14.7 Radiation
3 min read•june 18, 2024
plays a crucial role in heat transfer, allowing energy to move through space without a medium. This process involves visible light, radio waves, and . The amount and color of emitted depend on an object's temperature and surface properties.
The describes how radiant heat power relates to an object's temperature. As temperature increases, the peak of emitted radiation shifts, changing the color we perceive. This phenomenon explains why heated objects glow and stars have different colors.
Electromagnetic Radiation and Heat Transfer
Heat transfer by electromagnetic radiation
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- Electromagnetic radiation transfers energy through space as waves without requiring a medium (vacuum)
- Includes visible light, radio waves, X-rays, and infrared radiation
- occurs when an object emits electromagnetic waves due to its temperature
- Energy carried by these waves can be absorbed by another object, increasing its temperature
- Amount of thermal radiation emitted depends on object's temperature and surface properties
- Hotter objects emit more thermal radiation than cooler objects
- Objects with higher (ability to emit thermal radiation) radiate more energy than those with lower (matte black vs shiny metal)
Temperature and radiated color relationship
- Color of radiated energy related to temperature of emitting object
- As temperature increases, peak wavelength of emitted radiation shifts to shorter wavelengths ()
- , where is peak wavelength, is absolute temperature, and is Wien's displacement constant ( m·K)
- Cooler objects emit most radiation in infrared region, not visible to human eye (room temperature objects)
- As temperature increases, object begins to emit radiation in visible spectrum
- Color changes from red to orange to yellow to white to blue as temperature increases (heated metal, stars)
- Sun, with surface temperature of ~5,800 K, emits most radiation in visible region, peaking in yellow-green part of spectrum
Stefan-Boltzmann law for heat transfer
- Describes relationship between rate of heat transfer by radiation and temperature of an object
- Total radiant heat power (energy per unit time) emitted by an object proportional to its absolute temperature raised to the fourth power
- Equation for Stefan-Boltzmann law:
- is radiant heat power (watts)
- is emissivity of object's surface (unitless, 0 to 1)
- is ( W·m·K)
- is surface area of object (m)
- is absolute temperature of object (K)
- To calculate net heat transfer rate between two objects:
- is absolute temperature of hotter object
- is absolute temperature of cooler object
- Small changes in temperature result in significant changes in rate of heat transfer by radiation due to fourth-power dependence on temperature (doubling temperature increases radiation by factor of 16)
Electromagnetic spectrum and radiation properties
- The encompasses all types of electromagnetic radiation, including radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays
- Each type of radiation is characterized by its wavelength and
- Wavelength is the distance between consecutive wave crests
- Frequency is the number of wave cycles passing a fixed point per second
- Photons are the fundamental particles of electromagnetic radiation, carrying discrete amounts of energy
- radiation refers to the theoretical perfect emitter and absorber of electromagnetic radiation
- Real objects approximate blackbody behavior to varying degrees
- and of radiation:
- Absorption occurs when an object takes in electromagnetic energy
- Emission is the process by which an object releases electromagnetic energy
Key Terms to Review (21)
Absorption: Absorption is the process by which a substance or energy is taken up and incorporated into a system. This term is particularly relevant in the context of various physical phenomena, including radiation, sound, and light, where absorption plays a crucial role in the behavior and interactions of these forms of energy.
Beat frequency: Beat frequency is the frequency at which two waves of slightly different frequencies interfere with each other, resulting in a modulation pattern perceived as a periodic variation in amplitude. It is calculated as the absolute difference between the frequencies of the two interfering waves.
Blackbody: A blackbody is an idealized physical object that absorbs all electromagnetic radiation that falls on it, regardless of the angle or wavelength of the radiation. It is considered the perfect absorber and emitter of radiation, and its behavior is used as a reference for understanding the properties of real-world materials and their interactions with light.
De Broglie wavelength: The de Broglie wavelength is the wavelength associated with a particle and is inversely proportional to its momentum. It highlights the wave-particle duality of matter.
Electromagnetic Radiation: Electromagnetic radiation is a form of energy that is transmitted through space or a medium in the form of electric and magnetic fields oscillating perpendicular to each other. It encompasses a wide range of wavelengths and frequencies, from radio waves to gamma rays, and plays a crucial role in various physical phenomena and applications.
