1.6 Mechanisms of Heat Transfer
3 min read•june 24, 2024
transfer is all about how energy moves from hot to cold. It's crucial for understanding everything from keeping your coffee warm to cooling your computer. There are three main ways moves: , , and .
Each method has its own quirks and formulas. happens through direct contact, involves fluid movement, and uses electromagnetic waves. Understanding these helps us tackle real-world heat problems in buildings, cooking, and electronics.
Mechanisms of Heat Transfer
Mechanisms of heat transfer
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- Conduction involves heat transfer through direct contact between particles of matter, occurring in solids, liquids, and gases as heat flows from higher to lower regions, with the rate depending on , material properties (), cross-sectional area, and distance
- Convection transfers heat by the movement of fluids (liquids or gases), combining effects of conduction and fluid motion, with two types: driven by forces from temperature differences (hot air rising) and induced by external means (fans, pumps), and the rate depends on fluid properties (density, viscosity, ), velocity, surface area, and temperature difference between the surface and fluid
- Radiation transfers heat through electromagnetic waves without requiring a medium (can occur in a vacuum), emitted and absorbed by all objects, with the rate proportional to the fourth power of absolute temperature () and depending on surface properties (, ), area, and view factor (geometric relationship between emitting and absorbing surfaces)
Formulas for heat transfer rates
- Conduction follows : , where is (W), is thermal conductivity (W/m·K), is cross-sectional area (m²), and is temperature gradient (K/m), simplifying for steady-state conduction through a plane wall to , with and as surface temperatures (K) and as wall thickness (m)
- The , which is the heat transfer rate per unit area, can be calculated as
- Radiation follows the Stefan-Boltzmann law: , where is surface (0 ≤ ≤ 1), is the (5.67 × 10⁻⁸ W/m²·K⁴), is surface area (m²), and and are absolute temperatures (K) of the surface and surroundings
Heat transfer in real-world scenarios
- in buildings involves conduction through walls, windows, and roofs, convection from air leakage and natural convection within the building, and radiation from solar radiation through windows and heat emission from surfaces
- The effectiveness of insulation is often characterized by its , which is the reciprocal of thermal conductivity
- Cooking on a stove involves conduction from the heating element to the pot, convection from the pot to the food by the motion of boiling water (pasta) or air (oven), and radiation from the heating element and pot to the surrounding environment
- Heat sinks in electronics involve conduction from the electronic component (CPU) to the heat sink, convection from the heat sink to the surrounding air often enhanced by fans (computer case), and usually negligible radiation from the heat sink to the environment compared to convection
- The efficiency of heat transfer in this scenario is influenced by the between the heat sink and the surrounding air
Additional heat transfer concepts
- is a material property that measures the rate at which heat diffuses through a material, combining thermal conductivity, density, and
- The heat transfer coefficient quantifies the heat transfer between a solid surface and a fluid, incorporating the effects of both conduction and convection in a single parameter
Key Terms to Review (46)
Absolute temperature scale: An absolute temperature scale is a thermodynamic temperature scale that uses absolute zero as its null point. The two most common absolute temperature scales are Kelvin and Rankine.
Absorptivity: Absorptivity is a measure of a material's ability to absorb electromagnetic radiation, such as light or heat, that falls upon its surface. It is a crucial property in the study of heat transfer mechanisms, as it determines how much of the incident radiation is absorbed by a material rather than being reflected or transmitted.
Adiabatic: Adiabatic refers to a process or system in which there is no transfer of heat or mass between the system and its surroundings. In other words, an adiabatic process occurs without any exchange of heat with the environment.
Adiabatic compressions: Adiabatic compression is a process in which a gas is compressed without any heat exchange with its surroundings. The internal energy of the gas increases, resulting in an increase in temperature.
Buoyancy: Buoyancy is the upward force experienced by an object submerged in a fluid, which counteracts the weight of the object. This force arises due to pressure differences in the fluid that act on different parts of the submerged object. The concept of buoyancy plays a crucial role in understanding how objects interact with fluids and is essential for explaining phenomena such as floating, sinking, and the behavior of gases in liquids.
Conduction: Conduction is the transfer of heat through a material without any movement of the material itself. It occurs due to the collision and diffusion of particles within the substance.
