Heat Transfer: Definition And Formula

In order to understand the heat transfer definition, it is important to understand what heat is and how it is formed. When the temperature between the two systems is different, the interaction between those two systems will vary. This difference in an interaction caused by the difference in temperature is called heat. 

Two important things are considered here: 

1. i) temperature difference 
2. ii) interaction between the systems. 

Even if one of these two is absent, heat will not be produced. Hence, heat transfer is the interaction or exchange of energy and entropy from one system to another. The SI unit of heat transfer is expressed as joule (j) and calorie (cal) as heat is a form of energy.

Heat transfer definition

The heat transfer mechanism is heat transferred from one place to another. In other words, the energy and entropy are conveyed from one system to another system.

Heat transfer formula

The heat that is transferred from one place to another is measured by using the heat transfer formula mentioned below:

Q=mcΔT

Where: 

Q is the heat transfer (quantity of heat in joules),

m is the mass of the substance,

c is the specific heat capacity which is the amount of energy needed to change the temperature of 1 kg of a substance by 1unit of temperature,

ΔT is the temperature change.

Modes of heat transfer

There are three mechanisms through which heat transfer occurs:

1. Conduction
2. Convection
3. Radiation

1.Conduction: 

Conduction is the simple mode of heat transfer. In this mechanism, the heat transfers from one part of the body to another due to the temperature difference between them. Conduction occurs in solids. The molecule with greater energy will pass over some of its energy to the molecule possessing lesser energy.

In simple words, conduction could be defined as the heat transferred from the hotter end to the colder end of an object. It is usually a slow process.

A simple example of conduction is when you hold a glass of hot water in your hand, the heat will be transferred from the glass to your hand.

Gases are poor thermal conductors, whereas liquid is an intermediate between solids and gases. 

2.Convection: 

When heat transfer occurs because of the movement of atoms and molecules, which carry heat from one place to another, it is called convection. In simple words, the heat transfer that occurs due to the actual movement of the matter is called convection. Convection occurs only in fluids and rarely in gases.

The movement of atoms or molecules is due to either the change in density or the forced movement of the liquid. Convection can be natural or forced.

• Natural convection: 

The heat is transferred mainly due to gravity. When water is heated in a utensil, not all the water gets heated simultaneously. Instead, the water at the bottom of the utensil, which is closer to flames, gets heated first and expands, having less density. As a result, the hotter water goes up because of buoyancy, and the water on the sides goes down and gets heated. Similarly, once this water becomes hot, it goes up, bringing the water from the sides to the bottom to get heated. Thus, the whole water gets heated. 

From this, we understand that heat transfer is caused by the changes in the density of the liquid in natural convection.

• Forced convection

The fluid is made to move by a force of physical factors like a compressor, stirrer, pipe or pump etc. Examples of forced convection include forced-air heating systems in the home and the human blood circulation system in which the heart acts as a pump circulating blood to all parts of the body.

3.Radiation:

For radiation, no material is required as a heat transfer medium, unlike conduction and convection. In radiation, the heat transfer occurs by the transfer of heat by electromagnetic waves from one place to another by photons of wavelength greater than 10000 Å. 

The solids above 0° K can radiate and absorb heat to and from their surroundings. Radiant energy is the form of energy that is radiated by electromagnetic waves. 

The radiant heat may be reflected or transmitted, or absorbed. Many liquids and gases are transparent to it.

The radiation heat transfer does not require any material to transfer heat because, in an electromagnetic wave, the electric and magnetic fields interact in space and time. Electromagnetic waves can be travelled at the speed of light. All bodies emit radiant energy no matter if it is solid, liquid or gas.

Thermal radiation occurs when electromagnetic waves are emitted due to their temperature, just like radiation emitted by a filament lamp’s light. When the thermal radiation is emitted, it falls on another body and may partly get absorbed and partly reflected. 

The heat a body can absorb by radiation depends on a body’s colour. Black colour absorbs and emits radiant energy more effectively than lighter colours.

Conclusion:

In this article, we discussed heat transfer definition and heat transfer formula and different modes through which heat transfer occurs. The transfer of energy and entropy from one place to another is called heat transfer. 

The heat transfer formula is Q=mcΔT. Heat is transferred through conduction, convection and radiation. The heat transfer of solids occurs through the mode of conduction. The heat transfer of liquids occurs through the mode of convection by natural or forced method, whereas the heat transfer of solid, liquid and gas occurs through radiation mode.