JEE Adv. 2013: $K\left[ {\frac{x}{{{{({x^2} + {y^2})}^{3/2}}}}\hat i + \frac{y}{{{{({x^2} + {y^2})}^{3/2}}}}\hat j} \right]$ Sunday, September 22, 2013 Solution The equation of the circle is x^{2}+y^{2}=a^{2} Hence, (D)