Conduction: Conduction is the transfer of thermal energy through a material without the involvement of any bulk motion of the material. It occurs when heat flows from a region of higher temperature to a region of lower temperature within a material or between materials in direct contact, due to the kinetic energy of vibrating atoms and free electrons.
Constant-volume gas thermometer: A constant-volume gas thermometer measures temperature by observing the pressure of a gas held at constant volume. The relationship between pressure and temperature is governed by the ideal gas law.
Convection: Convection is the transfer of heat through the movement of fluids (liquids or gases) caused by temperature differences. It involves the physical motion of the fluid, carrying energy from one place to another.
Convection: Convection is a mode of heat transfer that involves the movement of a fluid, such as air or water, to transport thermal energy from one location to another. It is a crucial mechanism in the exchange of heat between a solid surface and a fluid in motion.
Emissivity: Emissivity is a measure of how effectively a surface emits thermal radiation compared to an ideal blackbody. It ranges from 0 to 1, where 1 represents a perfect emitter.
Emissivity: Emissivity is a measure of an object's ability to emit thermal radiation, relative to the amount of radiation emitted by a perfect blackbody at the same temperature. It is a crucial parameter in understanding the mechanisms of heat transfer, particularly radiation heat transfer.
Forced convection: Forced convection is a mechanism of heat transfer where fluid motion is generated by an external source like a pump, fan, or mixer. This process enhances heat transfer efficiency compared to natural convection.
Forced Convection: Forced convection is the process of heat transfer through a fluid (liquid or gas) that is induced by an external force, such as a pump or fan, to move the fluid. This method enhances the heat transfer rate compared to natural convection by increasing fluid motion, which disrupts thermal layers and allows for more efficient energy transfer between surfaces and the moving fluid. The significance of forced convection lies in its widespread application in various systems, including heating and cooling systems, where efficient thermal management is essential.
Fourier's Law: Fourier's law is a fundamental principle in heat transfer that describes the relationship between the rate of heat conduction and the temperature gradient within a material. It is a key concept in understanding the mechanisms of heat transfer, particularly in the context of thermal conductivity.
Greenhouse effect: The greenhouse effect is the process by which certain gases in Earth's atmosphere trap heat, preventing it from escaping into space and thereby warming the planet. It is a crucial factor in maintaining Earth's temperature but can lead to global warming if intensified.
Heat: Heat is a form of energy transfer between systems or objects with different temperatures. It flows from the hotter object to the cooler one until thermal equilibrium is reached.
Heat: Heat is a form of energy that is transferred from a hotter object to a cooler object due to a temperature difference. It is a fundamental concept in thermodynamics that describes the flow of thermal energy and its effects on matter.
Heat Equation: The heat equation is a partial differential equation that describes the distribution of heat (or variation in temperature) in a given region over time. It is a fundamental tool in the study of heat transfer and is widely used in various fields, including physics, engineering, and materials science.
Heat flux: Heat flux is defined as the rate of heat energy transfer per unit area, typically measured in watts per square meter (W/m²). It quantifies how much thermal energy passes through a given surface in a specific time frame, and is a crucial concept when analyzing various mechanisms of heat transfer such as conduction, convection, and radiation. Understanding heat flux helps in evaluating energy efficiency in systems involving temperature changes and thermal interactions.
Heat Transfer Coefficient: The heat transfer coefficient is a measure of the rate of heat transfer between a surface and a fluid, or between two surfaces in contact. It quantifies the amount of heat that is transferred per unit time and per unit area, for a given temperature difference.
Heat Transfer Rate: Heat transfer rate refers to the amount of thermal energy that is transferred per unit of time between two systems or a system and its surroundings due to a temperature difference. It is a measure of how quickly heat is being transferred and is a crucial concept in understanding the mechanisms of heat transfer.
Infrared radiation: Infrared radiation is a type of electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves, ranging from approximately 700 nm to 1 mm. It is commonly associated with thermal radiation emitted by objects due to their temperature.
Insulation: Insulation is a material or method used to reduce the rate of heat transfer. It works by minimizing conduction, convection, and radiation.
Isothermal: Isothermal refers to a process or condition in which the temperature remains constant while heat is transferred into or out of a system. In an isothermal process, the system's internal energy does not change because any heat added to the system is balanced by an equal amount of work done by the system, allowing it to maintain a steady temperature throughout.
