Q.Give the advantage of AC in long distance power transmission with an illustration.

The transmission-loss problem. Electric power, generated at large power stations often located far from the towns and cities that consume it, must travel long distances through transmission lines, unavoidably losing a fraction of it to Joule heating $I^2R$ along the way. Reducing this loss by lowering the line's resistance R (using thicker copper/aluminium wire) is technically possible but far too costly to be economically viable across a whole national grid.

The AC solution -- reduce current, not resistance. Because generated power is alternating, transformers can step its VOLTAGE up (and correspondingly current DOWN, since $P=VI$ is fixed) cheaply and efficiently, and because loss scales as $I^2R$ -- the SQUARE of current -- even a modest voltage increase produces a dramatically larger loss reduction than an equivalent reduction in R would (worked numerically: transmitting the same 2 MW power at 10 kV loses 80% of it to $I^2R$ heating, but transmitting it instead at 100 kV -- a mere tenfold voltage increase -- cuts that loss to just 0.8%, a hundredfold improvement, because the current itself falls by a factor of 10 and loss falls by the square of that factor). …