Extension
4 describe the movement of the electrons in an a. c. circuit by relating it to the
changing potential difference of the supply.
(3 marks)
5
look back to the diagram in question 2. when the original trace was made the
oscilloscope's volts/div dial was set at 0.5 volts/div and the time base was set
at 5 ms/div. calculate the frequency of the supply.
(3 marks)

Athin electrical heater is inserted between a long circular rod and a concentric tube with inner and outer radii of 20 and 40 mm. the rod (a) has a thermal conductivity of ka = 0.15 w/(m*k), while the tube (b) has a thermal conductivity of kb = 0.8 w/(m*k) and its outer surface is subjected to convection with a fluid of temperature t= -15°c and heat transfer coefficient 60 w/(m2 *k). the thermal contact resistance between the cylinder surfaces and the heater is negligible. (a) determine the electrical power per unit length of the cylinders (w/m) that is required to maintain the outer surface of cylinder b at 15°c. (b) what is the temperature at the center of cylinder a?