Chapter 12, Problem 17P

a. A flux of $\text{0}{\text{.2 × 10}}^{\text{-4}}\text{ Wb}$ will establish sufficient attractive force for the armature of the relay in FIg. 12.44 to close the contacts. Determine the required current to establish this flux level if we assume that the total mmf drop is across the air gap.

b. The force exerted on the armature is determined by the equation

$\text{F(newtons)=}\frac{1}{2}.\frac{B^2{}_{\text{g}}A}{\mu o}$ whereBg is the flux density within the air gap and A is the common area of the air gap. Find the force in newtons exerted when the flux $\varphi$ specified in part(a) is established.