric and magnetic fields in these waves is quite similar to that of a very long taut string, one end of which is rapidly moved up and down in a periodic fashion Any point along the string will be observed to... north pole of the needle will be near the north pole of the magnet release the needle ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………… 4 THE EXISTENCE OF/ BY A SIMPLE EXPERIMENT lines of force exist this may be demonstrated ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………… 5 TO THE SOUTH POLE OF THE MAGNET... traced out by the needle the lines are drawn the lines indicate lines of magnetic force ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………… 7 THAT IS/ TEND TO/ WHICH the force acts along definite lines magnetic poles will be driven along certain lines 117 these lines are called lines of force ………………………………………………………………………………………… ………………………………………………………………………………... testify to the changing nature of Earth's magnetic field and to the recurring upwelling of magma resulting from the movement of tectonic plates above a seething molten planetary core The conclusion: Mars too might have experienced tectonic activity 3 Magnetism and you Anywhere there is an electric current, there is a magnetic field: in a bolt of lighting, in the circuits of a hand calculator, and even . job of matter of objection to pleasure in /of point of/ in possibility of problem in /of worry about prospect of purpose of/ in question about /of reason for. besides by by means of despite for in in addition to in favor of in spite of instead of on account of since through what about with 112 before how about on