IIT JEE Advanced & Mains Physics 2018 : Electricity : Rank file / expected questions / Physicsmynd Elite Series / Page II
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10 . A soap bubble of radius 1 cm , which is at a potential of 90 volts , collapses to a drop of radius 1 mm . Find the change in its electrostatic energy?
11.From a sphere of uniform charge density 
and radius R ,a small spherical hollow region of radius r is cut out at a distance d from the center of it .
(a) What is the field E at the center of the hollow sphere ?
12 .In addition ,also find the potential 
at above stated point.
13 . Suppose ,in the above case , a particle of charge q is moved from infinity to the center of the spherical shell through the cut out hole . Find the work needed ? [take the thickness of the shell to be t .]
14 .Find the the electric field and potential at the center of a ring of radius r whose net charge +Q is uniformly distributed on it.
15. Calculate the point(s) on the axis of a charged ring where the electric field is maximum.
16.Figure below shows the discharge curve of a Capacitor . From the graph the valve of time [ t ] when 
is-
A ] 
B ] 
C ] 
D ] 
17 ] Drift Velocity
The primary charge carriers in sea water are 
and 
ions. Consider a non conducting pipe of 3 meters connected to a 15 volt battery at it’s both ends by an electrode.This pipe is then filled with seawater..Find the average drift velocity of the ions, in cm/s?
Note : The resistivity of seawater is about 
and there are about 
of each ions .
18 ] Consider the following arrangement .-
An air-filled parallel-plate capacitor of area A , with plates a and b are kept on a smooth surface in such a manner ,that it’s one end is fixed and the other is connected to a spring having a force constant k .The charges on the plates are respectively Q + and Q – .Then the expansion of the spring is –
A ] 
B ] 
C ] 
D ] 