Electromagnetic spectrum: The electromagnetic spectrum is the range of all types of electromagnetic radiation, which includes visible light, radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. This spectrum is crucial because it encompasses the various forms of energy that travel through space at the speed of light and affects many aspects of physics, including radiation, magnetism, wave production, and energy transfer.
Emission: Emission is the process by which energy is released in the form of electromagnetic radiation or particles. This term is particularly relevant in the context of radiation and the ray aspect of light, as it describes the mechanisms by which these phenomena occur.
Emissivity: Emissivity is a measure of an object's ability to emit infrared energy or thermal radiation. It ranges from 0 to 1, where 1 represents a perfect blackbody that emits the maximum possible radiation.
Emissivity: Emissivity is a measure of a material's ability to emit thermal radiation compared to an ideal black body at the same temperature. It is a crucial factor in understanding heat transfer through radiation, which is one of the three fundamental modes of heat transfer along with conduction and convection.
Frequency: Frequency is a fundamental concept in physics that describes the number of occurrences of a repeating event per unit of time. It is a crucial parameter in various areas of study, including radiation, oscillations, waves, sound, and electromagnetic phenomena.
Greenhouse effect: The greenhouse effect is the process by which certain gases in Earth's atmosphere trap heat, preventing it from escaping into space. This natural phenomenon maintains Earth's temperature at a level suitable for life.
Infrared Radiation: Infrared radiation is a type of electromagnetic radiation with wavelengths longer than those of visible light, but shorter than those of radio waves. It is invisible to the human eye but can be detected as heat. Infrared radiation plays a crucial role in the study of radiation and the electromagnetic spectrum.
Net rate of heat transfer by radiation: The net rate of heat transfer by radiation is the difference between the amount of thermal radiation emitted by a body and the amount absorbed from its surroundings. It depends on factors such as temperature, surface area, and emissivity.
Radiation: Radiation is the transfer of energy through electromagnetic waves without requiring a medium. It can occur in a vacuum and is responsible for the heat we receive from the sun.
Single-photon-emission computed tomography(SPECT): Single-photon-emission computed tomography (SPECT) is a nuclear imaging technique that provides 3D images of functional processes in the body. It uses gamma rays and a rotating camera system to capture detailed images of internal organs.
Stefan-Boltzmann constant: The Stefan-Boltzmann constant is a physical constant denoted by the symbol $$ ext{σ}$$, which relates to the power radiated by a black body in thermal equilibrium per unit area as a function of its temperature. This constant is crucial for understanding how objects emit thermal radiation, allowing us to quantify the energy radiated by an ideal black body across all wavelengths based on its absolute temperature raised to the fourth power, represented mathematically as $$P = ext{σ} imes A imes T^4$$. It connects to various phenomena including the behavior of stars, climate science, and thermodynamics.
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 fundamental principle connects temperature and radiation, showing how hotter objects emit more energy, which is crucial for understanding heat and radiation transfer in various contexts.
Stefan-Boltzmann law of radiation: The Stefan-Boltzmann law of radiation states that the total energy radiated per unit surface area of a black body is directly proportional to the fourth power of the black body's absolute temperature. Mathematically, it is expressed as $E = \sigma T^4$, where $E$ is the energy radiated, $\sigma$ is the Stefan-Boltzmann constant, and $T$ is the absolute temperature.
Thermal Radiation: Thermal radiation is the electromagnetic radiation emitted by a body due to the thermal energy (heat) of its particles. It is a fundamental mode of heat transfer, distinct from conduction and convection, and is a key concept in the study of radiation physics and nuclear weapons.
Wavelength: Wavelength is a fundamental characteristic of waves, representing the distance between consecutive peaks or troughs in a wave. It is a crucial parameter that describes the spatial extent of a wave and is closely related to other wave properties such as frequency and speed.
Wien's Displacement Law: Wien's displacement law describes the relationship between the temperature of a blackbody and the wavelength at which it emits the most thermal radiation. It states that the product of the blackbody's temperature and the wavelength of its peak emission is a constant.