Joule: The joule (J) is the fundamental unit of energy in the International System of Units (SI). It represents the amount of work done or energy expended when a force of one newton acts through a distance of one meter. The joule is a versatile unit that can be used to quantify various forms of energy, including thermal, electrical, and mechanical energy.
Natural convection: Natural convection is the transfer of heat through a fluid (liquid or gas) caused by molecular motion and density differences within the fluid. It occurs without any external forces like fans or pumps.
Net rate of heat transfer by radiation: Net rate of heat transfer by radiation is the amount of thermal energy exchanged between objects due to electromagnetic radiation per unit time. It depends on the temperature and emissivity of the objects involved.
R factor: The R factor, or R-value, measures the thermal resistance of a material. It quantifies how well a material can resist the flow of heat.
Radiation: Radiation is the transfer of energy through electromagnetic waves or particles. It does not require a medium to travel and can occur in a vacuum.
Radiation: Radiation refers to the emission or transmission of energy in the form of waves or particles through space or a medium. It is a fundamental mechanism of heat transfer that plays a crucial role in the topics of heat transfer, specific heat, calorimetry, and the mechanisms of heat transfer.
Rate of conductive heat transfer: The rate of conductive heat transfer is the amount of thermal energy transferred per unit time through a material due to temperature differences. It depends on the material's thermal conductivity, cross-sectional area, temperature gradient, and thickness.
Specific heat capacity: Specific heat capacity is the amount of heat energy required to raise the temperature of one unit mass of a substance by one degree Celsius (or one Kelvin). It plays a crucial role in understanding how different materials respond to heat and is key in processes like heat transfer, calorimetry, and thermodynamics.
Stefan-Boltzmann Constant: The Stefan-Boltzmann constant is a physical constant that describes the total amount of energy radiated per unit surface area of a black body in unit time at all wavelengths. It is a fundamental constant in the field of thermal radiation and plays a crucial role in the understanding of heat transfer mechanisms.
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 emissive power, $\sigma$ is the Stefan-Boltzmann constant, and $T$ is the absolute temperature.
Temperature: Temperature is a measure of the average kinetic energy of the particles (atoms or molecules) in a substance. It quantifies the degree of hotness or coldness of an object and is a fundamental concept in thermodynamics that is closely related to the transfer of heat energy.
Temperature Gradient: The temperature gradient refers to the rate of change in temperature over a given distance or direction. It represents the spatial variation in temperature within a system or material, indicating the direction and magnitude of heat flow.
Thermal conductivity: Thermal conductivity is a material's ability to conduct heat. It quantifies the rate at which heat energy passes through a material given a temperature gradient.
Thermal Conductivity: Thermal conductivity is a material property that describes the ability of a substance to transfer heat. It is a measure of how quickly heat can flow through a material, and it is a crucial factor in understanding heat transfer processes.
Thermal Diffusivity: Thermal diffusivity is a material property that describes the rate at which heat can diffuse or spread through a material. It is a measure of how quickly a material can conduct heat and reach thermal equilibrium with its surroundings. This property is crucial in understanding heat transfer processes and the behavior of materials in various thermal applications.
Thermal equilibrium: Thermal equilibrium is the state in which two or more objects in thermal contact no longer exchange heat, resulting in a uniform temperature throughout the system. This occurs when the temperatures of the objects are equal.
Thermal Equilibrium: Thermal equilibrium is a state in which two or more objects or systems have reached the same temperature and no longer exchange heat energy. This concept is fundamental to understanding temperature, thermometers, heat transfer, and the behavior of thermodynamic systems.
Thermal Resistance: Thermal resistance is a measure of a material's ability to resist the flow of heat. It quantifies how effectively a material or system impedes the transfer of thermal energy from a hotter region to a cooler one, and is an important concept in the study of heat transfer, specific heat, and calorimetry.
Thermographs: Thermographs are devices that automatically record temperature over time. They are used to monitor and document temperature fluctuations in various environments.
Thermometer: A thermometer is a device used to measure and indicate the temperature of a substance or environment. It is a fundamental tool in the study of temperature and thermal equilibrium, heat transfer, and mechanisms of heat transfer.
Watt: The watt is the unit of power, measuring the rate at which energy is generated or consumed. It is a fundamental unit in the study of energy, work, and heat transfer, and is widely used in various scientific and engineering applications.