Energy Transfer Via Radiation


Radiation is the transfer of thermal energy from one place to another by means of electromagnetic radiation, without the need of an intervening material medium.

All matter radiate thermal energy in all directions in amounts determined by their temperature, where the energy is carried by photons, such as the infrared, visible and X-ray portions of the electromagnetic spectrum. These photons warm up anything that absorbs them.

  • All objects emit radiation when their temperature is above absolute zero, primarily in the form of infrared radiation. However, in the case of extremely hot bodies, such as the Sun, light and ultraviolet radiation are also present.
  • In order for an object to maintain a constant temperature, the rate at which energy is emitted from the object must match the rate at which it receives energy.
  • If the average amount of energy radiated is lower than the energy absorbed, the object’s temperature will increase. Conversely, if the average energy emitted exceeds the energy absorbed, the object’s temperature will decrease.

Radiation is the only process that does not need a medium to transfer the energy.

Factors Affecting The Rate Of Radiation

Colour and texture of surface
Black, matt surfaces are good in both absorbing and emitting radiation.
Shiny or polished white surfaces are poor absorbers because they act better as reflectors, and hence, poor emitters.

Surface temperature
The hotter the object, the more energy it radiates.

Surface area
The grater the area, the more energy it radiates.

Cooling Rate Of An Object

When an object’s surface temperature surpasses that of its surroundings, it releases radiation more rapidly than it takes in radiation from the environment. Consequently, the object undergoes cooling until a point is reached where the emission and absorption rates equalize, leading to a stable temperature. The greater the disparity between the object’s surface temperature and its surroundings, along with an increased surface area, the higher the amount of radiation emitted and, consequently, the faster the cooling rate.

Applications Of Radiation

The greenhouse effect provides a means to grow plants that need a warm environment in cold countries. Short infrared radiation from the sun passes easily though the glass panels of a greenhouse, and is absorbed by the plants and soil inside. The plants in turn also radiate energy, but with a much longer wavelength. This radiation is reflected by the glass panels. Thus the temperature inside the greenhouse increases until it reaches a thermal equilibrium suitable for plants to grow.

A layer of aluminium sheet is placed below the roof tiles to keep the air temperature inside the building steady. In the day, the aluminium sheet reflects the radiation and keeps the building cooler. In the night, it reduces emitting radiation from the inside and keeps the interior warm.

An infrared thermometer captures the thermal radiation released by an object, transforming it into an electrical signal. The object’s temperature can be deduced from the detected radiant power, and the result is displayed digitally. This method is non-contact, enabling temperature measurement from a distance. Infrared thermometers find common use in monitoring the well-being of arriving passengers at airports.

Table Showing Differences Between Conduction, Convection & Radiation

MethodMechanismExamplesMediumMaterial Medium Necessary?
ConductionHeat transfer through direct contact. Thermal energy is transferred due to the vibration and movement of atoms or molecules.Heating a metal rod at one end and feeling the other end getting warm; cooking food in a pan.Solids, but also occurs in liquids and gases albeit less efficiently.Yes
ConvectionHeat transfer through the movement of fluids (liquids or gases). Warmer parts of the fluid rise while cooler parts sink, creating a circulation pattern.Boiling water; atmospheric circulation causing wind.Liquids and gases.Yes
RadiationTransfer of energy through electromagnetic waves. Heat is transferred directly without affecting the medium it passes through.Sunlight warming the Earth; heat from a fire felt from a distance.Can occur through vacuum, as well as through transparent solids, liquids, and gases.No

Thermal Energy Transfer Process In Vacuum Flask

Vacuum flask can store and maintain temperature (either hot or cold) of the contents in the flask by reducing heat transfer in or out through conduction, convection and radiation.

Type of heat transferHow heat transfer is reduced
ConvectionVacuum between the double glass walls
ConductionVacuum between the double glass walls.

Insulated cover and stopper

RadiationShiny silvered inner surface of the glass walls

Thermal Energy Transfer Process In Car Radiator

Both conduction and radiation processes are at play in a car radiator, serving to disperse the heat produced in the engine. The radiator contains a fluid that moves between the engine block and the radiator itself. Conduction facilitates the transfer of thermal energy to the fluid as it flows over the engine block. Upon reaching the radiator, the fluid undergoes a conduction-based thermal energy transfer to the radiator, which then emits energy in the infrared spectrum to the surroundings. The radiator, designed with a black surface and a large area, proves to be an effective emitter of radiation. This process ensures the cooling of the fluid before it circulates back to the engine block.

Thermal Energy Transfer Process In Fires

Heating a room with a wood- or coal-burning fire involves both radiation and convection processes. Thermal energy emanates from the burning wood or coal, warming objects in the room as they absorb the heat. The air in direct contact with the hot wood or coal becomes heated and ascends due to its lower density compared to the colder air above. Simultaneously, cooler air descends to replace it, initiating a convection current that transfers additional thermal energy into the room.

Worked Examples

Example 1

What category of radiation does thermal radiation fall under? Explain the energy transfer process of thermal radiation.

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Thermal radiation is a form of electromagnetic radiation that is emitted due to the temperature of an object. It is a process by which the object releases energy in the form of electromagnetic waves as a result of its temperature, without the need for a medium to transfer the heat. This type of radiation encompasses a broad spectrum of wavelengths, including infrared radiation.

As an object gets hotter, its atoms and molecules gain energy, leading to increased movement and acceleration. This heightened thermal motion results in the emission of electromagnetic waves, with the wavelength distribution determined by the temperature of the object. Objects at higher temperatures emit shorter-wavelength radiation, often extending into the infrared region.

In summary, thermal radiation is a specific manifestation of electromagnetic radiation linked to the thermal energy of an object, and it plays a crucial role in heat transfer and energy exchange in various natural and industrial processes.

Example 2

What causes less likelihood of frost on a cloudy night compared to a clear night?

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Cloudy nights are less likely to experience frost compared to clear nights because clouds act as a thermal blanket, reducing the radiative heat loss from the Earth’s surface. On clear nights, the absence of clouds allows for efficient radiative cooling, where heat from the ground is radiated into the atmosphere and space. This rapid cooling can lead to the condensation of moisture in the air, forming frost.

In contrast, clouds act as insulators by trapping some of the outgoing longwave radiation emitted by the Earth’s surface. This trapped radiation is then re-radiated back to the surface, mitigating the cooling effect. As a result, the presence of clouds reduces the temperature drop at the surface, making it less conducive to the formation of frost on a cloudy night compared to a clear night.

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