An experimental power source for spot welding is designed to deliver current as a ramp function of time: I = 100,000 t, where I = amp and t = sec. At the end of the power-on time, the current is stopped abruptly. The sheet metal being spot welded is low carbon steel whose unit melting energy = 10 J/mm3 . The resistance R = 85 micro-V. The desired weld nugget diameter = 4 mm and thickness = 2 mm (assume a disc-shaped nugget). It is assumed that 1/4 of the energy generated from the power source will be used to form the weld nugget. Determine the power-on time the current must be applied in order to perform this spot-welding operation.