The matter is present around us, in three states, solid, liquid and gas. The conversion of matter from one state to another is termed as a change in state, that takes place due to the exchange of heat between the matter and its surroundings. So, heat is the transition of energy from one system to another, due to the difference in temperature, which occurs in three different ways, that are conduction, convection and radiation.
People often misconstrue, these forms of heat transfer but, they are based on diverse physical interaction to transfer energy. To study the difference between conduction, convection and radiation, let’s take a look at the article provided below.
Content: Conduction Vs Convection Vs Radiation
- Comparison Chart
- Definition
- Key Differences
- Conclusion
Comparison Chart
| Meaning | Conduction is a process in which transfer of heat takes place between objects by direct contact. | Convection refers to the form of heat transfer in which energy transition occurs within the fluid. | Radition alludes to the mechanism in which heat is transmitted without any physical contact between objects. |
| Represent | How heat travels between objects in direct contact. | How heat passes through fluids. | How heat flows through empty spaces. |
| Cause | Due to temperature difference. | Due to density difference. | Occurs from all objects, at temperature greater than 0 K. |
| Occurence | Occurs in solids, through molecular collisions. | Occurs in fluids, by actual flow of matter. | Occurs at a distance and does not heats the intervening substance. |
| Transfer of heat | Uses heated solid substance. | Uses intermediate substance. | Uses electromagnetic waves. |
| Speed | Slow | Slow | Fast |
| Law of reflection and refraction | Does not follow | Does not follow | Follow |
Definition of Conduction
Conduction can be understood as the process, which enables direct transfer of heat through the matter, due to the difference in temperature, between adjacent parts of the object. It happens when the temperature of the molecules present in a substance increase, resulting in vigorous vibration. The molecules collide with surrounding molecules, making them vibrate too, resulting in the transportation of thermal energy to neighbouring part of the object.
In simple terms, whenever two objects are in direct contact with one another, there will be a transfer of heat from the hotter object to the colder one, which is due to conduction. Further, the objects which permit heat to travel easily through them are called conductors.
Definition of Convection
In science, Convection implies the form of heat transfer, by real movement of matter, that occurs only in fluids. Fluid alludes to any substance, whose molecules move freely from one place to another, such as liquid and gases. It happens naturally or even forcefully.
Gravity has a great role to play in natural convection such that when the substance is heated from below, leads to the expansion of the hotter part. Due to buoyancy, the hotter substance rises as it is less dense and the colder substance replaces it by sinking at the bottom, due to high density, which when gets hot moves upward, and the process continues. In convection, on heating up the substance, it’s molecules disperse and moves apart.
When the convection is performed forcefully, the substance is compelled to move upwards by any physical means such as the pump. E.g. Air heating system.
Definition of Radiation
The heat transfer mechanism in which no medium is required is called radiation. It refers to the movement of heat in waves, as it does not need molecules to travel through. The object need not be in direct contact with one another to transmit heat. Whenever you feel heat without actually touching the object, it is because of radiation. Moreover, colour, surface orientation, etc. are some of the surface properties on which radiation depends greatly.
In this process, the energy is transmitted through electromagnetic waves called as radiant energy. Hot objects generally emit thermal energy to cooler surroundings. Radiant energy is capable of travelling in the vacuum from its source to the cooler surroundings. The best example of radiation is solar energy that we get from the sun, even though, it is miles aways from us.
The substantial differences between conduction, convection and radiation are explained as under:
- Conduction is a process in which heat is transported between parts of a continuum, through direct physical contact. Convection is the principle, wherein heat is transmitted by currents in a fluid, i.e. liquid or gas. Radiation is the heat transfer mechanism, in which the transition takes place through electromagnetic waves.
- Conduction shows, how heat is transferred between objects in direct contact, but Convection reflects how heat travels through liquids and gases. As against this, radiation indicates how heat travels through places having no molecules.
- Conduction takes place as a result of the difference in temperature, i.e. heat streams from high-temperature area to low temperature area. Convection happens due to the variation in density, such that the heat moves from low-density region to high-density region. On the contrary, all object release heat, having a temperature more than 0 K.
- Conduction usually occurs in solids, through molecular collision. Convection occurs in fluids by mass motion of molecules in the same direction. In contrast, Radiation takes place through the vacuum of space and does not heat up the intervening medium.
- The transfer of heat is through heated solid substance, in conduction, whereas in convection the heat energy is transmitted by way of intermediate medium. Unlike, ration uses electromagnetic waves to transfer heat.
- The speed of conduction and convection is slower than radiation.
- Conduction and convection do not follow the law of reflection and refraction, whereas, radiation obeys the same.
