A metal disk lies in the xy-plane, centered about the origin, and rotates with a constant angular velocity about the z-axis. There is a uniform 0.0314 T magnetic field parallel to the z-axis. The radius of the disk is 1.56 m. At what rate is the disk turning if an E of 3.86 V develops between the center of the disk and a point on its rim

 w = -101 rad / s

Explanation:

For this exercise we will use Faraday's law

          E = - dФ / dt

where the magnetic flux is

          Ф = B. A = B A cos θ

In this case, the angle between the magnetic field and the normal to the disk is zero, cos 0 = 1, they indicate that the field is constant, let's find the area

The area rotated by the disk is

         A = ½ r s

if we express the angles in radians

         θ = s / r

         s = r θ

where is the arc supported

         A = ½ r (r θ)

let us substitute in the Faraday equation

          E = - d (B ½ r² θ) / dt

          E = - ½ B r² dθ/dt

the definition of angular velocity is

          w = dθ/dt

          E = - ½ B r² w

           w = - 2E / B r²

let's calculate

         w = - 2 3.86 / (0.0314 1.56²)

         w = -101 rad / s

answer: the answer is a