4. (a) Sketch the H-B hysteresis curves for both soft and hard magnets. Label the key parameters of the curves. (5 marks) (b) With reference to your answer for part (a) explain the different applications for soft and hard magnets. (4 marks) (c) An iron bar with a coercivity of 7000 A/m is to be demagnetised. If the bar is inserted into a cylindrical wire coil which is 0.2 m long and comprises of 200 turns, calculate the electric current required to demagnetise the bar. (4 marks) (d) Describe, with the aid of a diagram, the underlying principles of a thermoelectric device in power generation mode. Identify the problems that need to be solved to maximise the performance of thermoelectric generators. (7 marks)