4.1 Fundamental concepts of thermal radiation
3 min read•july 31, 2024
Thermal radiation is electromagnetic energy emitted by matter due to its temperature. It's a crucial heat transfer mode, distinct from conduction and convection, that doesn't need a medium to propagate.
Understanding thermal radiation's characteristics, like its temperature dependence and relation to the electromagnetic spectrum, is key. Concepts like , , and are essential for analyzing and designing thermal systems.
Thermal radiation characteristics
Nature and properties of thermal radiation
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- Thermal radiation is electromagnetic radiation emitted by matter due to its temperature
- Occurs at temperatures above absolute zero
- Emission results from internal energy of matter causing atoms and molecules to oscillate and emit photons
- Characteristics of thermal radiation depend on temperature and surface properties of emitting body
- Can be emitted, absorbed, or reflected by matter
- Processes governed by laws of thermodynamics and electromagnetism
Temperature dependence of thermal radiation
- Intensity of thermal radiation increases with temperature, following
- Total radiant heat power emitted from surface is proportional to fourth power of absolute temperature
- Wavelength distribution of thermal radiation varies with temperature
- Higher temperatures result in shift towards shorter wavelengths ()
- Thermal radiation does not require medium for transmission
- Can propagate through vacuum, unlike conduction and convection
Thermal radiation vs other heat transfer
Comparison with conduction and convection
- Thermal radiation is distinct mode of heat transfer, differing from conduction and convection
- Conduction transfers heat through direct contact between particles of matter
- Thermal radiation does not require physical contact between emitting and receiving bodies
- Convection transfers heat by bulk movement of fluids or gases, driven by temperature gradients and buoyancy forces
- Thermal radiation can occur in absence of any medium
Factors affecting heat transfer modes
- Rate of heat transfer by conduction and convection depends on temperature gradient and thermal properties of medium
- Rate of thermal radiation depends on absolute temperature and surface properties of emitting body
- Conduction and convection more significant at lower temperatures and shorter length scales
- Thermal radiation becomes dominant at higher temperatures and larger length scales
Electromagnetic spectrum and thermal radiation
Overview of electromagnetic spectrum
- Electromagnetic spectrum is range of all possible frequencies or wavelengths of electromagnetic radiation
- Includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays
- Thermal radiation occupies specific portion of electromagnetic spectrum
- Primarily in infrared region, with some overlap into visible and microwave regions depending on temperature
Relationship between wavelength, frequency, and energy
- Wavelength and frequency of electromagnetic radiation are inversely related
- Shorter wavelengths correspond to higher frequencies and higher energy photons
- Planck distribution describes spectral density of electromagnetic radiation emitted by blackbody at given temperature
- Shows variation of emitted energy with wavelength or frequency
- Peak wavelength of thermal radiation emitted by body shifts towards shorter wavelengths as temperature increases (Wien's displacement law)
Applications and importance
- Total energy emitted by body across all wavelengths of electromagnetic spectrum is proportional to fourth power of absolute temperature (Stefan-Boltzmann law)
- Understanding electromagnetic spectrum and its relation to thermal radiation is essential for applications
- Infrared thermography, remote sensing, and heat transfer analysis
Emissivity, absorptivity, and reflectivity in thermal radiation
Definitions and properties
- Emissivity (ε), absorptivity (α), and reflectivity (ρ) are surface properties describing how material interacts with thermal radiation
- Emissivity: ratio of thermal radiation emitted by surface to that emitted by ideal blackbody at same temperature (0 to 1)
- Absorptivity: fraction of incident thermal radiation absorbed by surface (0 to 1)
- Reflectivity: fraction of incident thermal radiation reflected by surface (0 to 1)
Kirchhoff's law and conservation of energy
- of thermal radiation: for body in thermal equilibrium, emissivity and absorptivity of surface are equal at any given wavelength and direction
- Conservation of energy principle: sum of absorptivity, reflectivity, and transmissivity (if applicable) for surface must equal 1
Modeling and applications
- Real surfaces often modeled as gray bodies, with wavelength-independent emissivity, absorptivity, and reflectivity
- Simplifies radiation heat transfer calculations
- Net radiation heat transfer between two surfaces depends on temperatures, surface areas, and emissivity values
- Described by Stefan-Boltzmann law and view factor relations
- Applying these concepts enables analysis and design of systems involving thermal radiation
- Insulation, solar collectors, devices
Key Terms to Review (19)
Absorption Coefficient: The absorption coefficient is a measure of how much radiation is absorbed per unit distance as it travels through a medium. It indicates the ability of a material to absorb energy, with higher values meaning greater absorption. This concept is crucial for understanding thermal radiation, as it helps explain how energy transfer occurs in various materials, particularly in scenarios involving participating media where interactions between radiation and matter become significant.
Absorptivity: Absorptivity is a measure of a material's ability to absorb thermal radiation when exposed to it. This property is significant because it influences how much energy a material can absorb from its surroundings, impacting thermal equilibrium and the overall thermal performance of systems involving radiation. It is important to note that absorptivity varies with wavelength and temperature, connecting it to the concepts of emissivity and reflectivity in thermal radiation.
Atmospheric absorption: Atmospheric absorption refers to the process where certain wavelengths of electromagnetic radiation, particularly thermal radiation, are absorbed by gases and particles present in the Earth's atmosphere. This phenomenon is crucial for understanding how heat is transferred and redistributed in the atmosphere, influencing temperature profiles and climate patterns.
Blackbody radiation: Blackbody radiation refers to the electromagnetic radiation emitted by a perfect blackbody, which is an idealized physical object that absorbs all incident radiation and re-emits energy in a characteristic spectrum solely dependent on its temperature. This phenomenon is crucial in understanding how objects emit thermal energy, with applications spanning from astrophysics to thermal engineering.
