Thermodynamics and heat transfer are two concepts of physics that are related but different. We explain the difference between heat transfer and thermodynamics by giving their definitions:
We define heat transfer as the transmission of a flow of energy in the form of heat from one body to another. There are different mechanisms to transfer heat, but heat flows from the hot body to the cold body according to the second law of thermodynamics.
On the other hand, thermodynamics is the part of physics that studies heat transfer, energy conversion, and the ability of systems to produce work. The laws of thermodynamics explain the global behavior of macroscopic systems in equilibrium situations.
Heat Transfer Mechanisms
There are three types of heat transfer: conduction, convection, and radiation:
Conduction of heat transfer: Heat conduction occurs when two objects at different temperatures are in direct contact. Heat passes from the hot to the cold object through the contact points.
Thermal Radiation Transfer: This heat transfer mechanism occurs through electromagnetic waves and can be transmitted through a vacuum. The amount of heat that is transferred depends on the wavelength. This is how solar radiation travels through the Solar System.
Convective heat transfer. The convection process is carried out through an intermediate medium that transports energy, usually liquids and gases such as air or water.
Heat Transfer by Conduction
Conduction heat transfer occurs when two substances are in direct contact and have a temperature difference.
Heat flows from particles with higher kinetic energy (temperature) to less energy-rich (cooler) particles. The heat flow depends on the temperature difference over the distance (the temperature gradient) and the internal resistance to the heat flow of the relevant material, called thermal conductivity or the coefficient of thermal conductivity. This principle is expressed in Fourier's law.
Example of Thermal Conduction
An example of heat transfer by conduction is when we touch a piece of ice with our hands. We feel the cold because we are transferring heat from our body to the ice.
Heat Transfer by Radiation
Transfer of thermal energy by radiation. This is the transfer of heat between two bodies that are not in contact with each other without using an intermediate. One body is hot and emits a lot of electromagnetic radiation, thus losing heat. The other body absorbs part of the incoming radiation and converts it to heat.
Example of Heat Transfer by Radiation
An example of radiant energy transmission is infrared radiation from the Sun. Solar energy is transmitted through space through electromagnetic waves.
Heat Transfer by Convection
Convective heat transfer is heat transfer through the displacement of a warm liquid, hot gas, cold liquid, or cold gas. When heat drives the flow, the extent of the flow can be expressed as the heat transfer coefficient.
There are two types of thermal convection:
Natural convection occurs without any artificial help. The movement of the fluid is produced by differences in density due to the temperature difference.
Forced convection occurs when the movement of the fluid is done through some artificial mechanism, such as a water pump.
Example of Heat Transfer by Convection
An example of convective heat transfer is the operation of a fan. When the fan is running, the circulating air carries away the hot air in contact with the skin and replaces it with cooler air.
Another example of convection occurs in a hot shower. The electric heater heats water by increasing its thermal energy. The hot water travels through the pipes until it reaches the shower, it falls on us transmitting the heat.
The Laws of Thermodynamics
The laws of thermodynamics are empirical principles that cannot be demonstrated because they are based on experience, not theoretical reasoning. They refer to systems in a state of equilibrium. There are four, although the most important are the first and the second: