A copper-target x-ray tube is operated at 40,000 volts and 25 mA. The efficiency of an x-ray tube is so low that, for all practical purposes, one may assume that all the input energy goes into heating the target. If there were no dissipation of heat by water- cooling, conduction, radiation, etc., how long would it take a 100-gm copper target to melt? (Melting point of copper = 1083C, mean specific heat = 6.65 cal/mole/"C, latent heat of fusion = 3220 cal/mole.)