detailed, easy to understand and fun to learn with pictures.. :)
- 1. Now we shall try to explain some observableevents on the earths surface from anunderstanding of the earths interior.We would like to emphasize, however thatscientists did not necessarily arrive at theseexplanations in the same manner. In fact, they used the knowledge gained fromstudies of these phenomena to build a modelof the earths structure.
2. Do you want to know moreabout Earths Magnetism? 3. As early as 2600 B.C., that Chinese knew that a certain kind of rock, when shaped into a rod and allowed to turned freely, pointed in a north-south direction.Ancient Greeks discovered a certain mineral they named magnetic could attract iron. 4. 1100 A.D. the freely turning magnet hadbecome a standard for navigation. 1400 A.D. sailors knew that the needle-shapedmagnet, now called a compass, did notalways point exactly to the north pole. It sometimes pointed a little to the eastor a little to west of north. 5. Do you want to know more about Gilberts Experiment? 6. Who is William Gilbert? Dr. William Gilbert Born:24 May 1544 Colchester Died:30 November 1603 (aged 59) London Occupation: Physician Known for: Studies of magnetism 7. William Gilbert, also known as Gilberd, (24 May 1544 30 November 1603) was an English physician, physicistand natural philosopher. He was an early Copernican,and passionately rejected both the prevailingAristotelian philosophy and the Scholastic method ofuniversity teaching. He is remembered today largely forhis book De Magnete (1600), and is credited as one ofthe originators of the term electricity. He is regarded bysome as the father of electrical engineering orelectricity and magnetism. While today he isgenerally referred to as William Gilbert, he also wentunder the name of William Gilberd. The latter was usedin his and his fathers epitaph, the records of the townof Colchester, and in the Biographical Memoir in DeMagnete, as well as in the name of The Gilberd Schoolin Colchester, named after Gilbert.A unit of magnetomotive force, also known asmagnetic potential, was named the gilbert in hishonour. 8. In 1600 William Gilbert published a bookentitled De Magnete wherein heexplained the behavior of the magneticrock. He proposed the earth itself was a hugespherical magnet with its poles situatedalmost at the geographic poles. 9. Gilbert discovered that when a compass isheld over different points of a sphericalmagnet, it behaves in about the same way asit does when it held at different parts of theearth. Thus, he concluded that the earth is aMAGNET. As a magnet, the earth is surrounded by amagnetic field whose lines of force meet at themagnetic poles. 10. Compass and the magnet actually alignthemselves to the line of the magneticforce. This alignment makes the compass pointnorth-south. Its because the earths magnetic poles arenot located exactly at the geographicpoles. At present the magnetic north pole islocated at 75N latitude and 100Wlongitude. 11. Magnetic south pole is at 66S latitudeand 140E longitude. Magnetic north pole is only about 1900km away from the geographic northpole a distance slightly less than 0.05%of the earths circumference. The angle by which a compass needleturns slightly away from the geographicnorth pole, whether east or west of it, iscalled the magnetic declination. 12. Magnetic DeclinationThese varies at different places on Earth. 13. End of the compass or magnet thatpoints north is called the north seekingpole or N-pole. Opposite poles attracts. Therefore, the north seeking pole mustbe the south pole of the compass ormagnet. End that points the south is the southseeking pole, o t the S-pole. 14. South seeking pole is actually the north poleof the compass or magnet. Thus, when a compass is placed near amagnet, the north-seeking pole of thecompass is attracted to the south-seekingpole, or the S-pole of the magnet. Horizontal compass is free to turn around avertical axis and a horizontal plane. However, if a compass were free to turnaround in all directions it should not onlypoint north-south but also tilt to one side soalong as it is not at the magnetic equator. 15. A compass constructed specifically toallow tilting is called tilting or dippingcompass. It turns on a horizontal axis and on avertical plane. The angle formed between the dippingneedle and the horizontal plane is calledthe magnetic inclination. 16. A Dipping Compass 17. Magnetic Inclination 18. Gilberts theorythat the earth is agiant magnet isnow accepted inmodern science.But it led toanother question:What caused theEarthsMagnetism? 19. Do you want to know more about magnetic field? 20. A magnetic field is a mathematicaldescription of the magnetic influence ofelectric currents and magnetic materials.The magnetic field at any given point isspecified by both a direction and amagnitude (or strength); as such it is avector field.