When I was going to college thermodynamics was dreaded by many engineering students. Apparently, it involved concepts difficult for many to grasp. For some reason it made perfect sense to me. Compared to most of my other engineering classes it was easy. I got a very high A in the class. That was decades ago, and I have forgotten a lot of it but I do have a good recollection of heat transfer.
That bright engineering students find it challenging means it should come as no surprise that people with little or no training in the subject would have misconception about how certain thermodynamic related physical phenomena occur. There have been many times on this blog I have made statements, or linked to articles, which described perfectly obvious observations. Some commenters declared them obliviously false. I didn’t want to take the time to explain why they were in error. It was just too much work for that particular situation.
It is now time to attempt explain certain things to people in terms and examples that will help them understand the physics of heat transfer. There are other sources on the web as well. But I will include example directly applicable to material on this blog.
The three* classic methods of heat transfer are:
- Thermal conduction (also called diffusion).
- Thermal convection.
- Thermal radiation.
The first two are relatively well understood at an intuitive level by nearly all functional people. It is the thermal radiation that I most want to address because of the clear lack of understanding I see in the comments here. I will explain the items 1 and 2 first to make the distinction from thermal radiation clearer. Please either stick with me or skip ahead if excessive boredom occurs.
Thermal conduction occurs when two objects of different temperature touch. Your finger touching an ice cube initiates the transfer of heat from your finger to the ice. The ice warms and when it reaches the melting point it changes phase from solid to liquid water.
If your finger is in a glass of liquid water and an ice cube thermal convection occurs**. Via conduction your finger warms the water touching your finger and because the warm water is slightly less dense than cold water*** the warm water rises. If the warm water is rising, then the cold water must be sinking. This creates a loop of water flow in the glass. It is slow enough that you cannot easily see it or feel it. If you were to put a drop of food coloring in the water, you probably could. The coldest water is next to the ice cube and the warmest is next to your finger. The water leaving the ice cube is replaced by water that recently left your warm finger. The warm water touching the ice cube conducts heat to the ice cube. This warms and melts the ice and cools the water causing it to sink. Heat is thus transferred from your finger to the ice.
Thermal convection occurs in gases as well as liquids. If you open a hot oven door with your face over the opening, you will feel an almost blast of hot air. No similar blast occurs at the crack at the bottom of the oven because cool air is rushing in. The hot air rises near a wood stove and cool air near the floor replaces it and forms a slow-moving loop of air. Soaring birds “ride the thermals” when different portions of the earth absorb more energy from the sun that others (for example dirt versus plants). This creates an updraft of air over the hotter earth which allows the birds to stay aloft with greatly reduced effort.
Thermal radiation occurs at all temperatures above absolute zero (-273.15 C or –459.67 F). But in our normal earthly circumstances most people are unaware of it because conduction and convection tend to dominate everyday life thermal transfers. Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter.
- Thermal radiation occurs across empty space as in from the sun to the earth.
- Thermal radiation occurs through gases as in from the sun to the surface of the earth.
- Thermal radiation occurs though solids as in the sun through glass.
The sun is not some magical generator. Microwave ovens emit a very specific frequency of identical electromagnetic waves which also transfer heat. The difference is only in the means by which the EM waves are creates, not in the nature of the waves.
The leaves of plants will sometimes get colder than the nearby air because they radiate their heat into outer space on a cold clear night when the air is still. Hence the air may be 35 F but the plant leaf can lose heat and drop to below 32F and get frost damage.
Orchard owner sometimes protect their crops by “heaters” which produce smoke to block the thermal radiation. The heaters do not produce enough heat to significantly heat the air. Owners also sometimes use large fans which move warmer air over the leaves. Via conduction the warmer air restores the heat lost by radiation.
The temperature of the sun is extremely high, and the thermal radiation occurs at high levels across a broad spectrum which includes the visible spectrum. The visible spectrum is what we call light. At the low frequency end of light, we call this thermal radiation red and even lower infrared. At the high frequency of light, we call this thermal radiation blue and even higher is ultraviolet.
Glass is not a magical solid conductor of thermal radiation. When exposed to thermal radiation all substances will do the following three things in various degrees with the incoming thermal radiation
- Transmit it through to the other side.