Emissivity: Emissivity is a measure of a material's ability to emit thermal radiation compared to that of a perfect blackbody at the same temperature. This dimensionless value ranges from 0 to 1, where 1 represents perfect emission, characteristic of blackbody radiation, and values less than 1 indicate varying degrees of emission efficiency. Understanding emissivity is crucial for analyzing heat transfer through thermal radiation and how different surfaces interact with thermal energy.
Geothermal radiation: Geothermal radiation refers to the heat energy that is emitted from the Earth’s interior, primarily in the form of thermal radiation. This energy transfer occurs as a result of the natural decay of radioactive isotopes and the heat from the Earth's formation, contributing to temperature variations on the surface and influencing geothermal systems.
Joseph Stefan: Joseph Stefan was an Austrian physicist known for his contributions to the field of thermal radiation, particularly the formulation of Stefan's Law, which relates to the emission of thermal radiation from a black body. His work laid the foundation for understanding fundamental concepts of thermal radiation and the mechanisms behind radiation exchange between surfaces.
Kirchhoff's Law: Kirchhoff's Law states that for any body in thermal equilibrium, the amount of thermal radiation emitted is equal to the amount of thermal radiation absorbed at a given wavelength. This law connects the concepts of emissivity, absorptivity, and the nature of thermal radiation, making it essential for understanding how energy is exchanged through radiation between surfaces.
Max Planck: Max Planck was a German physicist who is best known as the originator of quantum theory, which revolutionized our understanding of atomic and subatomic processes. His work on black-body radiation led to the formulation of Planck's law, which describes how electromagnetic radiation is emitted by a black body in thermal equilibrium, connecting closely with concepts of thermal radiation and radiation exchange between surfaces.
Pyrometer: A pyrometer is a type of temperature measuring device that can detect the temperature of an object without needing to be in direct contact with it. This non-contact capability is essential for measuring high temperatures in industrial settings, where traditional thermometers may not be suitable due to extreme conditions or the nature of the materials being measured. Pyrometers are particularly relevant when discussing thermal radiation because they operate on the principle of detecting the electromagnetic radiation emitted by hot objects, which relates directly to their temperature.
Radiative Cooling: Radiative cooling is the process by which an object loses heat by emitting thermal radiation, typically in the infrared spectrum. This phenomenon is crucial for understanding how bodies in different environments, like buildings and natural surfaces, can lose heat to the atmosphere. It plays a significant role in energy balance, temperature regulation, and can even impact weather patterns.
Radiative heat transfer: Radiative heat transfer is the process of energy transfer in the form of electromagnetic radiation, primarily occurring through infrared radiation emitted by surfaces at temperatures above absolute zero. This method of heat transfer does not require a medium, allowing energy to travel through a vacuum, and is significant in various applications, from climate science to engineering. It involves understanding how different surfaces emit and absorb radiation, which connects deeply with the concepts of thermal radiation, blackbody behavior, and how energy exchanges between surfaces can occur.
Reflectivity: Reflectivity is the measure of how much incident radiation is reflected off a surface rather than absorbed or transmitted. It plays a crucial role in understanding thermal radiation, as different materials will have varying abilities to reflect energy, which affects their temperature and heat transfer characteristics. The concept of reflectivity is integral to evaluating blackbody radiation and surface properties, as well as analyzing how radiation interacts with participating media.
Solar radiation: Solar radiation refers to the electromagnetic energy emitted by the sun, primarily in the form of visible light, ultraviolet light, and infrared radiation. This energy is crucial for life on Earth, as it drives various natural processes including photosynthesis and influences weather patterns and climate. Understanding solar radiation is essential for grasping the principles of thermal radiation, as it forms the basis for how energy is transferred from the sun to the Earth's surface.
Spectroscopy: Spectroscopy is the study of the interaction between matter and electromagnetic radiation. This technique enables the analysis of materials by measuring the spectrum of light they emit, absorb, or scatter, providing insights into their composition, structure, and other properties. The connections between spectroscopy and thermal radiation arise from how objects at different temperatures emit radiation that can be analyzed using spectroscopic techniques to understand their thermal behavior.
Stefan-Boltzmann Law: The Stefan-Boltzmann Law states that the total energy radiated per unit surface area of a black body is directly proportional to the fourth power of its absolute temperature. This relationship highlights how temperature plays a critical role in thermal radiation and is foundational in understanding concepts such as blackbody radiation and how surfaces emit or exchange heat with one another.
Surface Temperature: Surface temperature refers to the temperature of an object’s surface, which is crucial in understanding thermal radiation and energy transfer. It plays a significant role in determining how much thermal radiation an object emits, which is directly influenced by its emissivity and the surrounding environment. This concept is vital when discussing the properties of materials, their radiation characteristics, and how energy exchanges between different surfaces occur.
Thermal conductivity: Thermal conductivity is a material property that quantifies how well a substance can conduct heat. This property plays a crucial role in various heat transfer processes, influencing the rate at which heat moves through different materials and affecting energy efficiency in systems involving thermal management.
Wien's Displacement Law: Wien's Displacement Law states that the wavelength at which the emission of a blackbody spectrum is maximized is inversely proportional to its absolute temperature. This means that as the temperature of a blackbody increases, the peak wavelength of emitted radiation shifts to shorter wavelengths, indicating that hotter objects emit radiation more intensely at higher frequencies. This principle is fundamental in understanding thermal radiation and the behavior of idealized blackbodies, connecting temperature with electromagnetic radiation characteristics.