4. Electrical current of triangular waveform (amplitude lo and frequency @) flows through a long solenoid (n turns per unit length, radius a and length L). (a) What are the shape and amplitude of the induced emf? Draw it schematically. (You can us B=p nl) (b) What is the electric field at point P outside the solenoid, a distance r from the central axis, Plot its magnitude as a function of time