Earth's Magnetic Field & Magnetic Material

1 Mark Questions

1. Relative permeability of a material <math>\mu_r = 0.5</math>. Identify the nature of the magnetic material and write its relation of magnetic susceptibility. [Delhi 2014]

Ans. The nature of magnetic material is diamagnetic.

<math>\mu_r = 1 + \chi_m</math>

1. What are permanent magnets? Give one example. [Delhi 2013]

Ans. Permanent magnets are those magnets which have high retentivity and coercivity The magnetisation of permanent magnet is not easily destroyed even if it is handled roughly or exposed in stray reverse magnetic field, e.g. steel.

1. Where on the surface of the earth's vertical component of earth's magnetic field zero?

[Delhi 2013C, All india 2011]

Ans. At equator, vertical component will be zero.

1. The horizontal component of the earth's magnetic field at a place is B and angle of dip is

60°. What is the value of vertical component of the earth's magnetic field at equator? [Delhi 2012]

Ans.

<math>\langle \rangle</math> The horizontal and vertical components of the earth's magnetic field are perpendicular to each other.

Horizontal component of earth's magnetic field,

<math>H = B_e \cos 60^\circ = B</math> (given) <math>\Rightarrow</math> <math>B_{\rm e} \times \frac{1}{2} = B</math> or <math>B_e = 2B</math>

Vertical component of earth's magnetic field,

<math>\Rightarrow</math>

<math>V = B_e \sin 60^\circ</math> <math display="block">V = 2B \times \frac{\sqrt{3}}{2}</math> <math>V = \sqrt{3}B</math> ⇒

1. What is the angle of dip at a place where the horizontal and vertical components of the

earth's magnetic field are equal? [Foreign 2012]

Ans. . . The angle of dip is given by

<math>\delta = \tan^{-1} \left( \frac{B_V}{B_H} \right)</math>

<math>B_V</math> = vertical component of the earth's magnetic field.

<math>B_H</math> = horizontal component of the earth's magnetic field.

So, as <math>B_V = B_H</math>

Then, <math>\delta = \tan^{-1}(1) = 45^{\circ}</math>

<math>\therefore</math> The angle of dip will be <math>\delta = 45^{\circ}</math>.

(1) 6.A magnetic needle free to rotate m a vertical plane orients itself vertically at a certain place on the earth What are the values of

• horizontal component df the earth's magnetic field and

angle of dip at this place? [Foreign 2012]

Ans. (i) The coil is free to move in vertical plane, it means that there is no componerr A the earth's magnetic field in horizontal direction, so the horizontal component of the earth's magnetic field is 0.

1. The angle of dip is 90°.

[All India 2011]

1. Where on the surface of earth is the angle of dip 90°?

Ans. At poles, the angle of dip is 90°.

1. The permeability of a magnetic material is 0.9983. Name the type of magnetic material, it represents. [Delhi 2011]

Ans.