Multiple-Concept Example 8 reviews the approach that is used in problems such as this. A person eats a dessert that cont

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Multiple Concept Example 8 Reviews The Approach That Is Used In Problems Such As This A Person Eats A Dessert That Cont 1 (25.24 KiB) Viewed 37 times

Multiple Concept Example 8 Reviews The Approach That Is Used In Problems Such As This A Person Eats A Dessert That Cont 2 (39.37 KiB) Viewed 37 times

Multiple Concept Example 8 Reviews The Approach That Is Used In Problems Such As This A Person Eats A Dessert That Cont 3 (29.08 KiB) Viewed 37 times

Multiple Concept Example 8 Reviews The Approach That Is Used In Problems Such As This A Person Eats A Dessert That Cont 4 (42.09 KiB) Viewed 37 times

Multiple-Concept Example 8 reviews the approach that is used in problems such as this. A person eats a dessert that contains 230 Calories. (This "Calorie" unit, with a capital C, is the one used by nutritionists; 1 Calorie - 4186 J. See Section 12.7.) The skin temperature of the person is 37 °C and that of her environment is 16 °C. The emissivity of her skin is 0.71 and its surface area is 1.1 m². How many hours would it take her to emit a net radiant energy from her body that is equal to the energy contained in this dessert? Heat from environment Heat from person Number 2.6145e8 Units hr
A perfect vacuum would eliminate conduction, except where the vacuum container contacted the neck and base of the thermos with gaskets and low conducting spacers. Convection couldn't play a major role either, because it involves the mass movement of fluid. But here, the vacuum eliminates any convection currents which could transfer heat between the inner chamber and the outside. All matter emits blackbody electromagnetic radiation. The hotter the material, the more radiation it gives off. Electromagnetic radiation can propagate through a vacuum, as proven by the fact that sunlight reaches the Earth. Here's the point: Since the coffee is hotter than the air outside the thermos, the coffee and the inner surface of the chamber emit more blackbody radiation than they absorb from the air. This net outflow of radiation flows through the hollow chamber, from the inner surface to the outer surface. In this way, the coffee slowly loses energy; it cools off. Of course, coating the liner with reflective silver paint will minimize the transfer of heat by radiation, but not eliminate it. Even if the chamber of a thermos bottle contained a perfect vacuum, hot coffee in the thermos bottle would still cool off (though extremely slowly), due primarily to conduction. convection. radiation. Onone of the above.
Wool socks conduct heat poorly. For instance, they keep your feet warm by reducing the rate of heat flow from your feet to the cold floor. For this same reason, wool socks reduce the rate at which heat flows from the air into the cans. Remember, the cans warm up because they absorb heat from the surrounding air. By slowing down this process, the socks keep the cans cold. Some students choose A, reasoning that wool tends to make things warm. But wool doesn't make things warm. It merelykeepsthings warm-or cold-by slowing down the heat transfer between that thing and its environment. Chemistry student 1 wants to wrap the cold cans in thick wool socks. As compared to leaving the cans unwrapped, this plan will: Make the cans heat up more quickly than they otherwise would. Make no difference in how quickly the cans heat up. Make the cans heat up less quickly than they otherwise would. Prevent the cans from heating up at all.
If the cans were warmer than the surrounding air, then blowing air on the cans would cool them off, by increasing the rate of heat flow. Here's why. Conduction would cause a thin layer of hot air to form right next to the hot can. Blowing air on the can replaces that hot layer with a "fresh" layer of cool air, allowing the conduction process to proceed more efficiently. Similar reasoning shows that blowing hot air on a cool can causes the can to heat up more quickly than it otherwise would. As the higher-kinetic-energy air molecules bump into the lower-kinetic-energy can molecules, the air molecules give some of their energy to the can. As a result, the can heats up a little, and a thin layer of cooler air forms right next to the can. When hot air blows on the can, it replaces that cool layer with a "fresh" layer of hot air, allowing the conduction process to proceed more efficiently. For this reason, exposing the cool cans to a hot breeze makes the cans heat up more quickly than they otherwise would.. If you chose C, it's probably because you associate breezes with cooling. Well, breezes almost always cool people off, because we generate our own heat, and the breeze increases the rate of heat flow from our bodies to the air. But if the air were hotter than we are, and if we didn't produce our own heat, then a breeze would make us warmer. We'd be just like the cold cans on a warm day. Chemistry student 2 wants to keep the backpack open, so that air flows over the cans. This plan will: Make the cans heat up more quickly than they would if the backpack were closed. Make no difference in how quickly the cans heat up. Make the cans heat up less quickly than they would if the backpack were closed. Prevent the cans from heating up at all.