A) Calculate the cross-sectional area, in mm², of a piece of copper wire, 40 m in length
and having a resistance of 0.25 2. Take the resistivity of copper as 0.02 x 10-6 22 m.
b) A coil of 20 2 resistance is joined in parallel with a coil of R2 resistance. This
combination is then joined in series with a piece of apparatus A, and the whole circuit
connected to 100 V mains. What must be the value of R so that A shall dissipate 600
W with 10 A passing through it?
c) A load taking 200 A is supplied by copper and aluminium cables connected in parallel.
The total length of conductor in each cable is 200 m, and each conductor has a cross-
sectional area of 40 mm2. Calculate:
the voltage drop in the combined cables;
the current carried by each cable; .
the power wasted in each cable. Take the resistivity of copper and aluminium
as 0.018 um and 0.028 um respectively.

Acommercial grade cubical freezer, 4 m on a side, has a composite wall consisting of an exterior sheet of 5.0-mm thick plain carbon steel (kst= 60.5 w/m k), an intermediate layer of 100-mm thick polyurethane insulation (kins 0.02 w/m k), and an inner sheet of 5.0- mm thick aluminium alloy (kal polyurethane insulation and both metallic sheets are each characterized by a thermal contact resistance of r 2.5 x 104 m2 k/w. (a) what is the steady-state cooling load that must be maintained by the refrigerator under conditions for which the outer and inner surface temperatures are 25°c and -5°c, respectively? (b) for power saving purpose, which wall material should be increased/reduced in. thickness in order to reduce 50% of the cooling load found in part (a)? redesign the thickness of the proposed material. 177 w/m-k). adhesive interfaces between the q=575.93 w