Heat transfer is the exchange of heat or thermal energy between two physical systems with different temperatures. The rate of heat transfer completely depends on the temperature of the two bodies and the medium in which the two bodies are placed. This is the basic definition of heat transfer.
Generally, there are three fundamental modes of heat transfer namely Conduction, Convection in fluids and the last one radiation. In all the three modes of heat transfer, there will be an exchange of heat which causes primary change in the internal energy of the system. Energy can neither be created nor destroyed.
Let’s begin the topic from conduction. For instance, let us assume a cup of cold water and a hot iron bearing. The temperature of the cold water and hot iron bearing is different. When we drop the hot iron bearing into the cold water, the heat energy or thermal energy gets transferred from the hot iron bearing into the water constantly until a constant and a stationary temperature is attained by both the water and iron bearing. This type of heat transfer is known as conduction. Conduction may also be considered as diffusion but diffusion is not equal to conduction. In simple terminology, conduction is the transfer of heat between two bodies when there is a physical contact between them.
There are again two types of conduction. The first one is steady state conduction and the second one is transient conduction. Steady state conduction can be seen when the temperature difference driving the conduction is constant. Transient conduction can be seen when the temperature within the material changes with respect to time. These are the two main subcategories of conduction.
Most of the people think conduction is the only mode of heat transfer between two bodies but their assumptions are completely wrong. The conduction is the best and effective way of heat transfer there is another way which is approximately equal to the effect of conduction known as convection. In conduction we have placed the hot iron bearing into the cold water, where the heat got transferred from hot iron bearing to the cold water. Now let’s assume a hot iron bearing alone. Just keep it open to the atmosphere and conversion takes place automatically. The heat transfer in convection is also due to the difference of temperature between the two mediums, but we can observe that there is no physical contact between the two.
Basically heat cannot get transferred from liquids to gases through conduction. The only available mode of heat transfer is convection. Convection is the transfer of heat through the bulk of liquid and gases, generally can be termed as fluids. Even plasma comes under this aspect and can be taken place only through Convection.
The best example of convection is heating cold water through a pan kept on the stove. First the bottom layer of pan gets heated up which can be considered as the heating of the complete pan. The heat of the pan gets transferred to the bottom layer of water. We know that when its temperature increases, density decreases. So the cold water present on the top of the pan interchanges the place with hot water at the bottom.
There are two subcategories in convection namely Natural convection and Forced Convection. Natural convection includes the heating of the earth and atmosphere through the sun’s rays. Forced convection involves pumps, motors or any machines or systems that gets forced to other fluid. Home cooling air conditioners through fans comes under Forced Convection.
This is the final mode of heat transfer which involves Radiation. Radiation is usually the transfer of heat from one place to another place through electromagnetic waves. Radiation have no obstructions. They can even penetrate through solid objects and reach their targets. This does not even include the motion or movement of particles.
For example, let us consider the universe and the earth. The suns heat reaches the Earth surface only through radiation, as the radiation can also pass through the vacuum in the universe. The higher the temperature of the object, the higher will be its radiation. The Radiation is directly proportional to fourth power of temperature.