There are three concentric thin spheres of radii a,b, c (a>b>c). The outer surface charge densities of their surfaces are σ, -σ and σ respectively. Find the magnitude of electric field at r (distance from centre) such that a>r>b .

Please tell me how to tackle this problem?

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According to ur question,

The field is required at a point which lies between the spheres 1&2.

This point is an external point for the 1st sphere and internal point

for the 2nd & 3rd spheres.

Since, the charge is distributed over the sufrace of the spheres, so they

are conducting spheres. there is no electric field inside conducting spheres.

Hence, there is no electric field at the required point due to the spheres 2 & 3.

Only 1st sphere produces electric field at the point whose magnitude is equal to (σ/ε₀)

while u r solving this type of probs, plzz try to identify where the point is located.