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Heat Transfer: Test Your Understanding Teacher's Guide + ward ' s science What conditions are required for heat transfer between states of matter to occur? In what direction will heat transfer occur? In order for heat transfer between states of matter to occur, there must be a temperature difference between two interacting states of matter. Heat transfer will occur from the state of high temperature to the state of low temperature. What are the three kinds of heat transfer? How do they differ? The three ways in which heat transfer occurs is by conduction, convection, and radiation. Conduction involves the transfer of heat from one molecule to an adjacent one as an inelastic collision in the case of fluids, as oscillations in solid nonconductors of electricity, and as motions of electrons in conducting solids. Convection involves the transfer of heat by the mixing of molecules of a fluid with the body of the fluid after they have either gained or lost heat by intimate contact with a hot or cold surface. Radiation is the only form of heat transfer that does not require a conducting substance. Critical Thinking: Why do gases have a lower thermal conductivity than solids? Heat transfer by conduction requires contact between two molecules through an inelastic collision. In a solid phase material, molecules are closely packed together and can more easily collide with one another to transfer heat. The denser the material, the more easily heat can be transferred through its molecules. In a gas, molecules are more dispersed. Therefore, collisions are less likely, and heat would not be transferred as well as it would be in a solid. Critical Thinking: A baker is cooking two loaves of bread in an oven. One loaf is in a glass pan. The other loaf is in a metal pan that is otherwise identical to the glass pan. Which loaf of bread will be done cooking first? Why? Support your answer using principles of conduction, convection, and/or radiation. Sample answer: The bread in the metal pan will be finished cooking before the bread in the glass pan because metal is a better conductor of heat than glass. Each loaf of bread will be exposed to heat by radiation from the oven and also a small amount of conduction from contact between the pans and the oven rack. Convection of the air within the oven will also play a role in cooking by heating the bread and the sides of the pan. Ultimately, the thermal conductivity of the pan material and, subsequently, its ability to conduct heat to the uncooked bread will be the main factor influencing cooking time.