Heat Transfer: Conduction, Convection, Radiation; Thermal Conductivity

← Builds on Temperature, Scales, Zeroth Law, Heat Units

Terms you will need

conductionConduction is the transfer of heat through direct contact between objects or parts of an object.
convectionConvection is the transfer of heat by the movement of fluids, such as air or water, carrying heat from one place to another.
radiationRadiation is the transfer of heat through electromagnetic waves, which can travel through empty space and warm objects they hit.
thermal conductivityThermal conductivity is a measure of how well a material can transfer heat through it by conduction.
heat transferHeat transfer is the movement of thermal energy from a hotter object or area to a cooler one.

Heat transfer is the movement of thermal energy from one object to another due to a temperature difference. It plays a crucial role in understanding how energy moves in nature and in engineered systems. There are three primary mechanisms of heat transfer: conduction, convection, and radiation. Conduction occurs when heat is transferred through direct contact between particles in a solid. Convection involves the movement of heat through fluids (liquids or gases) by the physical movement of the fluid itself. Radiation is the transfer of heat through electromagnetic waves and does not require a medium. Understanding these processes is essential for analyzing energy systems, from everyday phenomena like cooking to complex industrial applications such as power generation. A key factor in conduction is thermal conductivity, which measures how well a material conducts heat. These principles are foundational in thermodynamics and are critical for designing and optimizing systems that rely on efficient heat management.

Figures (4)

Figure 1.5 (a) Thermal expansion joints like these in the (b) Trojan Bridge in Prague, Czech Republic allow bridges to change length without buckling. (credit: modification of works by “ŠJů”/Wikimedia Commons)

Figure 1.6 The curvature of a bimetallic strip depends on temperature. (a) The strip is straight at the starting temperature, where its two components have the same length. (b) At a higher temperature, this strip bends to the right, because the metal on the left has expanded more than the metal on the right. At a lower temperature, the strip would bend to the left.

Figure 1.7 In general, objects expand in all directions as temperature increases. In these drawings, the original boundaries of the objects are shown with solid lines, and the expanded boundaries with dashed lines. (a) Area increases because both length and width increase. The area of a circular plug also increases. (b) If the plug is removed, the hole it leaves becomes larger with increasing temperature, just as if the expanding plug were still in place. (c) Volume also increases, because all three dimensions increase.

Figure 1.8 This curve shows the density of water as a function of temperature. Note that the thermal expansion at low temperatures is very small. The maximum density at 4°C4°C is only 0.0075%0.0075% greater than the density at 2°C2°C, and 0.012%0.012% greater than that at 0°C0°C. The decrease of density below 4°C4°C occurs because the liquid water approachs the solid crystal form of ice, which contains more empty space than the liquid.

Particles vibrate and collide, passing kinetic energy to neighboring particles. The rate depends on thermal conductivity, a material property indicating how well it conducts heat. Higher conductivity means faster energy transfer.

Convection involves fluid motion carrying heat. Warmer, less dense fluid rises, cooler fluid sinks, creating circulation. Heat transfer depends on fluid properties and flow patterns.

This mechanism is key in liquids and gases, where particles can move freely. All objects emit radiation based on their temperature. The Stefan-Boltzmann law relates emitted power to temperature, showing higher temperatures produce more intense radiation.

Thermal conductivity governs conduction efficiency. Materials with high conductivity, like metals, transfer heat rapidly. Insulators, with low conductivity, resist heat flow.

These principles are essential for designing systems that manage heat effectively.

Key Points

Conduction is the transfer of heat through a material by the collision of particles and the movement of electrons within the material.
Convection is the transfer of heat by the movement of fluids or gases, where the warmer parts of the fluid rise and the cooler parts sink, creating a circulation pattern.
Radiation is the transfer of heat through electromagnetic waves, which can travel through a vacuum and do not require a medium to propagate.
Thermal conductivity is a measure of a material's ability to conduct heat, quantifying how effectively heat is transferred through the material by conduction.
Heat transfer is the movement of thermal energy from a region of higher temperature to a region of lower temperature, occurring through conduction, convection, or radiation.

Terms

Tap a term for a plain-language explanation.

Sources & licensing(4)

Olivier Cleynen, Engineering Thermodynamics — thermodynamicsbook.com/ (Creative Commons Attribution-ShareAlike 4.0)
Howard DeVoe, Thermodynamics and Chemistry, 2nd edition — www2.chem.umd.edu/thermobook/ (Creative Commons Attribution 4.0)
OpenStax — openstax.org/books/university-physics-volume-2/pages/1-introduction (Creative Commons Attribution 4.0)
OpenStax — openstax.org/books/university-physics-volume-2/pages/1-3-thermal-expansion (Creative Commons Attribution 4.0)