Thermal conduction

Thermal conduction is the process by which heat transfers from hotter objects to cooler objects through direct contact. Heat transfers only from hotter objects to cooler objects, never from cooler objects to hotter objects. Some materials conduct heat more efficiently than others. Materials that conduct heat extremely poorly are useful as insulators. They are used to trap heat or to keep heat out. The transfer of heat may also occur through convection or radiation.

Background

Thermal energy is energy that comes from heat. Heat is the transfer, or flow, of energy from a hot object to a colder object. Heat is a one-way street; it always moves from a warmer object to a cooler one. Temperature is different from heat. Temperature is a measure of an object's ability to transfer heat. The higher the object's temperature, the more likely the object is to transfer heat. As an object transfers heat to its surroundings, it gradually decreases in temperature. Heat is transferred through a variety of mechanisms, including conduction, radiation, and convection.

All objects are made of molecules. Heat is caused by the movement of molecules in an object. As the molecules move, they impact one another, which causes other molecules to move. At higher temperatures, molecules move faster, causing more collisions. When a fast-moving molecule collides with a slow-moving molecule, it transfers some of its kinetic energy (energy of movement) to the slow-moving molecule. The faster molecule slows down, while the slower molecule speeds up. A person who touches an object with a higher temperature feels the sensation of heat because the object forces molecules in the person's skin to vibrate faster. A person who touches an object with a lower temperature feels the sensation of cold because heat from the person's skin is transferred to the cold object, which causes molecules in the person's skin to slow down.

Heat transference functions differently when applied to different states of matter. The three most common states of matter are solid, liquid, and gas. In a solid, the molecules of an object are packed tightly together and neatly arranged. They do not move freely and are strongly bonded to one another. The molecules in a liquid are still tightly packed, but they move freely around one another. This is why liquids are able to flow and change their shape. In a gas, the molecules are spread far apart and are not bound to one another at all. For this reason, gases tend to expand until they fill a room.

Overview

Heat transference between two objects in direct contact is called thermal conduction. Because the molecules of a solid are in such close contact, vibrations and kinetic energy travel easily between them. This allows heat to efficiently travel from the warmer object to the cooler object. When two objects are pressed against each other, their molecules begin to collide. If one object is hotter than the other, its molecules are vibrating faster than the other. Because they are vibrating faster, they collide with the molecules of the cooler object. This transfers the kinetic energy held in the warmer object to the cooler object. Eventually, the molecules of both objects will vibrate at the same speed. At this point, collisions between the two objects will happen at the same rate, allowing the transfer of kinetic energy between the two objects to cease.

Some materials make more efficient conductors than other materials. This means that kinetic energy transfers through these material without much loss. Metals are particularly efficient conductors. They react to heat in an extreme manner, becoming much softer and more malleable.

Conduction can occur between liquids and solids but is significantly less efficient. The molecules in a liquid are not locked in place, so they have more space around them than molecules in a solid do. This reduces the number of collisions between molecules, drastically slowing the transfer of heat between the two objects. However, conduction can still occur. For example, hot coffee in a cold mug will transfer some of its heat to the mug. As a result, the temperature of the mug will increase, while the temperature of the coffee will decrease.

Conduction can also occur between solids and gases. Because the molecules in gases are not attached to one other and therefore can spread much farther apart, the rate of heat transference is further slowed. Many engineers and designers actually utilize this knowledge to create insulated areas. By creating air pockets inside items and homes, engineers and designers can create areas where thermal conduction is slowed. This allows body heat to build up inside warm jackets without being lost and for thermoses to keep liquids hot or cool. Many homes place insulating materials inside sections of drywall to slow the flow of heat into or out of the home, which helps homeowners maintain a static temperature.

Thermal conduction is distinctly different from radiation and convection, the other commonly discussed forms of heat transference. An example of convection is the spread of hot air throughout a room. Air rises as it warms and falls as it cools. The interaction between rising warm air and falling cool air creates air currents, which spread warm air throughout a room.

During radiation, an object produces electromagnetic waves. The waves carry energy away from an object. In most cases, radiation is caused when an object becomes unusually charged with electricity or some other form of energy. The accelerated random movements of the molecules in the object cause the characteristic electromagnetic waves to form. All warm objects radiate electromagnetic waves to their surroundings. When heated to their normal temperature, most of these waves are emitted in the infrared spectrum. Electromagnetic waves are emitted evenly in all directions, though they can be absorbed by other objects before reaching a destination. Unlike conduction and convection, radiation can heat other objects without passing through another medium. It functions just as well in the vacuum of space as it does through air and water. For example, the Sun heats Earth through radiation.

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