21.

Assertion:  The size and shape of the rigid body remains unaffected under the effect of external forces. 

Reason:  The distance between two particles remains constant in a rigid body.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

The dimensions of specific resistance are

• [ M L² T^-2 A^-1 ]

• [ M L³ T^-3 A^-2 ]

• [ M L³ T^-2 A^-1 ]

• [ M L² T^-2 A^-2 ]

Reciprocal of impedance is

• susceptance

• conductance

• admittance

• transconductance

Four charges arc arranged at the corners of a square as shown in the figure. The direction of electric field at the centre of the square is along

• DC

• BC

• AB

• AD

A conducting sphere of radius R carrying charge Q lies inside an uncharged conducting shell of raius 2R. If they are joined by a metal wire, the amount of heat that will be produced is

• $\frac{1}{4{\mathrm{\pi \epsilon }}_0} \frac{{\mathrm{Q}}^2}{4\mathrm{R}}$

• $\frac{1}{4{\mathrm{\pi \epsilon }}_0} \frac{{\mathrm{Q}}^2}{2\mathrm{R}}$

• $\frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{{\mathrm{Q}}^2}{\mathrm{R}}$

• $\frac{2}{4{\mathrm{\pi \epsilon }}_0} \frac{{\mathrm{Q}}^2}{3\mathrm{R}}$

Black  holes  in  orbit  around  a  normal  star  are  detected  from  earth  due  to  the  frictional  heating  of  infalling  gas  into  the  black  hole,  which  can  reach  temperatures  greater  than  10⁶ K.  Assuming  that  the  infalling  gas  can  be  modelled  as  a  blackbody  radiator,  then  the  wavelength  of  maximum  power  lies

• in the visible region

• in the X-ray region

• in the microwave region

• in the gamma-ray region of electromagnetic spectrum

What  is  the  energy  stored  in  the  capacitor  between terminals  a and  b of the  network shown in the figure? ( capacitance of each capacitor C = 1 μF)

• 12.5 μJ

• Zero

• 25 μJ

• 50 μJ

A  moving  coil  galvanometer  has  a resistance  of  900 Ω.  In order  to send  only 10 % of  the  main  current through  this  galvanometer,  the  resistance  of  the required  shunt  is

• 0.9 Ω

• 100 Ω

• 405 Ω

• 90 Ω

A  current  I₁  carrying  wire  AB  is  placed  near  another   long  wire  CD  carrying  current I₂. If  wire  AB  is  free  to  move,  it  will  have

• rotational motion only

• translational motion only

• rotational as well as translational motion

• neither rotational nor translational motion

A  coil  of  wire  of  a  certain  radius  has  100 turns and  self  inductance  of  15  mH.  The  self  inductance  of  a  second  of  a  second  similar  coil  of  500  turns  will be

• 75 mH

• 375 mH

• 15 mH

• none of these