IIT-JEE 2011 · Paper 2 · Physics Q31

PhysicsMultiple correct+4Question 31 of 60

Which of the following statement(s) is/are correct?

If the electric field due to a point charge varies as

r^{-2.5}

instead of

r^{-2}

, then the Gauss law will still be valid.

The Gauss law can be used to calculate the field distribution around an electric dipole.

If the electric field between two point charges is zero somewhere, then the sign of the two charges is the same.

The work done by the external force in moving a unit positive charge from point A at potential

V_A

to point B at potential

V_B

(V_B-V_A)

Answer and worked solution

Official answer: C, D

(A) ✗ Gauss's law needs the inverse-square law: with $E\propto r^{-2.5}$ the flux through a sphere, $\propto r^{-0.5}$ , depends on its radius.
(B) ✗ Gauss's law holds, but a dipole has no symmetry that lets $E$ be taken out of the integral.
(C) ✓ Between two charges the fields cancel only if they point opposite ways, i.e. the charges have the same sign.
(D) ✓ Moving a unit charge slowly, $W_\text{ext}=\Delta U=V_B-V_A$ .

(C), (D)

Topics:Electric flux and Gauss's law Electric potential

IIT-JEE 2011 · Paper 2, Physics question 31