CLASS XII PHYSICS

Hot Questions

UNIT 01: ELECTROSTATICS

1 Marks Questions:

Q.1    A certain region has cylindrical symmetry of electric field. Name the charge distribution producing such a field.

Q.2    Represent graphically the variation of electric field with distance, for a uniformly charged plane sheet.

Q.3    How will the radius of a flexible ring change if it is given positive charge?

Q.4    Five Charges of equal amount (q) are placed at five corners of a regular hexagon of side 10 cm. What will be the value of sixth charge placed at sixth corner of the hexagon so that the electric field at the centre of hexagon is zero ?.

Q.5    Two conducting spheres of radii r₁ & r₂ are at same potential. What is the ratio of charges on the spheres?.

Q.6    Why do we use nitrogen or methane gas in Van-de-Graff generator ?

Q.7    An electric charge q is placed at one of the corner of a cube of side ‘a’. What will be the electric flux through its one of the face?

Q.8    A square surface of side L meters is in the plane of the paper. A uniform electric field E (volts/m), also in the plane of paper, is limited only to lower half of the square . What will be the electric flux (in SI units)           associated with the surface.                          

                                                                                            

Q.9  The distance of the field point on the equatorial plane of a small electric dipole is           halved. By what factor will the electric field change for the dipole?

2 Marks Questions:

                                                   

Q.10  A charged particle is free to move in an electric field. Will it always move along an electric line of force? Justify your answers?.

(2 marks)

Q.11 A charge Q is divided in two parts q and Q - q separated by a distance R. If force between the two charges is maximum, find the relationship between q & Q.

(2 marks)

3 marks question:

Q12. A parallel plate capacitor   is charged to potential V  by a source of emf . After removing  the source, the separation between the plates is doubled . How will the following change electric field change on each plate potential difference capacitance of the capacitor Justify your answer

Q 13  If  N drops of same size ,each having the same charge ,coalesce to form a bigger drop . How will the following vary with respect to single small drop?

      (i)Total charge on bigger drop

      (ii) Potential on the bigger drop

      (iii) The capacitance on the bigger drop

Q14 Work done to move a charge along a closed path inside an electric field is always zero, using this fact, prove that it is impossible to produce an electric field in which all limes of force would be parallel lines and density of their distribution would constantly increase in a direction perpendicular to the lines of force.

Q 20. Obtain the formula for electric field due to a long thin wire of uniform linear charge density  without using Gauss’s law.

       ANSWERS/HINTS

1 mark question

Q.1    Uniform linear charge distribution

Q.2    E is constant with r.

Q3.    Increases due to repulsion

Q.4    6th Charge is Q

Q.5    Q₁/Q₂=R₁/R₂

Q.6    It transfers the leakage of Charge to earth through earthed     steel chamber

Q.7    Q/24 έ₀

Q.8    Zero

Q9.    E_x=E_z<E_y

Q.10  E a 1/r³       if r=r/2      , E= 8 times

2 marks question

Q 11  work done is independent of  path

    w= ¼πε_Ο   q₁ q₂ ( 1/r₁- 1/r₂)    putting the values & ans 15 J

Q 12 if charge is positive & at rest in electric field then it will move along electric line of force. If charge has initial velocity making some angle with electric field than it will along parabolic path.

Q 13      Ф'= 4Ф

         Q + q₁+ q₂+q₃/ ε_o= 4 X (q₁+q₂+q₃)/ε_o

          putting the values & finding Q = 3*8.854 μC

Q 14.   F=K q(Q-q) / r 2

    for max. &min.dF / dq=0    , q=Q/2

Q 15. All are in parallel

        C= ε_oA/3d+ε_oA/6d+ ε_oA/9d= 11ε_oA/18 d

3 marks question

Q.16  a. E same

          b. Q same

          c. V same

          d. C is halved with reasons

Q.17  i.        N times the charge on small drop

          ii.       N 2/3 times the potential on small drop

          iii       N 1/3 times the capacitance on small drop

Q.18  If q is moved along abcd then W_abcda = 0

          W_ab+ W_bc+ W_cd +W_da = 0

          as E perpendicular to bc & da

          so W_bc = W_da = 0

          therefore W_ab= -W_cd

          But W_ab can never be equal to W_cd as the lines of force are closer to cd

          therefore W_cd > W_ab

          therefore W_abcda is not equal to 0 hence such electric field E is          impossible

Q.19  i         As the charge move closer the charge on large sphere `                              is redistributed as shown in diagram

          ii        As the spheres move more closer than the charge is                                   redistributed as shown in diagram

          iii       Behaviour of force between 2 cm & 1 cm :

                             force of repulsion increases upto 1.4 cm &  F rep. is                                  max. at r=1.4 cm

                             If 1.2 cm<r<1.4 cm F rep. is decreasing Fatt. increases                                        due to inductive effect.

                             At r=1.2 cm F rep. = F att. & if r<1.2 cm force is                                       strongly attractive

Q.20  X=q/length = q/l change on dl length

          dq = λ dl At point l

dE = 1 / 4 π ε_o * dq /op2 = 1/ 4π ε_o   * λ dl / r2 +l2 l= rtan Q

          find dl .dE = 1 / 4 π ε_o λ cos q dq/ r

          integrate between -π/2 to +π /2 than E is λ/2π ε_o r