Why is heat transferred through a solid by conduction

All matter is made from atoms — either single ones or those bonded in groups known as molecules. These atoms and molecules are always in motion. If they have the same mass, hot atoms and molecules move, on average, faster than cold ones. Even if atoms are locked in a solid, they still vibrate back and forth around some average position. In a liquid, atoms and molecules are free to flow from place to place.

Within a gas, they are even more free to move and will completely spread out within the volume in which they are trapped. Put a pan on a stovetop and turn on the heat. The metal sitting over the burner will be the first part of the pan to get hot. They also vibrate farther back and forth from their average position.

As they bump into their neighbors, they share with that neighbor some of their energy. Think of this as a very tiny version of a cue ball slamming into other balls during a game of billiards. As a result of collisions with their warmer neighbors, atoms start moving faster. In other words, they are now warming. These atoms, in turn, transfer some of their increased energy to neighbors even farther from the original source of heat.

This conduction of heat through a solid metal is how the handle of a pan gets hot even though it may be nowhere near the source of heat. Convection occurs when a material is free to move, such as a liquid or a gas.

Again, consider a pan on the stove. Put water in the pan, then turn on the heat. The double-pipe heat exchanger consists of one tube inside another tube with one fluid flowing inside the inner tube and the other flowing in the annular space between tubes.

In both cases, the tube walls serve as the heat-exchange surface. Heat exchangers consisting of spaced flat plates with the hot and cold fluids flowing between alternate plates are also in use. Each of these exchangers essentially depends upon convection heat transfer through the fluid on each side of the heat-exchange surface and conduction through the surface.

Countless special modifications, often also utilizing radiation for heat transfer, are employed for a variety of purposes. In these exchangers, the fluid streams may flow parallel concurrently or in mixed flow. In most cases, the temperatures of the various streams remain essentially constant at a given physical location, and the process is said to be a steady-state process.

As the fluids move through the heat exchangers, unless there is a phase change, the fluids are continuously changing in temperature, and the temperature gradient from one stream to the other may be continuously varying. To determine the amount of heat exchange surface needed for a given process, the designer must evaluate the effective temperature gradient for the particular conditions and particular heat exchanger design. Often the heating or cooling of an object is desired.

In this case, the object does not remain at a constant temperature, and such a process is an unsteady, transient, or time-varying system. The heating or cooling of food in ovens and refrigerators, respectively; the heating of steel billets in metallurgical furnaces; the heating of bricks in a kiln; and the calcination of gypsum are all examples of transient processes.

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Contributors include more than 10, highly qualified scientists and 46 Nobel Prize winners. Physics Thermodynamics and heat Heat transfer. Key Concepts Hide The transfer of heat can occur in three ways: conduction, convection, and radiation.

Heat transfer occurs between states of matter whenever a temperature difference exists and heat transfer occurs only in the direction of decreasing temperature, meaning from a hot object to a cold object.

Although the mechanisms and laws governing the three modes of heat transfer are quite different, all three modes can occur during a single process. An important industrial device for enabling heat transfer between fluids is called a heat exchanger.

Water boiling on a stove offers an example of all three modes of heat transfer occurring at the same time. Heat conducts through the metal fixture of the stove into the bottom of the pot of water. Heat also radiates from the flame of the gas burner to the bottom of the pot.

Convection transfers the heat from the bottom of the pot to the water. Conduction Heat conduction involves the transfer of heat from one molecule to an adjacent molecule as an inelastic collision in the case of fluids, as oscillations in solid nonconductors of electricity, and as motions of electrons in conducting solids such as metals.

Convection Like conduction, heat transfer by convection is due to molecular motion. See also: Density Convection heat transfer is vital to an enormous number of engineering applications and systems found in nature. See also: Heat convection Heat transferred by convection can involve single-phase fluids for example, gases or liquids or fluids that undergo a phase change.

Radiation All materials, regardless of temperature, emit radiation in all directions. See also: Blackbody ; Light ; Photon ; Volt-ampere Liquids and gases absorb and emit radiation with more distinct spectral characteristics. Design considerations The principles governing the three modes of heat transfer can be used to design and analyze systems. Heat exchangers Heat transfer is a goal in many industries. See also: Heat exchanger In power plants, oil refineries, and chemical plants, two commonly used designs are tube-and-shell and double-pipe heat exchangers.

Margaret Wooldridge Ralph H. Test Your Understanding Hide What conditions are required for heat transfer between states of matter to occur? In what direction will heat transfer occur? What are the three modes of heat transfer? How do they differ? Critical Thinking: Why do gases have a lower thermal conductivity than solids? Critical Thinking: In tube-and-shell heat exchangers from a heat exchanging perspective, why are bundles of multiple small tubes used on the interior to transfer heat to the external fluid rather than one large tube to carry the same volume of fluid?

Critical Thinking: A baker is cooking two loaves of bread in an oven. One loaf is in a glass pan. The other loaf is in a metal pan that is otherwise identical in size and shape as the glass pan.

Which loaf of bread will be done cooking first? You may already have access to this content. When the free electrons absorb heat energy, they move much faster.

As they move through the metal, free electrons crash into metal ions. Some of the kinetic energy of the free electron is absorbed by the ions and it vibrates faster and with greater amplitude. This process is very much faster than conduction caused by just passing vibrations from atom to atom. Hence, conduction in metals is faster than in non-metals. Conduction in a solid. A metal bar is heated up. The heat energises the atoms and vibration increases. The vibration spreads throughout the solid heating up the entire bar but this is a very slow process.