When thermal energy is transferred from one system to another through heat dispersal, the phenomenon is called heat transfer.
This can be done in conduction, convection, and radiation.
Key Takeaways
- Conduction is the transfer of heat through direct contact between two objects, while convection is the transfer of heat through the movement of fluids or gases.
- Radiation transfers heat through electromagnetic waves or particles without a medium or direct contact.
- Conduction, convection, and radiation are all essential mechanisms of heat transfer and are present in various natural and technological processes.
Conduction vs Convection vs Radiation
The difference between conduction, convection, and radiation is about the way how heat is transferred from a zone with higher kinetic energy to a zone with lower kinetic energy. In conduction, this happens through direct physical contact between two objects. Convection, on the other hand, occurs when the heat is transferred through the movement of molecules. Radiation does not require physical contact between two objects like the other methods.
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Comparison Table
| Parameter Of Comparison | Conduction | Convection | Radiation |
|---|---|---|---|
| Definition | Process of heat transfer between objects through direct physical contact. | Process of heat transfer through a fluid medium such as liquids or gas. | Process of heat transfer through electromagnetic waves. |
| Method | Heat is transferred due to molecular collision when solids come into contact with each other. | The flow of the fluids transfers heat. | Heat is transferred through the radiation emitted by bodies without needing a medium. |
| Cause | Heat travelling from a high-temperature area to a low-temperature area. | Heat travelling from a low-density area to a high-density area. | The energy emitted by bodies through atoms and molecules’ rotational and vibrational movements. |
| Medium | Heated solids. | Intervening substances like fluids. | Electromagnetic waves. |
What is Conduction?
The heat transfer process through direct contact between two objects is called conduction.
When the molecules of one object absorb heat energy, they start moving quickly, and while doing so, they come into contact with neighbouring objects, and an energy transfer occurs.
Conduction is a widespread process. The simple act of touching a hot vessel causes conduction to occur, and heat is transferred from the vessel to your hand.
For conduction to happen, some factors must be kept in mind.
The first is the temperature gradient, which describes the direction in which the heat is flowing and the transfer rate.
The conduction process from a hot source to a cold source (or a source lacking in heat energy) continues until both bodies reach a state of thermal equilibrium.
Another critical factor is the size of the objects involved. Larger objects require more heat to be warmed up, but at the same time, they lose heat faster.
This is because the bigger their surface area is, the more they come into contact with the open air. The physical properties of the objects must also be taken into account.
If you use a wooden spoon while cooking, you’ll notice that the spoon does not become hot. This is because wood is a lousy conductor.
However, if you were to use a metal spoon, heat would be transferred to it very quickly because the metal is a good conductor. Poor conductors are also called insulators. They prevent energy from flowing away from the source.
For example, polar bears are able to survive in arctic regions because their fur serves as an insulator that traps heat inside the body.
What is Convection?
When the mass motion of a fluid due to the heated fluid moving away from the source heat occurs, it carries energy with it. This is also a form of heat transfer, and it is called convection.
This process happens because heat decreases the density of fluids like air and water. The loss of density causes the fluid to rise, thereby creating convection currents that can transfer energy.
As the heated layers of the fluid rise, the colder layers, which still retain their density, descend towards the heat source until they become heated and begin to rise.
There are two kinds of convection – spontaneous and forced. In the former, convection occurs naturally due to buoyancy. The difference in temperature causes a difference in densities.
For example, when heat from the sun warms up the earth, the sea absorbs most of the energy, but it takes more time to become warm as opposed to land.
Therefore, the air above the land loses density faster, creating a low-pressure area over coastal areas.
But the area over the sea has higher pressure, which makes air move from the higher pressure area to the lower pressure area, i.e., from sea to land.
This is why the breeze near the sea is generally stronger. Forced convection is caused using an external source like a fan or a geyser. It is related to Newton’s law of cooling the equation, which is as follows:
P = dQ/dt = hA(T-T0)
Here P = dQ/dt is the rate of transfer of heat. The h is the convection heat-transfer coefficient. A is the surface area of the material which is exposed.
T refers to the temperature of the object in the fluid, and T0 refers to the temperature of the fluid made to undergo the convection process.
What is Radiation?
Unlike conduction and convection, which require physical contact between two bodies, radiation is the heat transfer that occurs even when the bodies don’t come into contact or are separated in space.
Everything in the universe is made up of atoms which together form molecules. The rotation and vibration of atoms and molecules ensure that all substances continue to emit energy through electromagnetic radiation.
Electrons with high energy at high atomic levels descend to levels where the energy is lower. Whatever energy is lost along the way is emitted as electromagnetic radiation.
When an atom absorbs energy, its electrons climb to higher energy levels. Therefore when the rate at which energy is absorbed is balanced by the rate at which it is emitted, the substance’s temperature will not change.
If the former is greater than the latter, the temperature will rise; if it is lower, the temperature will also decrease.
A typical example of heat transfer through radiation is the sun. It does not come into contact with other planets, nor is there a physical medium for heat transfer.
Yet we can feel its warmth because of the electromagnetic radiation it emits, which allows its rays to reach the earth.
Main Differences Between Conduction, Convection, and Radiation
- Conduction is the transfer of heat between two objects through direct physical contact.
- In convection, heat is transferred by a fluid medium such as a liquid or a gas.
- The transition of heat through electromagnetic waves is called radiation.
- Heat travelling from an area with a high temperature to an area with a low temperature is the cause of conduction.
- Heat moving from an area with low density to a high density causes convection.
- Radiation occurs because the rotational and vibration movements of the atoms and molecules which make up bodies cause energy to be transmitted constantly.
- https://www.sciencedirect.com/science/article/pii/0142727X94000144
- https://asmedigitalcollection.asme.org/heattransfer/article-abstract/85/4/318/414710
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Piyush Yadav has spent the past 25 years working as a physicist in the local community. He is a physicist passionate about making science more accessible to our readers. He holds a BSc in Natural Sciences and Post Graduate Diploma in Environmental Science. You can read more about him on his bio page.
