Problem Set #8
Goal of Problem Set #8: This assignment is meant to help you understand:
  1. the relationships between thermal energy, heat, and temperature,
  2. the physics of combustion,
  3. the mechanisms of heat transfer: conduction, convection, and radiation,
  4. the black body spectrum,
  5. how the rates of heat transfer depend on temperatures.
In a move combining business and pleasure, you have opened a pizza parlor, with delivery service, in your off-grounds apartment. Being a responsible citizen, you have naturally obtained all the appropriate licenses. And you've developed a Frisbee-shaped plastic carton so that you can deliver the pizzas by flinging them across town with a high-tech catapult. Your slogan is "When you're a-drag-en, get a lift from our pizza," and your poster features that mythical creature flying around and cooking pizza with its fiery breathe (get it? "a-drag-en" = "a dragon" and lift and drag are both aerodynamic terms? I know, things are pretty bad when I have to explain my own jokes.) However, our focus today is not on making pizzas fly; it's on cooking them.
1. You cook your pizzas in a wood-burning brick oven. While it seems that the oven would be most energy efficient if it were perfectly sealed and didn't exchange any air with the rest of the apartment, the fire would go out if you sealed it. Why?
2. To make sure the fire doesn't go out, the oven has an arched opening on one side and a chimney coming out of its top and passing up through the roof. When the fire is burning, the oven develops a natural draft: air enters the side opening and exits through the chimney. What force or imbalance in forces propels this draft?
3. Brick is a wise choice for the oven walls because the outside of the oven remains cool to the touch even when the inside surface of the oven is quite hot. If you had used metal oven walls, you'd burn your fingers on the outside of the oven. Explain briefly what physical difference between brick and metal makes brick the better choice for the oven.
4. The brick walls are dark in color, particularly in the infrared. What heat transfer mechanisms convey heat from the burning wood to the oven walls?
5. Both the burning wood and the pizzas rest on opposite sides of the oven's flat bottom surface. Why don't convection or conduction directly heat the pizzas?
6. This cooking system rarely burns the bottom of the pizza crust. Why?
7. Since the wood and pizzas are side-by-side on the oven bottom, they don't "see" one another very well. In particular, the pizza has almost no view of the glowing wood coals. What steps are involved in the flow of heat from the wood to pizzas? How are the oven walls involved?
8. If you were to install a big fan inside the oven to circulate the air artificially, the cooking process would change significantly. Why?