# What Is Conduction?

Conduction refers to the transfer of energy through the movement of particles that are in contact with each other. In physics, the word "conduction" is used to describe three different types of behavior, which are defined by the type of energy being transferred:

• Heat conduction (or thermal conduction) is the transfer of energy from a warmer substance to a colder one through direct contact, such as someone touching the handle of a hot metal skillet.
• Electrical conduction is the transfer of electrically charged particles through a medium, such as electricity traveling through the power lines in your house.
• Sound conduction (or acoustic conduction) is the transfer of sound waves through a medium, such as vibrations from loud music passing through a wall.

A material that provides good conduction is called a conductor, while a material that provides poor conduction is called an insulator.

## Heat Conduction

Heat conduction can be understood, on the atomic level, as particles physically transferring heat energy as they come into physical contact with neighboring particles. This is similar to the explanation of heat by the kinetic theory of gases, though the transfer of heat within a gas or liquid is usually referred to as convection. The rate of heat transfer over time is called the heat current, and it is determined by the thermal conductivity of the material, a quantity that indicates the ease with which heat is conducted within the material.

For example, if an iron bar is heated at one end, as shown in the image above, the heat is understood physically as the vibration of the individual iron atoms within the bars. The atoms on the cooler side of the bar vibrate with less energy. As the energetic particles vibrate, they come into contact with adjacent iron atoms and impart some of their energy to those other iron atoms. Over time, the hot end of the bar loses energy and the cool end of the bar gains energy, until the entire bar is the same temperature. This is a state known as thermal equilibrium.

In considering heat transfer, though, the above example is missing one important point: the iron bar is not an isolated system. In other words, not all of the energy from the heated iron atom is transferred by conduction into the adjacent iron atoms. Unless it's being held suspended by an insulator in a vacuum chamber, the iron bar is also in physical contact with a table or anvil or another object, and it is also in contact with the air around it. As air particles come into contact with the bar, they too will gain energy and carry it away from the bar (though slowly, because the thermal conductivity of unmoving air is very small). The bar is also so hot that it is glowing, which means that it is radiating some of its heat energy in the form of light. This is another way in which the vibrating atoms are losing energy. If left alone, the bar will eventually cool down and reach thermal equilibrium with the surrounding air.

## Electrical Conduction

Electrical conduction happens when a material allows an electrical current to pass through it. Whether this is possible depends on the physical structure of how the electrons are bound within the material and how easily the atoms can release one or more of their outer electrons to neighboring atoms. The degree to which a material inhibits the conduction of an electrical current is called the material's electrical resistance.

Certain materials, when cooled to nearly absolute zero, lose all electrical resistance and allow electrical current to flow through them with no loss of energy. These materials are called superconductors.

## Sound Conduction

Sound is physically created by vibrations, so it is perhaps the most obvious example of conduction. A sound causes the atoms within a material, liquid, or gas to vibrate and transmit, or conduct, the sound through the material. A sonic insulator is a material whose individual atoms do not easily vibrate, making it ideal for use in soundproofing.

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