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Question 12 Marks
Define two magnetic elements of the earth.
Answer
Declination : The angle between the magnetic meridian and the geographic meridian at a place is called the declination at that place and is denoted by $\alpha$. The magnetic meridian of that earth may be due east or due west of its geographic meridian. Hence, it is expressed as $\alpha^{\circ}$ east or $\alpha^{\circ}$ west. In figure, < GOM, the declination is $\alpha^{\circ}$ east.
Inclination : This is the angle made by the magnetic field of the earth with the horizontal. It is generally denoted by denoted by $\delta$. In figure, < DOA is the angle of dip. The angle of dip at a place is measured by an instrument called dip circle, which has a compass needle that can rotate freely in a vertical plane.
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Question 22 Marks
What are Eddy currents? Give its two uses.
Answer
Eddy currents are loops of electrical current induced within cenductors by a changing magnetic field in the conductor according to faraday's law of induction.
Uses : (i) Eddy currents are used to heat electrically conductive materials by transferring heat over short distances without the objects touching.
(ii) Eddy currents are used to identify defects and breaks in metal parts.
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Question 32 Marks
Write expressions for magnetic field $\vec{B}$ on axis and equator of a short bar magnet. State the direction of $\vec{B}$ else.
Answer
$B_{axis}=\frac{\mu_{0}}{4\pi}(\frac{2M}{r^{3}})$ and $B_{equator}=\frac{\mu_{0}}{4\pi}(\frac{M}{r^{3}})$
where $\mu_{0}=4\pi \times 10^{-7} N/A^{2}$ is the permeability of free space or air. M = magnetic moment of the magnet and r = distance of the point of observation from the middle point of the magnet.
Direction of $\vec{B}_{axis}$ will be away from the magnet and direction of $\vec{B}_{equator}$ will be parallel to the axis of the magnet directed from North to South pole.
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Question 42 Marks
Differentiate between electric lines of force and magnetic lines of force.
Answer
Electric and magnetic fields differ in the following respects :
(i) Electric fields can be due to charges at rest or moving charges but the magnetic field is basically due to moving charges (or current) only.
(ii) Electric field lines in free space do not form closed paths but magnetic field lines form closed paths.
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2 Marks Questions - Physics STD 12 Science Questions - Vidyadip