Heat transfer – conduction

Heat transfer – Conduction is one of the subjects in Thermal engineering which is primarily concerned with the exchange, use, and conversion of heat or in other words thermal energy between physical bodies. Exchange of thermal energy due to the difference in temperatures. This heat transfer takes place when there is a temperature difference. Let’s discuss this topic in detail with relevant examples for better understanding. Transfer of thermal energy from more energetic to less energetic particles due to their interaction is called conduction heat transfer. We have two types of energy transferred in materials like thermal energy and microwave energy. Let’s discuss this thermal energy and modes of transfer in detail.

Conduction 

• Heat is transferred between adjacent molecules during the heat conduction process, which is a slow process.

• The flow of heat energy during the conduction process is by direct contact. Therefore, heat energy is emitted from a warmer object and is absorbed by a colder object. The best example of this phenomenon is heating a frying pan kept on a stove.

• The transfer of heat within solid materials and between two solid materials occurs because of heat conduction.

• Metals are highly conductive substances. The transfer of heat occurs because the molecules of the hotter substance begin to vibrate faster than the molecules of the cooler object. Thermal equilibrium is maintained during heat transfer because the heat absorbed by one object is balanced by the heat emitted by another object.

• The factors affecting heat transfer rate through conduction are temperature and the thickness of the substance through which the heat energy is transferred. Therefore, the thicker the material of the substance, the longer it will take to transfer heat energy through it. Therefore, when we feel cold during the night, we sleep with a thicker blanket.

• The thermal conductivity of a material refers to the constant of proportionality K. A material will conduct more heat if the value of K for that material is high.

• The SI unit of thermal conductivity K is (J s-¹ m-¹ K-¹) or W m-¹ K-¹.

• Conductors are instruments/tools through which heat energy can easily flow, whereas an insulator is an instrument that doesn’t allow heat energy to transfer.

• We can observe in our day-to-day lives several combinations of instruments that have a good conductor and a good insulator. For example, cooking pots are made from metals such as copper or brass, which are great conductors. But if we notice closely, their handles are made from insulators such as plastic foam or wood.

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. 

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. It is inversely proportional to the length of the bar. This is denoted by the formula:

H = KA.ΔT/L

Here H is amount of transferred heat

K is thermal conductivity

A is the area of the surface

L is the distance

ΔT 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

Conclusion :

Heat transfer refers to the phenomenon of transferring energy from one point to another. This specific mechanism includes heat conduction, convection, and radiation heat transfer. Conduction is the process of transfer of heat during the heat conduction process, which is a slow process.  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.