A sphere of radius R has charge Q. The electric field strength at distance rR is Ei.
A) What is the ratio Ef/Ei of the final to intitial electric field strengths if Q is halved?
B) if R is halved?
C) if r is halved (but is still R). Each part changes only one quantity, the other quanties have their initial values

Question

A sphere of radius R has charge Q. The electric field strength at distance r>R is Ei. 
A) What is the ratio Ef/Ei of the final to intitial electric field strengths if Q is halved?
B) if R is halved?
C) if r is halved (but is still > R). Each part changes only one quantity, the other quanties have their initial values

anonymous · Answer

A sphere can be treated as a point charge as long as you are past the surface of the sphere (satisfied by r>R)
A) 1/2. 
B) 1
C) 4