Q22 of 26 Page 1

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.


Resistivity() of a material is the resistance offered by the material to the the flow of charges. It is defined as the resistance(R) of the material of a unit area(A) per unit length(l).



The following diagram shows the resistivity versus temperature plots of the conductor and semiconductor.


In case of a conductor, the resistivity increases as the temperature increases. Here, the number of charge carriers is independent of the temperature. Increasing temperature decreases the relaxation time or the time between two collisions. Thus, more collisions occur which increases the resistance.



In a semiconductor, the density of charge carriers increases as temperature increases. This effect overcompensates for the increase in relaxation time. Hence, the resistivity decreases.


More from this chapter

All 26 →
20

(a) The light from a clear blue portion of the sky shows a rise and fall of intensity when viewed through a polaroid which is rotated. Describe, with the help of a suitable diagram, the basic phenomenon/process which occurs to explain this observation.

(b) Show how light reflected from a transparent medium gets polarized. Hence deduce Brewster’s law.


 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 .

 23 SECTI

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 .


 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.