A helicopter has 4 rotor blades attached to the central axis of rotation. (Like a + sign with the axis of rotation at the center.) Each rotor blade is 5.2m long and spins at 6 revolutions per second perpendicular to the earth's magnetic field of 0.047T. What is the emf induced between the axis and the tip of a rotor blade? This is what I did. The tip of a rotor blade sweeps out a circular path so: v = 2πr/T = 2πrf = 2π(5.2)(6) = 196.035m/s emf = Blv = 0.047(5.2)(196.035) = 0.048V A friend of mine had this solution. Φ = BA = 0.047π(5.2)^2 = 0.004Wb emf = -N∆Φ/∆t = -N∆Φf = -1(0.004)(6) = 0.024V I told him his solution didn't work because he is using the area of the entire circular path instead of the area of the blade (which cannot be calculated by the way). Also he is using the initial flux as the change in flux which isn't correct either. So who is right? Or are we both wrong?