Q12 of 26 Page 1

(a) A point charge q moving with speed v enters a uniform magnetic field that is acting into the plane of the paper as shown. What is the path followed by the charge q and in which plane does it move?

(b) How does the path followed by the charge get affected if its velocity has a component parallel to?


(c) If an electric field is also applied such that the particle continues moving along the original straight-line path, what should be the magnitude and direction of the electric field ?



(a) The magnetic force experienced by the charged particle is given by,



Where, velocity is perpendicular to the magnetic field. Thus, the point charge travels to perpendicular to both velocity and magnetic field.


(b) The magnetic force experienced by the charged particle is given by,



When velocity has a component parallel to, angle between them is zero.


So, 0


As there is no magnetic force acting on the particle it will move in a straight line.


(c) The magnetic force experienced by the charged particle is given by,



As the particle is moving in a straight-line magnetic force is balanced by electric force,



Thus, magnitude of electric field is,


E=vB


The direction of electric field is negative z axis.


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10

A metal rod of square cross-sectional area A having length l has current I flowing through it when a potential difference of V volt is applied across its ends (figure I). Now the rod is cut parallel to its length into two identical pieces and joined as shown in figure II. What potential difference must be maintained across the length 2l so that the current in the rod is still I?


 11

(a) Define torque acting on a dipole of dipole moment p placed in a uniform electric field

Express it in the vector form and point out the direction along which it acts.


(b) What happens if the field is non-uniform?


(c) What would happen if the external field is increasing (i) parallel to and (ii) anti-parallel to ?


 13

(i) Which segment of electromagnetic waves has highest frequency? How are these waves produced? Give one use of these waves.

(ii) Which em waves lie near the high frequency end of visible part of em spectrum?


Give its one use. In what way this component of light has harmful effects on humans?


 14

In the following diagram, an object ‘O’ is placed 15 cm in front of a convex lens L1 of focal length 20 cm and the final image is formed at ‘I’ at a distance of 80 cm from the second lens L2. Find the focal length of the lens L2.