Conduction, Convection and Radiation

Heat transfer refers to a process wherein heat flows due to the temperature differences between two or more bodies from the higher temperature to lower temperatures. Scientists of the early eighteenth century were of the view that every object in the world consists of an invisible fluid called caloric. This fluid had the property of flowing from a hotter object to a relatively cooler object. They even assigned properties such as weight to it. However, later scientists such as Thompson and Joule refuted the idea of caloric. According to them, the heat was more like a phenomenon caused as a direct consequence of the molecular motion of particles due to kinetic energy. It was the more sophisticated and correct understanding of the concept of heat. According to it, the more the kinetic temperature of particles at the molecular level, the higher the temperature. For instance, the faster the hands are rubbed, the warmer they become; or the faster the vehicle is moving, the hotter the tyres get.

Heat Transfer

Heat transfer is a phenomenon that is taking place constantly around us in nature. Something or the other is losing heat, and something or the other is gaining it. That is what it essentially is, in layman’s terms.

In a more scientific language, heat transfer is “the physical act of thermal energy being exchanged”. The heat transfer depends upon the temperature and the kinetic energy of particles at a molecular level. 

There are essentially three modes of heat transfer, based on the medium. The methods are conduction, convection and radiation.

Conduction

It is the most common form of heat transfer and occurs due to physical contact between particles. Conduction involves molecular collisions between the particles to transfer the heat. Since, temperature or heat essentially, just refers to the motion of particles at the molecular level, the faster the objects collide, the faster the heat transfer occurs. The particles with greater speed will collide with lesser speed particles, that is, heat transfer will take place from a body of higher temperature(more kinetic energy of molecules) to a body with lower temperature(lesser kinetic energy of molecules). As a result of this system of collision, the second particle which initially had lower kinetic energy will gain kinetic energy. Hence, the second object’s temperature would increase as a direct consequence of the increase in kinetic energy at the molecular level. This exchange would continue on till the time both the systems attain a state of thermal equilibrium. Conduction occurs primarily in solids.

Heat conduction depends upon a number of factors; namely, temperature gradient, cross section of material and length of travel path, along with physical matter properties. Temperature gradient describes the flow of heat from hotter to colder objects. The greater the length of the path and cross-sections, the greater is the energy required to heat it and the easier it is for the object to lose heat. Physical properties describe the type of materials which absorb heat at different rates.

The following equation calculates the rate of conduction:

Q=[k.A.(Thot- Tcold)]d

Q= Heat transferred word per unit time

k= Thermal conductivity of barrier

A= Heat transfer area

Thot= Temperature of hot object

Tcold= Temperature of cold object

d= Thickness of barrier

Convection

The mode of heat transfer in fluids is called convection. The part of the fluid nearest to the source is heated first. The heated fluid expands becoming less dense and rises, heating the entire fluid in the process. Actually, the molecules expand on heating(thermal energy) and the fluid needs to expand by the same volume which is the reason why the fluid rises(displaces). As the hot fluid rises, the colder and denser fluid around it is pushed below and gets eventually heated as well and then rises. The cycle repeats, eventually heating the entire fluid in the process.

The following equation calculates convection rate:

Q=hc.A.(Ts-Tf)

Q=heat transferred per unit time

hc=convective heat transfer coefficient

A=heat transfer area of surface

Ts=temperature of surface

Tf=temperature of fluid

Radiation

The mode of heat transfer without the requirement of medium is called radiation. It occurs in the form of electromagnetic waves which propagate the thermal energy. It occurs as a result of random movement of atoms and molecules. The charged particles in these atoms such as the protons and electrons emit energy in the form of electromagnetic waves. All objects radiate energy in some form or the other, depending on how hot the object is.

The following equation calculates radiation

P=e.σ.A.(Tr4-Tc4)

P=net radiated power

A=radiating area

Tr=temperature of the radiator

Tc=temperature of surroundings

e=emissivity

σ=Stefan’s Constant

Comparison and Differences

Mode of Transfer

• Conduction- Due to physical contact(collision) between particles
• Convection- Due to differences in density
• Radiation- Due to the emission of electromagnetic waves caused by the random motion of atoms and molecules

Process of Transfer

• Conduction-Energy(thermal) flows from hotter to colder objects.
• Convection-Movement of particles due to density differences propagates the transfer of thermal energy
• Radiation-Thermal energy is propagated by the emission of electromagnetic waves from charged particles

Medium

• Conduction- occurs primarily in solids
• Convection- occurs in fluids
• Radiation- no medium required; all objects radiate energy

Speed

• Conduction- It is a slow process
• Convection- It is also a slow process
• Radiation- It occurs at a relatively quicker pace

Conclusion

Heat transfer is the process of the flow of heat (thermal energy) from a higher temperature to a lower temperature till thermal equilibrium is established. The three modes of heat transfer are conduction, convection and radiation. Conduction occurs in solids, and convection in fluids while radiation does not need a medium.