Some problems(electric field.electric potential)

[maria curie]

please help mee, I have an exam tomorrow
1) Two long,thin cylindirical shells of radii r1 and r2,respectively,are oriented coaxially (one cylindwer is centered inside the other).The cylinders carry equal and opposite linear charge densities lambda ,describe the resulting electric field inside the smaller cylinder ,btween the cylinders,and outside the larger cylinder.
2)Two large ,thin metallic plates are placed parallel to each other ,seperated by 11 cm.The top plate carries a uniform charge density of 6.5*10(-6).While the bottom plate carries a uniform charge density of -4.8*10(-6)C.What is the electric field halfway btw plates?
an the last one
3)A solid sphere of radius R has uniform charge density p(rho).Calculate the total potential energy by calculating the energy required to bring a spherical shell of thickness dr and charge density p(rho) from infinity to a distance r from the sphere's center in the potential due to a uniformly charged sphere of radius r.
ı didnt understand especially the last one.do we use rho in the shell??

 

[maria curie]

heyy I am not an idiot..I tried to solve the first one.. but I don't know true or false,I have some opinions about the third one, but I couldn't understand why rho is given for spherical shell, Isn't is used in volumetric shape (I want to mean that ..for instance rho *4pi/3*r3=Q for sphere..) .BUT, in this question 'shell' is mentioned...Maybe I couldn't imagine the shape of the question..that's all I hope there is anybody who can understand what I said . because english is not my mother tongue ,is only my universtiy language.. THANKS ...

 

[berkeman]

What equations should you use for calculating the electric field and electric potential given some charge distribution? How did you set up these equations for these geometries?

 

[maria curie]

gauss' law for electric field,
especially the third question is confused mee. ooh ..hmm E.dA=Q/e0 ok ı know..for shell what can ı do? dQ=RHO*4*Pİ*r2*dr ı didnt understand this equation.isnt rho used for 3D shapes? is shell 2D isn't it?