A 5-cm-diameter copper sphere is initially at a uniform temperature of 200◦C. It is suddenly exposed to an environment at 20◦C having a heat-transfer coefficient h=28W/m2 · ◦C. Using the...

A 5-cm-diameter copper sphere is initially at a uniform temperature of 200◦C. It
is suddenly exposed to an environment at 20◦C having a heat-transfer coefficient
h=28W/m2 · ◦C. Using the lumped-capacity method of analysis, calculate the time
necessary for the sphere temperature to reach 90◦C.