Ncert Exercises & Solutions

The magnetic field of the earth can be modelled by that of a point dipole placed at the center of the earth. The dipole axis makes an angle of 11.3° with the axis of the earth. At Mumbai, declination is nearly zero. Then,

1. the declination varies between 11.3° W to 11.3° E
2. the least declination is 0 $°$
3. the plane defined by the dipole axis and the earth axis passes through Greenwich
4. declination averaged over the earth must be always negative

(a) Hint: Recall the concept of Earth's magnetism.

Step 1: For the earth's magnetism, the magnetic field lines of the earth resemble that of a hypothetical magnetic dipole located at the center of the earth.

Step 2: The axis of the dipole does not coincide with the axis of rotation of the earth but is presently tilted by approximately 11.3

°

with respect to the later. This results in two situations as given in the figure ahead.

Hence, the declination varies between 11.3° W to 11.3° E.

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