Application: Heating Food in a Microwave Oven. A frozen pizza ismarketed to be heated in a microwave oven. As
an engineer you are asked to write heating instructions,specifically how long should it be heated to reach a proper
temperature. The average residential microwave oven is 50 cm wide,40 cm deep, and 30 cm high. The oven has a low
and a high heating level. At low, it produces a time averaged powerof 500 W, whereas at high, the power is 1000 W.
We will assume the pizza is placed flat on the bottom of the ovenand any power coupled into the pizza enters from
above. The pizza is 25 cm in diameter, 1.5 cm thick, and is 75 %water by volume. The microwave oven heats the
water in the pizza. The frozen pizza is at –20 C and must be heatedto 75 C. Heat capacity of water is 4.1885 J/
(gK) and the latent heat (of melting ice) is 334 J/g (heat capacityis the energy needed to raise the temperature of one
gram of substance (water in this case) by 1 degree Kelvin, andlatent heat is the energy required to melt a gram of ice
at 0 C to water at 0 C). Assume that the heat transfer from theelectromagnetic waves to the pizza is 80 % efficient
and calculate:
(a) The time it takes to heat the pizza on the low setting of theoven.
(b) The time it takes to heat the pizza on the high setting of theoven.
(c) The cost in electricity to heat the pizza if a kW h costs$0.16.