[nb 1] The magnetic field is mostcommonly defined in terms of the Lorentzforce it exerts on moving electric charges.There are two separate but closely relatedfields to which the name magnetic fieldcan refer: a magnetic B field and amagnetic H field. 21. Magnetic fields are produced by movingelectric charges and the intrinsic magneticmoments of elementary particles associatedwith a fundamental quantum property, theirspin. In special relativity, electric and magneticfields are two interrelated aspects of a singleobject, called the electromagnetic field tensor;the aspect of the electromagnetic field that isseen as a magnetic field is dependent on thereference frame of the observer. In quantumphysics, the electromagnetic field is quantizedand electromagnetic interactions result fromthe exchange of photons. 22. Magnetic fields have had many uses inancient and modern society. The Earthproduces its own magnetic field, which isimportant in navigation. Rotating magneticfields are utilized in both electric motors andgenerators. Magnetic forces giveinformation about the charge carriers in amaterial through the Hall effect. Theinteraction of magnetic fields in electricdevices such as transformers is studied inthe discipline of magnetic circuits. 23. What is this Permanent Theory? 24. The simplest theory on the origin of theearths magnetism is that the center ofthe earth is one huge permanentmagnet made of iron and nickel. Earths core consists mainly of these twomainly materials, these theory seemsacceptable at first. Laboratory studies show that iron loses itsmagnetism at 770C and nickel at 358C. 25. NegativeExperimentWhat is Negative Experiment? 26. Another hypothesis was proposed byP.M.S. Blackett, an English physicist. He claimed that the magnetism of thesun, the stars and the earth was causedby their rotation. He set out to prove his theory bydeveloping a highly sensitive instrumentfor measuring magnetism, themagnetometer. 27. Blackett failed, however, to prove histheory. His report entitled NegativeExperiment, became widely acclaimedbecause of the full description of themethod he used and the precisemeasurements he made. 28. Although he did not succeed in provinghis theory, the experiment was not acomplete failure because of themagnetometer that he developed. This instrument later proved to be a mostuseful tool when Blackett began to studythe magnetism of rocks. 29. Do you want to know more about Dynamo Theory? 30. Walter Elsasser and Edward Bullarddeveloped a hypothesis that presentedthe earth as an electromagnet ratherthan a permanent magnet. They attributed the earths magnetism toelectric current produced in the earthsinterior, which in turn is continuallyproduced by the movement of the liquidouter core. 31. The inner and outer core move withdifferent speeds. It is this difference in speed that is thesource of magnetism. This is known as the Dynamo Theory. 32. A dynamo, or generator, is a device forproducing electricity by rotating anelectrical conductor across magnetic linesof force. If electricity is to be produced continuously,there should be a continuous rotation of theconductor. In the earth, the energy us thought to comefrom convection currents set in motion byheat produced by radioactivity in the core. 33. The dynamo theory involves a chain ofcomplex processes taking place in theearths core. Conditions necessary if the earth is to worklike a dynamo: the core of the earth must consist of a materialthat conducts electricity as well as metal does; the material must be in liquid form; The conducting liquid must flow in some way,the stirring processes providing the energyneeded to sustain the field. 34. 1940s, scientists became interested inpaleomagnetism, or the history of theearths magnetism. Development of instruments formeasuring magnetism in rocks togetherwith the refinements made in thetechniques for establishing rock agesgreatly aided scientists in their study offossil magnets. 35. These are bits of magnetic materials (iron) inrocks which have preserved the magnetismof the earth at the time the rocks wereformed. Many rocks contain iron-bearing mineralswhich are naturally magnetic. Studies of rocks in one place but whichwere formed at different geologic timesreveal varying magnetic directions andstrengths. 36. One interpretation states the earth isfixed and it is the magnetic poles whichare shifting in position. Another interpretation is that themagnetic poles are fixed and it is thewhole earth that moves with respect tothese poles. A third interpretation is that themagnetic poles are fixed only the upperlayers of the earth are shifting about. 37. Fossil rocks also distinguish between thenorth and the south pole. There is a strong evidence that the polesof the earths field have reversed manytimes in the past. The dynamo theory presents onepossible explanation: the direction ofthe magnetic field is influenced by thedirection of the movement of molteniron in the core.