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4.14 Heat Transfer

The idea of thermal equilibrium is one of the most important and simple ideas in understanding the physical universe around us. Heat always flows so as to equalize temperature differences. Let’s look at the three different ways that heat can be transferred from warm material to colder material: conduction, convection, and radiation. These three processes are happening around us all the time.

Conduction occurs when heat is transferred through atomic collisions. This can occur in a solid or a liquid; the atoms or molecules of a gas are too far apart for conduction to be effective. We have seen some examples of conduction — ice cooling down water, hot metal burning your finger.

As an analogy for conduction, suppose the balls on a pool table are all stationary and spread at random over the table. You stand at one end of the table and give the balls at that end high speed. Every time a ball comes within your reach you give it an extra kick and speed it up. You are acting like a heat source, creating "hot" material at the end of the table while leaving the other end "cold." Energy would be transferred among all the balls as they hit each other. Eventually, the velocities all over the table would be about the same, as "heat" transferred to the "cold" end, creating equal temperatures at both ends. You have given the balls at the far end energy without ever leaving your end of the table! This energy traveled by conduction. In the same way, if you (unwisely) held a metal poker with one end in a fire, heat would travel up the poker by conduction and you would be rapidly forced to drop it.

Not all materials are equally good conductors. In general, metals are good conductors while plastics and porous materials like wood are poor conductors. Suppose you have a metal and a plastic surface at room temperature. They are both at the same temperature, so why does the metal surface feel cooler? You know that your body is hotter than room temperature, so heat will try to flow from your finger to the material you touch. The long and complex molecules in plastic are inefficient at conducting heat, so your finger is not affected much. Metal atoms are very efficient at conducting heat. The result is that heat is rapidly transferred from your finger to the metal. Your finger cools down and to you the metal feels cold! When we want to trap heat, we use a material that is a poor conductor — Styrofoam for a coffee cup and wool for a sweater, for example.

Boiling water. At first, the water is heated through the heating element and the pan, but at 100? C, heat is transferred through the water via convection. Click here for original source URL.

Convection occurs when heat is transferred by the movement of masses of material. Since solids cannot move freely, convection is only effective in liquids and gases. When warm material mixes with cooler material, energy is transferred to the cooler material. Convection is a very efficient way to transfer energy. Think about it the next time that you take a bath. As you pour hot water into the bath, you could wait for the heat to diffuse out towards you by the collisions of water molecules (conduction), or you could speed up the process by stirring the hot water into the cooler water (convection). Now consider what happens when you boil a pan of water. At first, the temperature rises with no visible motion of the water. During this phase, heat is transferred from the heating element to the pan and from the pan to the water by conduction. As the water reaches 100° C, heat can no longer be transferred fast enough by conduction, so it begins to move by rolling convection motions. We see the water boil.

Many familiar weather conditions are caused by convection. The Sun heats the ground, which heats the air just above it. Warm air is less dense than cold air so it rises, cools, and then sinks again toward the ground. This movement of air in great circulating patterns is a good example of convection. Sit and watch a cumulus cloud some afternoon — they are the puffy, fair weather clouds. As warm air rises it will reach an altitude where liquid water droplets can condense from the gaseous water vapor in the air (just as water beads form on the outside of a cold glass of water). The droplets, of course, form what we see as a cloud. The slow billowing of a cumulus cloud is evidence of convection.

Just as in other modes of heat transfer, convection is energy flow that tries to equalize the temperature difference between two places. The Sun heats up the ground, and convection transfers heat upward through the atmosphere. The turbulence we sometimes experience while flying is a sign of convection. As Newton predicted, convection increases with an increasing temperature difference between the ground and the air. Turbulence is generally worse in summer than in winter. Convection is not only all around us, it is beneath our feet too. Geological activity is largely the result of churning convective motions of molten rock!

Thermal radiation in visible light can be seen on this hot metalwork. Click here for original source URL.

Radiation is another important mode of heat transfer. We will confine our attention to thermal radiation, which is the energy emitted by any object that relates to its temperature. The word "thermal" refers to heat: the hotter an object is, the more thermal radiation it emits. Heat transfer by radiation is quite different from the types of heat transfer we have already discussed. Conduction takes place in a solid or a liquid. Convection takes place in a liquid or a gas. Both of these mechanisms require a material medium to transmit the energetic motions of atoms and molecules. Radiation can take travel through a gas but it can also travel through a pure vacuum. Our most familiar experience of heat transfer by radiation is our daily exposure to the Sun. The Sun's radiation travels across space and strikes the Earth, providing the Earth's main source of heat. Without the Sun's thermal radiation, the Earth would be in a deep freeze.