CBSE     12      PHYSICS      IMPORTANT     UNIT-1
UNIT 1
1.Why do the electrostatic field lines do not form closed loops?
2.In Millikan’s oil drop experiment, what should be the nature of the oil used?
3.Calculate the number of electrons passing through the cross section of a conductor in the time interval of 3 minutes,if the current passing through it is 0.2 A ?
4.Two point charge q1 and q2 are located at point (a,0,0 ) and (0,b,0) respectively .Find the electric fields due to both these charges at the point (0,0,c)?
5.Define capacitive reactance .Write its S.I.  Unit?
6.Two identical parallel plate capacitors A and B are connected to a battery of V volts with the switch S closed. The switch is now opened and the free space between the plates of the capacitors is filled with a dielectric of dielectric constant k. Find the ratio of the total energy stored in both capacitors before and after the introduction of the dielectric. 
7.A.Derive an expression for the electric field E due to a dipole of length ‘’2a, at a point distant r form the centre of the dipole on the axial ?
B. Draw a graph of E versus r for r>>a?
C.If  this dipole were kept in a uniform external electric field E0 ,Diagrammatically represent the position of the dipole in stable and unstable equilibrium and write the expressions for the torque acting on the dipole in both the cases ?
8. Use Gauss’s theorem to find the electric field due to a uniformly charge infinitely large plane thin sheet with surface charge density σ?
9.An infinitely large thin plane sheet has a uniform surface charge density ₊
ợ. Obtain the expression for the amount of work done in bringing a point charge q form infinity to a point distant r, in front of the charged plane sheet