(11%) Problem 4: A thin plastic ring of radius R = 0.39 m is sprayed with electrically charged paint in a manner that half of the ring has a constant line charge density of + and the other half has constant line charge density of -, where = 3.1 mC/m. - +2 33% Part (a) Enter an expression for the magnitude of the electric field at the center of the ring. (Note: E is not zero even though the total charge on the ring is clearly zero.)