Explanation:
The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from the north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves. The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded. No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
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A rectangular coil of 300 turns has an average area of average area of 25 cm × 10 cm. The coil rotates with a speed of 50 cps in a uniform magnetic field of strength 4 
about an axis perpendicular of the field. The peak value of the induced e.m.f. is (in volt)
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(a) 3000 π |
(b) 300 π |
(c) 30 π |
(d) 3 π |
Two copper balls, each weighing 10g are kept in air 10 cm apart. If one electron from every 
atoms is transferred from one ball to the other, the coulomb force between them is (atomic weight of copper is 63.5)
|
(a) 2.0 |
(b) 2.0 |
(c) 2.0 |
(d) 2.0 |
The charge given to a hollow sphere of radius 10 cm is 3.2 × 10–19 coulomb. At a distance of 4 cm from its centre, the electric potential will be
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(a) 28.8 × |
(b) 288 volts |
(c) 2.88 volts |
(d) Zero |
Assertion : Current is passed through a metallic wire, heating it red. When cold water is poured on half of its portion, then rest of the half portion become more hot.
Reason : Resistances decreases due to decrease in temperature and so current through wire increases.
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(a) If both assertion and reason are true and the reason is the correct explanation of the assertion. |
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(b) If both assertion and reason are true but reason is not the correct explanation of the assertion. |
|
(c) If assertion is true but reason is false. |
|
(d) If the assertion and reason both are false. |
