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Answer the following questions:
(a) The earth's magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?
(b) The earth's core is known to contain iron. Yet geologists do not regard this as a source of the earth's magnetism. Why?
(c) The charged currents in the outer conducting regions of the earth's core are thought to be responsible for earth's magnetism. What might be the 'battery' (i.e., the source of energy) to sustain these currents?
(d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth's field in such distant past?
(e) The earth's field departs from its dipole shape substantially at large distances (greater than about $30,000 \mathrm{~km}$ ). What agencies may be responsible for this distortion?
(f) Interstellar space has an extremely weak magnetic field of the order of $10^{-12}$ T. Can such a weak field be of any significant consequence? Explain.
[Note: Exercise $5.2$ is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]
(a) The earth's magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?
(b) The earth's core is known to contain iron. Yet geologists do not regard this as a source of the earth's magnetism. Why?
(c) The charged currents in the outer conducting regions of the earth's core are thought to be responsible for earth's magnetism. What might be the 'battery' (i.e., the source of energy) to sustain these currents?
(d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth's field in such distant past?
(e) The earth's field departs from its dipole shape substantially at large distances (greater than about $30,000 \mathrm{~km}$ ). What agencies may be responsible for this distortion?
(f) Interstellar space has an extremely weak magnetic field of the order of $10^{-12}$ T. Can such a weak field be of any significant consequence? Explain.
[Note: Exercise $5.2$ is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]
Solution:
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Verified Answer
(a) Yes, due to the motion of its plates and core the earth's magnetic field does shift. Noticeable changes occur over a large period of time (few hundred years ) and variations are there even in small durations of time (few years) which cannot be ignored.
(b) The core contains molten iron (magnetic domains are destroyed in molten form) which is not ferromagnetic hence it cannot contribute to magnetism of earth.
(c) No, one really knows.
(d) By study rock magnetism i.e., the solidification of rocks leads to trapping of materials in their magnetised states. Analysing these we can get clues to the history of geomagnetism.
(e) At large distances external factors such as solar winds, ionospheric particles etc. influences the earths magnetic field, hence it gets distorted.
(f) From cyclotron relation $r=\frac{m v}{e . B}$ we can say that small field can affect the path of moving charged particle whose radius is large eg. cosmic rays.
(b) The core contains molten iron (magnetic domains are destroyed in molten form) which is not ferromagnetic hence it cannot contribute to magnetism of earth.
(c) No, one really knows.
(d) By study rock magnetism i.e., the solidification of rocks leads to trapping of materials in their magnetised states. Analysing these we can get clues to the history of geomagnetism.
(e) At large distances external factors such as solar winds, ionospheric particles etc. influences the earths magnetic field, hence it gets distorted.
(f) From cyclotron relation $r=\frac{m v}{e . B}$ we can say that small field can affect the path of moving charged particle whose radius is large eg. cosmic rays.
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