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If a current I enters at point A, what is the magnitude of the electric field at a point located a distance r from A?

1. ρI/(8πr²)
2. ρI/r²
3. ρI/(2πr²)
4. ρI/(4πr²)

Correct answer: ρI/(2πr²)

Solution

Current entering at a point on the surface of a conducting medium spreads radially through a hemisphere of area 2*pi*r^2, giving current density J = I/(2*pi*r^2). The field is E = rho*J = rho*I/(2*pi*r^2).

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