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There is a hollow metal sphere, initially uncharged, put in a uniform field. A field strength E0 is required to break the sphere. If the radius of the sphere is doubled, but the thickness of its walls is not changed, then what is the value of the electric field needed to break it up?

In Griffiths 3rd ed, eg 3.8, I saw that a charge density proportional to E and to cos(theta) is built up in the sphere..on integrating this over each hemisphere, I found that one hemisphere is positive and the other negative..but with equal magnitude of charge proportional to E and radius(r) squared. Thus, force will be proportional to E squared and r squared.

Since, in the second case, the thickness of the walls remains the same, I reasoned that the force to break it apart should be the same as before.

Now, force depends on (E x r)^2...

So..if r doubles,,we need to use only E0/2...

But my answer does not match with the given solution..where am I going wrong?