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ISC Class 12 Physics Solved Question Paper 2005

Short Answer Type

21. An electric dipole is placed in a uniform external electric field E. Show that the torque acting on the dipole is given by

, where p is the dipole moment.

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22.

A potential difference of V volt is applied across a copper wire of length I and diameter d. How will the drift velocity be affected if

(i) V is double (ii) I is doubled.

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23. State Biot-Savart law. Derive an expression for magnetic field at the centre of a circular current carrying coil of radius r.

Biot-Savart law :

Magnetic field due to small current element ‘dl’ at any point P is given by,

dB

dB space proportional to space dl space and dB space proportional to space 1 over straight r squared dB space proportional to space sin space straight theta Therefore comma space dB space proportional to space fraction numerator straight i space dl space sin space straight theta over denominator straight r squared end fraction dB space equals space straight K space fraction numerator straight i space dl space sin space straight theta over denominator straight r squared end fraction For space free space space comma space straight K space equals space fraction numerator straight mu subscript straight o over denominator 4 straight pi end fraction dB space equals space fraction numerator straight mu subscript straight o over denominator 4 straight pi end fraction space fraction numerator straight i space dl space sin space straight theta over denominator straight r squared end fraction

In vector form,

Magnetic field at the centre of current carrying coil : Consider a circular coil of radius r, carrying current i.

According to Biot-Savart law, magnetic field due to current element dl at centre is given by,

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24. Distinguish between diamagnetic and paramagnetic substance in terms of its relative permeability (μ_r) and magnetic susceptibility (x).

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25. A current of 4A flows in a coil when connected to a 12 V dc source. If the same coil is connected to a 12 V, 8 Hz ac source, a current of 2.4 A flows in the coil. Calculate the inductance of the coil.

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26. In the diagram given below (figure), find the current drawn from the battery:

(i) just when the switch S is closed.

(ii) when sufficient time has elapsed after closing the switch.

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27.

Microwaves, X-rays, U.V. rays, infrared rays.

Which of the above electromagnetic waves has—

(i) the shortest wavelength.

(ii) the lowest frequency.

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28. Derive Snell’s law of refraction using Huygen’s principle.

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29. In a Youngs double slit experiment with monochromatic light, fringes are obtained on a screen placed at a distance D from the slits. If the screen is moved by 5 x 10^-2 m towards the slits, the change in the fringe width is 3 x 10^-5 m If the distance between the slits is 10^-3m, calculate the wavelength of the light used.

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