(a) Explain the principle of conservation of charge and its relationship to Kirchhoff's current law. (6)

(b) A 5 kVA, 200/400 V transformer has iron losses of 40 W and full-load copper losses of 100 W. Find (10)

(i) efficiency at full load, unity pf;

(ii) efficiency at half load, 0.8 pf;

(iii) the load at which maximum efficiency occurs.