Physics · Ch 3 — Magnetism and Magnetic Effects of Electric Current
Earth's Magnetic Field and Magnetic Elements
Earth's Magnetic Field and Magnetic Elements
A freely suspended magnetic compass needle comes to rest approximately along the Earth's geographic north-south direction. The needle's north pole is actually attracted toward the Earth's magnetic south pole, which happens to lie near the geographic north pole -- this is why "north-seeking" pole terminology can be confusing. The branch of physics dealing with the Earth's field is called geomagnetism or terrestrial magnetism.
Day and night occur because the Earth spins about its geographic axis; the vertical plane through this axis is the geographic meridian, and the great circle perpendicular to the geographic axis is the geographic equator. Similarly, the straight line joining the Earth's two magnetic poles is the magnetic axis; the vertical plane through it is the magnetic meridian, and the great circle perpendicular to it is the magnetic equator. William Gilbert's 1600 proposal that the Earth itself is one giant bar magnet fails because the Earth's interior is far too hot for any material to retain permanent magnetism. A later idea attributed to Gover suggested instead that the Earth's field arises from hot, ionised air -- solar heating near the equatorial belt makes air rise and drift toward the northern and southern hemispheres, becoming electrified along the way and magnetising ferromagnetic material near the Earth's surface. Neither of these -- nor any of the many other theories proposed since -- fully explains the origin of the Earth's field even today.
Three quantities -- the elements of the Earth's magnetic field -- specify it completely at any point on the surface:
- Magnetic declination () -- the angle between the magnetic meridian and the geographic meridian at that point. It is small in India (about , i.e. west of true north, at Chennai) and grows larger at higher latitudes.
- Magnetic dip or inclination () -- the angle the Earth's total field makes with the horizontal, in the magnetic meridian (about at Chennai). …
What this figure shows. A cutaway sketch of the Earth showing that its geographic north and south poles do not coincide with its magnetic poles: the Earth's magnetic south pole sits close to the geographic north pole, and the magnetic north pole sits close to the geographic south pole. Curved field lines run from the magnetic north pole around to the magnetic south pole outside the body of the Earth, exactly as they would for a giant bar magnet buried inside it, which is why a compass n …
What this figure shows. Two vertical planes are drawn through a point on the Earth's surface: the geographic meridian, which contains the true geographic axis, and the magnetic meridian, which contains the Earth's magnetic axis. The two planes do not coincide, and the angle between them, marked D, is the declination; the diagram also shows the geographic equator and the magnetic equator as two separate great circles that are close to, but no …
What this figure shows. A vertical cross-section through the magnetic meridian at some point on the Earth, showing the Earth's total field vector B tilted down into the ground at an angle I measured from the horizontal direction BH. The diagram resolves B into its horizontal leg BH (=B cos I) and vertical leg BV (=B sin I), which is exactly how a dip circle measures t …
Worked out. Given BH = 0.15 G and BV = 0.26 G at some place, the angle of dip follows from tan I = BV/BH = 0.26/0.15 = 1.732, so I = tan^-1(1.732) = 60 degrees. The resultant field is the vector sum of the two perpendicular components, B = sqrt(BH^2 + BV^2) = sqrt(0.15^2 + 0.26^2) G = sqrt(0.0225+0.0676) G = sqrt(0.0901) G, which comes out to about 0.3 G. This illustrates how a dip-circle reading of I together with a separately measured BH lets you rec …