1. The problem statement, all variables and given/known data B = 0.02T L = 40 cm w = 10 rad/s a: Electromotive force between O and C = ? b: If electromotive force is the same - so what is the velocity? I found 2 ways of solution. See "3. The attempt at a solution" 2. Relevant equations φ = BA ε = -dφ(t)/dt 3. The attempt at a solution So I found 2 ways: 1. What every one would do - by Electric: 2. By the magnetic flux. But wait - There is no area. So I found that if I would take in problem a the area of what the rod had passed (part of a disc) I would get the exact solution. For problem b if I would take the area of a triangle I will get the solution too. My question - Why does the second way is also true? How can it be explained (For both problem a and b)? Thanks a lot!