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CBSE Class 12 Physics Solved Question Paper 2011

Short Answer Type

Define electric dipole moment. Write its S.I. unit.

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Where on the surface of Earth is the angle of dip 90°?

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A hollow metal sphere of radius 5 cm is charged such that the potential on its surface is 10 V. What is the potential at the centre of the sphere?

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How are radio waves produced?

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Two bar magnets are quickly moved towards a metallic loop connected across a capacitor ‘C’ as shown in the figure. Predict the polarity of the capacitor.

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A thin straight infinitely long conducting wire having charge density $straight lambda$ is enclosed by a cylindrical surface of radius r and length l, its axis coinciding with the length of the wire. Find the expression for the electric flux through the surface of the cylinder.

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Plot a graph showing the variation of coulomb force (F) versus , where r is the distance between the two charges of each pair of charges: (1 C, 2 C) and (2 C – 3 C). Interpret the graphs obtained.

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Write the expression for Lorentz magnetic force on a particle of charge ‘q’ moving with velocity v in a magnetic field B . Show that no work is done by this force on the charged particle.

A steady current (I₁) flows through a long straight wire. Another wire carrying steady current (I₂) in the same direction is kept close and parallel to the first wire. Show with the help of a diagram how the magnetic field due to the current I₁ exerts a magnetic force on the second wire. Write the expression for this force.

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9.
What are eddy currents? Write any two applications of eddy currents.

Eddy currents are the currents induced in a metallic plate when it is kept in a time varying magnetic field. Magnetic flux linked with the plate changes and so the induced current is set up. Eddy currents are sometimes so strong, that metallic plate becomes red hot.

Applications:

In induction furnace, the metal to be heated is placed in a rapidly varying magnetic field produced by high frequency alternating current. Strong eddy currents are set up in the metal produce so much heat that the metal melts. This process is used in extracting a metal from its ore. The arrangement of heating the metal by means of strong induced currents is called the induction furnace.
Induction Motor: The eddy currents may be used to rotate the rotor. When a metallic cylinder (or rotor) is placed in a rotating magnetic field, eddy currents are produced in it. According to Lenz’s law, these currents tend to reduce to relative motion between the cylinder and the field. The cylinder, therefore, begins to rotate in the direction of the field. This is the principle of induction motion.

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