Conduction

Introduction

Heat transfer refers to a transfer of energy from one part of an object to another, or from one object to another, arising due to a difference in temperature. There are 3 kinds of heat transfer: conduction, convection, and radiation. 

Conduction

Conduction is the mechanism of transfer of heat between two adjacent parts of a body because of their temperature difference. For instance, if one end of a metal rod is held above a flame, the other end, which has not been in contact with the flame, will soon turn hot as well. This is due to conduction, enabling the heat to travel through the entirety of the rod. The molecules at regions which are hotter have more kinetic energy than those which are not hot. Some part of this energy is passed on to nearby molecules, these nearby molecules will in turn pass it on to their neighbors. Gasses are known to be poor conductors of heat, whereas liquids have intermediate conductivity, and solids have the most amount of conductivity because the free electrons of metals enable the transferring of heat from one point to another very quickly and efficiently. The thermal conductivity thus depends on the state and nature of the conductor. 

Thermal conduction

Quantitatively, thermal conduction can be described as the rate of time at which heat flows in a given material while set at a particular temperature. It is also possible to maintain parts of an object at different temperatures. For instance, a metallic rod, when cross-sectioned into two equal parts, can be submerged in large reservoirs of the desired temperatures. This will enable the metallic rod to maintain different temperatures at the ends which are submerged. The ideal condition would be to insulate the sides so that no heat is exchanged between the sides and surroundings. When a steady state has been achieved after some time, the temperature decreases. The reservoirs at both ends supply heat at constant rates which is transferred through the bar. Experimentally, it is found that during this steady state, the rate at which heat flows is proportional to the difference in temperature at both ends and the cross-sections. It is inversely proportional to the length of the bar. This is denoted by the formula:

H=KA öT/L

Here H is amount of transferred heat

K is thermal conductivity

A is the area of the surface

L is the distance between two isothermal surfaces

T is difference in temperature

The greater the value for K is for a given material, the more rapidly it will conduct heat. The thermal conductivities for various materials are listed in the table below. These values are constant at a normal temperature range but may vary slightly with higher or lower temperature. 

TABLE 1

Conduction examples

After comparing the thermal conductivity of metals and non-metals, you can notice some stark differences in how they transfer heat. Wood is a weak conductor, whereas metals have higher conduct abilities and conduct the heat that is present very quickly. One of the conduction examples is why some cooking pots have copper underneath. Copper is a great conductor of heat and thus, promotes a uniform distribution of heat all throughout the container for optimal cooking. Plastic foam, on the other hand, is a bad conductor of heat because it contains pockets of air. Heat retention and transfer of heat are important factors determining everyday life decisions. Concrete houses get extremely hot during summers as it is a great conductor of heat, however less than metals. That is why most inhabitants give a layer of insulation to prevent overheating of their houses. This keeps the house cool. In cases like those of nuclear reactors, heat transfer is crucial as elaborate systems need to be planned and installed so that the huge amounts of energy that nuclear fission produces is transited as soon as possible and efficiently. This prevents the core from overeating.  

Types of conduction in physics

There are 2 types of conduction: steady-state conduction and transient conduction

Steady-state conduction

A material is said to be in steady-state when the temperature at its cross sections on any position remains constant with time. The steady-state is different from a state of thermal equilibrium in which temperature at any position should be the same. The temperature of the material in a steady-state varies with position. During this state, there is zero absorption or emission of heat at the cross-sections. The extreme ends of the material are steady at specific temperatures and all other surfaces are covered with insulators so that heat cannot escape through any wall. This also enables the same amount of heat to flow through the cross-sections at particular intervals of time. 

Transient or non steady-state conduction

During a transient or non steady-state conduction, the temperature changes or varies along any part of the material at any given point of time. The major determinant of the conduction in this case is the material’s time-dependent of temperature. Transient conduction generally occurs when a change in temperature is newly introduced on either the outer edges of the material or inside. Thus, the change in temperature is brought about by the sudden entry of a new source of heat within the particular material or object. 

Fourier’s law

Known alternately as the law of heat conduction, it postulates that the  “the rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat flows.” The law further has integral and differential forms. The integral form is characterized by the amount of energy that moves in or out of a particular object. The differential form, the rate of flow or local energy fluxes are considered. 

Conclusion

In this article, we learned about thermal conductivity, its example, types and Fourier’s law of conductivity. Heat can be transferred by three different mechanisms such as conduction, convection and radiation. Conduction is the mechanism of transfer of heat between two adjacent parts of a body because of their temperature difference. There are 2 types of conduction: steady-state conduction and transient conduction.