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Thermal Energy, Temperature, and How Heat Moves. Definition of Temperature Definition of Heat How heat, or thermal energy, can move. Imagine a Gas….
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Thermal Energy, Temperature, and How Heat Moves • Definition of Temperature • Definition of Heat • How heat, or thermal energy, can move
The temperature of a large assembly of particles (solid, liquid, gas) is proportional to the kinetic energy per particle (- with some assumptions, it’s the equation above)k = Boltzmann’s constantm = mass of the particle (usually an atom or a molecule)<V2> is the average of the squared velocities of all the particlesT = the temperature in KelvinsAdded up for the entire object, this constitutes the Thermal Energy of that object
The Thermal Energy of an Object, or the “Heat” it contains can flow from one place to another, by three mechanisms… • Radiation • Conduction • Convection
Heat Transfer by Radiation • Photons have energy, which they carry away from their source, and deposit where the photon is absorbed • How? One way, we have already examined: electron de-excitation = emission, followed by absorption by another atom and putting it into an excited state. But there are other ways…
Thermal radiators emit a broad spectrum of photons. Two objects near each other will radiate at each other according to their temperature according to this version of the Stefan Boltzmann equation… (In the real world, objects are not truly “black”, because of their texture and the complex way that molecules in a solid can vibrate; the radiation may not get out easily and there will be a certain amount of self-absorption. Such an object is called a “grey body” and emits with an emissivity factor (the little “e” above)
The hotter object will radiate more and receive less, and hence will cool by radiation heat loss
Heat Transfer by Conduction • This is the simplest to understand, I think. The moving particles bang up against neighbors who in turn bang around with about the same vigor, who bang up against their neighbors, etc. • Until the fast-jiggling particles on the hot side have spent some of their energy by making their distant neighbors jiggle more • Meaning, those distant neighbors are hotter, and the fast-jiggling particles are cooler.
The rate at which heat flows by conduction is proportional to the gradient in the temperature, to the conductivity of the material, and the area across which the heat is flowing. • Conductivity k; related to the “R factor” for insulation materials. Some materials allow this bumping together of neighboring atoms more easily than other materials. • Metal has a very high k, styrofoam very low k
Conduction: General Features • Heat flows by conductivity rapidly when the material is dense and the atoms very close. Like liquids (water for example) and metals especially. • Heat flow by conduction is less in low density materials, like (to some extent) atmospheres. • Heat flow by conduction is faster when the gradient in temperature is larger (more temperature difference across the material)
Heat Transfer by Convection • Both conduction and convection happen within a massive medium (radiation heat transfer, however, can happen best when there’s no medium) • So, how does convection differ from conduction and why doesn’t conduction account for ALL heat transfer in a medium?
Convection Physically Moves the Hot Material to a Colder Place • Here’s Why - When heat input to a material is too fast for that material to conduct it away to surrounding material, that material will instead get so hot that its density drops enough that cooler, denser material can fall by gravity and replace, ie force upward, that hot lower density material. • Convection requires Gravity! A fact not nearly stressed enough in the literature!
