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A ring of radius R carries charge uniformly. Along the line passing through its centre and perpendicular to its plane, the electric field attains its maximum magnitude at a point located a distance h from the centre. What is h?

1. R/√5
2. R/√2
3. R
4. R√2

Correct answer: R/√2

Solution

The axial field of a ring is E = kQx/(x^2+R^2)^(3/2). Setting dE/dx = 0 gives x^2 = R^2/2, so the field peaks at h = R/sqrt(2).

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