![]() You might expect to use about a 4 kg (7–10 lb) bag of ice per day. Significance The result of 3.44 kg, or about 7.6 lb, seems about right, based on experience. The greater the distance between hot and cold, the more time the material takes to transfer the same amount of heat. Heat transfers from the left to the right by a series of molecular collisions. ![]() (Figure) shows a slab of material with a higher temperature on the left than on the right. In contrast, a molecule in the higher-temperature region (left side) has high energy before collision, but its energy decreases after colliding with a low-energy molecule at the contact surface.Ī third quantity that affects the conduction rate is the thickness of the material through which heat transfers. In this illustration, a molecule in the lower-temperature region (right side) has low energy before collision, but its energy increases after colliding with a high-energy molecule at the contact surface. Collisions occurring at the contact surface tend to transfer energy from high-temperature regions to low-temperature regions. Molecules in two bodies at different temperatures have different average kinetic energies. Because the number of collisions increases with increasing area, heat conduction is proportional to the cross-sectional area-a second factor in the equation. Thus, the rate of heat transfer increases with increasing temperature difference If the temperatures are the same, the net heat transfer rate is zero. The cumulative effect of all collisions is a net flux of heat from the hotter body to the colder body. In a metal, the picture would also include free valence electrons colliding with each other and with atoms, likewise transferring energy. If two molecules collide, energy transfers from the high-energy to the low-energy molecule. (Figure) shows molecules in two bodies at different temperatures, and for “hot” and “cold.” The average kinetic energy of a molecule in the hot body is higher than in the colder body. (credit: Giles Douglas)Ī molecular picture of heat conduction will help justify the equation that describes it. Insulation is used to limit the conduction of heat from the inside to the outside (in winter) and from the outside to the inside (in summer). Each method has unique and interesting characteristics, but all three have two things in common: They transfer heat solely because of a temperature difference, and the greater the temperature difference, the faster the heat transfer ( (Figure)). In this section, we examine these methods in some detail. The snowshoers wear clothes designed with low conductivity to prevent heat flow out of their bodies. Convection carries some heat to them, but most of the air flow from the fire is upward (creating the familiar shape of flames), carrying heat to the food being cooked and into the sky. In the illustration at the beginning of this chapter, the fire warms the snowshoers’ faces largely by radiation. A less obvious example is thermal radiation from the human body. An obvious example is the warming of Earth by the Sun. Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed.This type of transfer takes place in a forced-air furnace and in weather systems, for example. Convection is the heat transfer by the macroscopic movement of a fluid.(The matter is stationary on a macroscopic scale-we know that thermal motion of the atoms and molecules occurs at any temperature above absolute zero.) Heat transferred from the burner of a stove through the bottom of a pan to food in the pan is transferred by conduction. Conduction is heat transfer through stationary matter by physical contact.Yet every heat transfer takes place by only three methods: So many processes involve heat transfer that it is hard to imagine a situation where no heat transfer occurs. It may occur rapidly, as through a cooking pan, or slowly, as through the walls of a picnic ice chest. Whenever there is a temperature difference, heat transfer occurs. Just as interesting as the effects of heat transfer on a system are the methods by which it occurs. Solve problems using the formulas for conduction and radiation.Solve problems on the relationships between heat transfer, time, and rate of heat transfer.Explain some phenomena that involve conductive, convective, and radiative heat transfer.By the end of this section, you will be able to:
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