Q23 SECTI of 26 Page 1

Asha’s uncle was advised by his doctor to have an MRI (magnetic resonance imaging) scan of his brain. Her uncle felt that it was too expensive and wanted to postpone it. When Asha learnt about this, she took the help of her family and when she approached the doctor, he also offered a substantial discount. She thus convinced her uncle to undergo the test to enable the doctor to know the condition of his brain. The resulting information greatly helped his doctor to treat him properly.

Based on the above paragraph, answer the following questions:


(a)What according to you are the values displayed by Asha, her family and the doctor?


(b)What in your view could be the reason for MRI test to be so expensive?


(c)Assuming that MRI test was performed using a magnetic field of 0 �1 T, find the maximum and minimum values of the force that the magnetic field could exert on a proton (charge = ) that was moving with a speed of .


(a) Asha and her family are quite generous and caring. Asha is quick to help others in distress. The doctor is that of caring and understanding nature.


(b) Magnetic Resonance Imaging utilizes high magnitude magnetic fields to map soft tissues of the body. To produce such magnetic fields, expensive electromagnets are required and hence the cost is high.


(c) We know that the force is given by



Given,




Maximum force occurs when the velocity of proton is perpendicular to the field.




While minimum force occurs when the velocity of proton is parallel to the field.



More from this chapter

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21

(a) Show that the average power consumed in an inductor L connected to an a.c. source is zero. (b) In a series LR circuit, and the power factor of the circuit is . When a capacitor with capacitance C such that is put in series, the power factor becomes . Find out .

 22

Define the electric resistivity of a conductor.

Plot a graph showing the variation of resistivity with temperature in the case of a (a) conductor, (b) semiconductor.


Briefly explain, how the difference in the behavior of the two can be explained in terms of number density of charge carriers and relaxation time.


 24

(a) Derive the expression for the potential energy of an electric dipole of dipole moment placed in a uniform electric field .

Find out the orientation of the dipole when it is in (i) stable equilibrium, (ii) unstable equilibrium.


(b) Figure shows a configuration of the charge array of two dipoles. Obtain the expression for the dependence of potential on r for r >> a for a point P on the axis of this array of charges.


OR


(a)Define electric flux. Write its S.I. unit.


(b)Using Gauss’s law, obtain the electric flux due to a point charge ‘q’ enclosed in a cube of side ‘a’.


(c)Show that the electric field due to a uniformly charged infinite plane sheet at any point distant x from it, is independent of x.


 25

(a) Define a wavefront.

(b) Using Huygens’ principle, draw the diagrams to show the nature of the wavefronts when an incident plane wavefront gets


(i)reflected from a concave mirror,


(ii)refracted from a convex lens.


(c) Draw a diagram showing the propagation of a plane wavefront from denser to a rarer medium and verify Snell’s law of refraction.


OR


(a)A concave mirror produces a real and magnified image of an object kept in front of it. Draw a ray diagram to show the image formation and use it to derive the mirror equation.


(b)A beam of light converges at a point P. Now a lens is placed in the path of the convergent beam 12 cm from P. At what point does the beam converge if the lens is


(i)a convex lens of focal length 20 cm,


(ii)a concave lens of focal length 16 cm?