Question

You are boiling pasta and absentmindedly grab a copper stirring spoon rather than your wooden spoon. The copper spoon has a 20 mm × 1.5 mm rectangular cross section, and the distance from the boiling water to your 35∘C hand is 18 cm. How long does it take the spoon to transfer 40 J of energy to your hand?

Answer 1

Given
   area of cross section of copper spoon i s A = 20mm*1.5 mm

  
   temperature difference is DT = (100-35) = 65 0C

   length of the spoon is l = 18 cm,

   amount of heat should be transfer Q = 40 J

coefficient of thermal conductivity of copper k = 400 W/mk

we know that the thermal conductivity is Q/t = k*A*DT/l

          

               t = Q*l/k*A*DT
               t = (40*0.18)/(400*30*10^-6*65)s

               t = 9.23 s