What is Heat Transfer?

All the matter in this universe is made up of constantly moving particles that possess kinetic energy. Kinetic energy is caused by the vibrations and movements between particles, and potential energy is stored within the particles of that matter.

As the particles move faster, the kinetic energy increases and, due to this, the temperature increases. Temperature is a quantity that tells about the kinetic energy of the particles. Thermal energy, or heat, is the total energy of the particles in a material. Heat depends on the mass of the system. The greater the mass of the system, the larger the number of moving particles it has. Heat is a type of energy that flows from a higher temperature range to a lower temperature range. It can be transferred in three modes: conduction, convection and radiation.

According to thermodynamics, during heat transfer, thermal energy always moves from the hotter region to the colder region. That is, the molecules move at a rapid pace from a high-temperature range to a lower temperature range.

The specific heat is the amount of heat required to increase the temperature of 1kg of a material by one degree Celsius. Water has a high specific heat capacity because the water molecules form strong bonds with each other, forming unbreakable H-bonds, which take more heat energy to break. This is the reason why land heats up quicker during the day and cools quicker at night than water.

Heat Transfer Modes 

Conduction Heat Transfer

Conduction is the transfer of thermal energy through matter by direct contact of particles. This heat transfers when the molecules of the substance collide and there is a temperature difference between them. This process takes place in solids, liquids, and gases. For example, when a metal strip is heated at one end, the heat slowly travels to the other end.

While heating, the particles vibrate. These vibrations vibrate the adjacent particles and the process is passed on to the entire substance or system. This is why metal is colder than wood if it's kept at the same temperature. It is also known as heat conduction. A metal is a conductor that conducts heat and wood is an insulator that doesn't conduct heat. So, the molecules or particles move with a low-velocity rate and collide with the particle that has a high-velocity rate. This occurs due to the interaction of the adjacent atoms, electrons, and ions present in the system or substance. 

The equation of the rate of conduction can be calculated as

Q=-k∇T

where

Q is the heat flux density of the substance

-k is the conductivity of the material

∇T is the temperature gradient where ∇T =T_hot-T_cold / d where d is the distance of the higher and lower level of the substance.

Types of conductors include plasma, electrolytes, and superconductors.

Convection Heat Transfer

In thermodynamics, liquids and gases both flow in the same way, hence they are called fluids. When fluids get heated up, the heated fluid particles gain energy and they move more and spread out. The same number of particles take more space now, and the fluid becomes less dense. During this process, for the less dense area, the gravity creates a buoyant force that lifts the fluid upwards, hence the mass flows.

In this process, there is heat transfer and mass transfer. This process cannot occur in solid state substances. The cooler regions are denser than the warmer regions of the same fluid and the former will sink. The steady flow between the warm and cool parts in the fluids is called convection current. An example of this is the coastal sides of a territory, which are windier at the boundary between the land and sea. Another example is the location of the freezer on top of the fridge because the freezer cools the air at the top and this cold air refrigerates the food. The convection effect at the molecular level is that the molecules expand when there is an increase in temperature and thereby increase the volume of the fluid.

The equation of the heat transfer rate of convection can be calculated as,

Q=hc·A·ΔT

where

hc is the heat transfer coefficient

ΔT is the temperature difference between temperature of the surface and the temperature of the fluid.

Radiation Heat Transfer

Radiation, or electromagnetic radiation, is the transfer of energy by electromagnetic waves, like visible light and infrared radiation. Thermal radiation is also called radiant heat. Energy that travels by radiation is called radiation energy. This electromagnetic energy should be converted or transformed into thermal energy.

Only radiant energy is absorbed and converted into thermal energy. It takes place through a vacuum or transparent medium which is of solid or liquid state. The wavelength in the visible spectrum of the radiations emitted decreases and shortens the wavelength emitted when the temperature is increased. A common example is the sun heating the earth or food being cooked in a microwave or oven.  This radiation is measured by a thermocouple. 

Thermal resistance is a characteristic of heat that gives the temperature difference of the substance or material resisting the heat flow. Emmisity is the physical measuring quantity of an object to emit radiation or infrared energy. The equation of the heat exchange of thermal radiation is understood by Stefan-Boltzmann law formula. That is,

P=σ·e·A·(ΔT_r)⁴ 

where

P is the radiation power

σ is Stefan's constant

e is emissivity

ΔT_r  is the temperature difference factor or the heat exchanger which is difference between the temperature of radiator and the temperature of the environment of the object. 

Black Body Radiation

Certain surfaces are better at absorbing thermal radiation than others, and they are known as good absorbers. Good absorbers are good emitters. Matte black surfaces are the best absorbers of radiation. A particular system or object that absorbs all the radiation that falls on it and at all wavelengths and phases is known to be a black body, or the black body spectrum. Where the object's temperature is even and when this object emits the radiation, it is called black body radiation.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Physics.