Ice and canned beverages are present inside a polystyrene icebox. Initially, the inside temperature is 0 oC. The total surface area of the box through which conduction occurs is 1 m2, and each of its walls is 2 cm thick. The icebox is kept in the trunk of a car where the temperature is 35 oC. Some of the ice melts over a period of 24 hours. Calculate the amount of ice melted in kg in 24 hours. The heat required to melt 1 kg of ice is 334 kJ. The thermal conductivity of polystyrene foam is 0.01 W/m⋅oC. Assume steady conditions.
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Your feet feel cold as you walk barefoot across the living room carpet in your cold house and then step onto the kitchen tile floor. This result is intriguing, since the carpet and tile floor are both at the same temperature. The different sensation you feel is explained by the different rates of heat transfer: the heat loss during the same time interval is greater for skin in contact with the tiles than with the carpet, so the temperature drop is greater on the tiles.
Some materials conduct thermal energy faster than others. In general, good conductors of electricity (metals like copper, aluminum, gold, and silver) are also good heat conductors, whereas insulators of electricity (wood, plastic, and rubber) are poor heat conductors. Figure 2 shows molecules in two bodies at different temperatures. The (average) kinetic energy of a molecule in the hot body is higher than in the colder body. If two molecules collide, an energy transfer from the hot to the cold molecule occurs. The cumulative effect from all collisions results in a net flux of heat from the hot body to the colder body. The heat flux thus depends on the temperature difference ΔΤ = Τhot − Tcold. Therefore, you will get a more severe burn from boiling water than from hot tap water. Conversely, if the temperatures are the same, the net heat transfer rate falls to zero, and equilibrium is achieved. Owing to the fact that the number of collisions increases with increasing area, heat conduction depends on the cross-sectional area. If you touch a cold wall with your palm, your hand cools faster than if you just touch it with your fingertip.