What is the effective resistance R of a pn junction diode that obeys Shockley’s equation, as a function of the bias voltage V ? What is the resistance in the limit delta V approaches 0? I know the shockley equation, but how does reistivity relate to it? I'm assuming you can substitute it in the quation relating it to current, but I'm not sure of the espression. Last parts ok to. Resistance should go to zero, right? Then he turned off the electric field, and measured the radius of curvature R of the beam, as deflected by the magnetic field alone. What is the charge-to-mass ratio q/m of the particles in terms of E, B and R? This is to do with a cyclotron, initially with equal magnetic and electric fields, such that the net force was zero, and the electron only went perfectry striaght perpendicular to the forces. Then he turns off the electric field. This is like a mass spectrometer. I know tha R=mv/qB, so theres the relation of R and B, but how does E fit into all of this. E=qV, but I can't see anyway of fitting this in, sine we need the q for the ratio q/m.