1. The problem statement, all variables and given/known data A thin sheet in the shape of an annular semicircle has a positive surface charge density +σ as shown. What is the electric field at point P? Here is an illustration of the problem: http://postimg.org/image/630bpqwan/ 2. Relevant equations Gauss's Law: φ=Qenclosed/ε0 φ=∫E⋅dA Qenclosed=ε0*∫E⋅dA Coulomb's Law: E=(1/(4πε0))(Σq/(r2)) 3. The attempt at a solution If I draw a gaussian surface somewhere between the point P and the radius a, the charge enclosed would be zero. Can I assume that this would mean that the electric flux through that surface is zero, and, by extension, the electric field experienced at point P is zero, too? If I'm wrong about the Gauss's law approach, should I instead use a Coulomb's law approach and integrate the electric field generated by the semicircular sheet, minus the electric field generated by the semicircular cavity? Thank you.