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41. What is the value of the angle of dip at the magnetic equator?

At any place on magnetic equator, the compass needle remains horizontal & the value of dip is zero. Hence, the value of dip is minimum at a place on the magnetic equator.

 

42. Why should one consider declination while using a compass needle in most of the earth?

Declination at a place on the earth's surface is defined as the angle between the geographical meridian & magnetic meridian at that place. This value varies from place to place on earth's surface. In general, its value at a place is constant but changes very slightly with time. So, one should consider declination while using a compass needle in most of the earth.

 

43. Earth has its terrestrial magnetism. Does this mean that the core of the earth is a huge permanent magnet?

Earth has its terrestrial magnetism. No, this does not mean that the core of the earth is a huge permanent magnet.  Before, earth is believed to have a huge magnet whose north pole lies to geographical south pole & south pole lies to geographical north pole. But at a very high temperature inside the earth, ferromagnetic material lose their magnetism & becomes paramagnetic materials so the core of the earth is not a huge permanent magnet. The latest theory assumes that the magnetic field of the earth is due to electric current circulating in its core due to directional current arising from radioactive heating inside the earth. It is assumed that the earth behaves as of a magnet is placed inside the earth.

 

44. An iron piece, buried for a long time underground, facing north south gets magnetized. Why?

As the earth is believed to have a huge magnet whose north pole lies to geographical south pole & south pole lies to geographical north pole. The existence of the earth's magnetic field is the main cause for an iron piece, buried for a long time acquire magnetism. The earth's magnetic field is uniform & if the iron piece is buried, then it is in the uniform magnetic field. In that field, the magnetic domains of iron which are oriented randomly in all directions are forced to oriented in one direction along the magnetic field of earth. If an iron piece is buried for long time then magnetic domains of iron have lost their restoring power and cannot orient each domain in different direction as at initial stage. So, magnetic domains of iron are oriented in one direction. Hence, an iron piece, buried for a long time underground, facing north south gets magnetized. 

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Magnetism 1