2.3: Heat Transfer

Modes of Heat Transfer

A critical example of kinetic energy is thermal energy, which arises from the microscopic movement of many molecules within a system. We discussed how temperature is a measure of the average speed of atoms and molecules in a system. The faster the molecules move, the higher the temperature. When two objects (solid, liquid or gaseous) with different temperatures either come into contact or are placed close to each other, this thermal energy can be transferred from the object with higher energy to the object with lower energy. The transfer of thermal energy due to a temperature difference between two objects is known as heat. Heat is a form of energy in transit, and once transferred, it is stored as internal energy. Heat, being a form of energy, is given in standard units of Joules (J).

Figure \(\PageIndex{1}\): Conduction, convection, and radiation are all methods of heat transfer. When you're sitting by a fireplace, you feel the heat via radiation. Warm air and smoke rise from the fire via convection, and a marshmallow stick or a hot object heats up by conduction. (CC BY-SA 4.0; Kmecfiunit via Wikimedia Commons).

Figure \(\PageIndex{1}\) illustrates the three primary methods of heat transfer: conduction, convection, and radiation. However, there is a fourth mode of heat transfer, which is very important for the atmosphere: the absorption or release of latent heat.

Conduction

If you hold a metal rod over a fire, you feel the energy from the fire is immediately transferred to your hand, causing you to feel a burn. This is an example of conduction: energy is directly transferred through a substance without the movement of material. The molecules of the rod directly above the fire absorb heat from the flame and begin to vibrate faster due to the additional kinetic energy. This energy is transferred to the neighboring lower-energy molecules through vibrations until it reaches your fingers. Contact with these vibrating rod molecules causes the molecules in your fingers to begin vibrating, in a sensation that we perceive as heat. Certain materials are better conductors of heat than others. Metals, for example, conduct heat very efficiently, while air, which acts as an insulator, is a very poor conductor of heat. This is why air and other poor conductors, such as styrofoam, are often used as insulators. However, that does not mean that air cannot transfer thermal energy.

Convection

Another type of energy transfer is convection, which occurs in liquids and gases (both fluids) because their molecules can move freely, allowing currents to form naturally. Convection refers to the heat transfer by the macroscopic movement of a fluid. This occurs because lower-density fluid tends to rise over higher-density fluid, which sinks under the force of gravity, thereby transferring heat within the fluid. In Figure \(\PageIndex{2}\), a beaker is being heated from the bottom by a flame. The arrows show upward movement in the center, where the fluid is heated and therefore is less dense and buoyant. The cooler fluid at the top of the beaker is denser and sinks toward the bottom under gravity.

Convection occurs commonly in the atmosphere. The Sun heats the Earth's surface unevenly, resulting in certain regions being warmer than others. The air above the warmer regions receives energy from the ground via conduction. Since air is a poor conductor, it cannot transfer this energy to its neighboring molecules via conduction. Instead, the warm air rises due to positive buoyancy and is replaced at the surface by sinking cooler air, similar to the beaker in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): Convection within a fluid that is heated from the bottom. Warm fluid rises up near the middle, and is replaced at the bottom by colder fluid from the sides of the beaker. (CC BY 2.0; Siyavula Education via Flickr).

Radiation

When you stand outside in the Sun, you feel warm. Similarly, if you're sitting next to a fireplace or a bonfire, you feel its warmth without having to place your hands in it. This transfer of energy from the Sun or the fireplace is called radiant energy or radiation. Radiation is the transfer of energy via electromagnetic waves. Radiant energy can be transferred to an object without the space in between being heated. Electromagnetic waves do not need any matter to travel through and propagate at the speed of light in a vacuum, which is about 300,000 km·s^–1. In the atmosphere, this is the most important form of energy transfer.