Conclusion
Thermodynamics is the study of heat transfer and the changes related to it. Conduction is nothing but the heat transfer from the hotter part to the colder one. Convection is the heat transfer by up and down motion of the fluid. Radiation occurs when heat travels through empty space.
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Any matter which is made up of atoms and molecules has the ability to transfer heat. The atoms are in different types of motion at any time. The motion of molecules and atoms is responsible for heat or thermal energy and every matter has this thermal energy. The more the motion of molecules, more will be the heat energy. However, talking about heat transfer, it is nothing but the process of transfer of heat from a high-temperature body to a low temperature one. According to thermodynamic systems, heat transfer is defined as “The movement of heat across the border of the system due to a difference in temperature between the system and its surroundings.” Interestingly, the difference in temperature is said to be a ‘potential’ that causes the transfer of heat from one point to another.
Heat can travel from one place to another in several ways. The different modes of heat transfer include:
- Conduction
- Convection
- Radiation
Meanwhile, if the temperature difference exists between the two systems, heat will find a way to transfer from the higher to the lower system.
Conduction is defined as
The process of transmission of energy from one particle of the medium to another with the particles being in direct contact with each other.
An area of higher kinetic energy transfers thermal energy towards the lower kinetic energy area. High-speed particles clash with particles moving at a slow speed, as a result, slow speed particles increase their kinetic energy. This is a typical form of heat transfer and takes place through physical contact. Conduction is also known as thermal conduction or heat conduction.
The rate of conduction can be calculated by the following equation:
\(\begin{array}{l}Q = \frac{[K.A.(T_{hot}-T_{cold})]}{d} \end{array} \) |
Where,
- Q is the transfer of heat per unit time
- K is the thermal conductivity of the body
- A is the area of heat transfer
- Thot is the temperature of the hot region
- Tcold is the temperature of the cold region
- d is the thickness of the body
The coefficient of thermal conductivity shows that a metal body conducts heat better when it comes to conduction.
Following are the examples of conduction:
- Ironing of clothes is an example of conduction where the heat is conducted from the iron to the clothes.
- Heat is transferred from hands to ice cube resulting in the melting of an ice cube when held in hands.
- Heat conduction through the sand at the beaches. This can be experienced during summers. Sand is a good conductor of heat.
Convection is defined as
The movement of fluid molecules from higher temperature regions to lower temperature regions.
As the temperature of the liquid increases, the liquid’s volume also has to increase by the same factor and this effect is known as displacement. The equation to calculate the rate of convection is as follows:
Where,
- Q is the heat transferred per unit time
- hc is the coefficient of convective heat transfer
- A is the area of heat transfer
- Ts is the surface temperature
- Tf is the fluid temperature
Examples of convection include:
- Boiling of water, that is molecules that are denser move at the bottom while the molecules which are less dense move upwards resulting in the circular motion of the molecules so that water gets heated.
- Warm water around the equator moves towards the poles while cooler water at the poles moves towards the equator.
- Blood circulation in warm-blooded animals takes place with the help of convection, thereby regulating the body temperature.
Learn more about Convection
Radiant heat is present in some or other form in our daily lives. Thermal radiations are referred to as radiant heat. Thermal radiation is generated by the emission of electromagnetic waves. These waves carry away the energy from the emitting body. Radiation takes place through a vacuum or transparent medium which can be either solid or liquid. Thermal radiation is the result of the random motion of molecules in matter. The movement of charged electrons and protons is responsible for the emission of electromagnetic radiation. Let us know more about radiation heat transfer.
Radiation heat transfer is measured by a device known as thermocouple. A thermocouple is used for measuring the temperature. In this device sometimes, error takes place while measuring the temperature through radiation heat transfer.
As temperature rises, the wavelength in the spectra of the radiation emitted decreases and shorter wavelengths radiations are emitted. Thermal radiation can be calculated by Stefan-Boltzmann law:
|
P = e ∙ σ ∙ A· (Tr – Tc)4 |
Where,
- P is the net power of radiation
- A is the area of radiation
- Tr is the radiator temperature
- Tc is the surrounding temperature
- e is emissivity and σ is Stefan’s constant (σ = 5.67 × 10-8Wm-2K-4
Following are the examples of radiation:
- Microwave radiation emitted in the oven is an example of radiation.
- UV rays coming from the sun is an example of radiation.
- The release of alpha particles during the decaying of Uranium-238 into Thorium-234 is an example of radiation.
| SI system | Joule |
| MKS system | cal |
| Rate of transfer of heat | KW |
To know more about heat transfer in detail, click on the video below.
The different modes of heat transfer are:
- Conduction
- Convection
- Radiation
Example: UV rays coming from the sun.
Microwave radiation emitted in the oven.
SI unit of heat is Joules.
Electromagnetic radiations are emitted through the movement of charged electrons and protons.
It is known as convection.
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