- Absorb it.
- Reflect it.
The amount of transmission, absorption, and reflection depend on the substance and frequency of the thermal radiation. These differing amounts are each described by a number between 0.0 and 1.0 inclusively. These numbers are called coefficients. The sum of all these numbers will always be equal to 1.0 (conservation of energy). Hence clear glass, for visible light, may have a transmission coefficient of 0.90. That is, 90% of the thermal radiation in the visible spectrum passes through the glass. The reflection coefficient may be 0.08 and the absorption coefficient 0.02 for a total of 1.0. Colored glass absorbs and/or reflects some energy at certain frequencies and transmits most of the energy at other frequencies.
Clean water has a high transmission coefficient for visible light but is highly absorbing of a certain frequency in the microwave region of the spectrum. This is why microwave ovens can heat a cup of water. The water absorbs nearly all the microwave frequency thermal radiation which the water intercepts.
Brick, wood, and other common house construction materials transmit thermal radiation at frequencies we know as radio waves. You can easily listen to your radio and make cell phone calls inside your brick building. At visible light frequencies and normal wall thickness there is no human perceptible transmission.
Thermal radiation is also why a vacuum is not a perfect insulator. Even in the hard vacuum of deep space, far from stars or any other object a warm object will radiate its heat into the surrounding empty space as lower and lower frequencies of electromagnetic radiation until it approaches a temperature of absolute zero.
This is also why the earth cools at night. It radiates heat into outer space. If it didn’t get rid of heat at the same average rate at it absorbed, it from the sun it would either get warmer or colder until the thermal radiation at night increased or decreased to match the visible light (as well as thermal) radiation absorbed from the sun.
Absorption and retransmission is where things get most obscure in our ordinary life and is where the commenters have been going astray.
Taking the example of an ordinary brick in the sunlight. It transmits none of the light, reflects some of the light in the red region of the spectrum and absorbs the rest. The absorbed the light causes the brick to warm. Some of that thermal energy is transferred to it’s surroundings via conduction and convection. But some of it is emitted as thermal radiation. This thermal radiation will be at various frequencies depending upon the exact chemical composition of the brick, but most will be in the infrared region of the spectrum.
This change of frequency is how certain gases get classified as “greenhouse gases”. This is how paint can actually cool the substance it is painted on below the ambient air temperature.
I’ll explain the paint first since it is simpler and has less emotional content.
The back side of the paint receives thermal energy via conduction. Suppose this paint is on a building at 75 F. It emits thermal radiation out into its exterior environment with the clear empty (sun and moon transmit their own thermal radiation) sky being a very cold (many degrees below zero) heat sink. Normal paints absorb significant light energy as well as conduction gains from the air. But what if the paint had very low conduction ability on the outside but high conduction ability on the inside, and the paint also reflecting almost all light? The outgoing thermal radiation would dominate the incoming heat transfer from the air and sunlight. Hence, the paint would literally cool the building it was painted on without the use of any external power source.
Now let’s consider the case of water vapor in the atmosphere. This is transparent to visible light. Clouds are condensation and/or ice. This water vapor transmits visible light to the earth which absorbs it and retransmits infrared thermal radiation just like our brick. The water vapor in the atmosphere, just like our colored glass, blocks the thermal radiation via reflection and absorption. If the incoming high frequency energy zips through the water vapor in the atmosphere and the retransmitted low frequency outgoing energy is reflected back to earth and/or absorbed, then the earth will get warmer.
That is the extremely simple version of greenhouse gases. Things get really complicated when you throw in things like clouds which reflect significant portions of visible light as well as whether they are clouds of ice crystals or water droplets and their presence during the day versus night, the latitude, the type of surface (earth, water, forest, ice, etc.) they are shading, and probably many other things. Does water vapor and/or CO2 really cause “global warming”? I don’t know. I am skeptical of manmade changes of CO2 in earth’s atmosphere causing heating and I think water vapor is complex enough that modeling it accurately is probably currently impossible.
Venus, almost for certain, is far hotter because of its mix of atmospheric gases than it would be if the composition were something like 80% nitrogen (earth) instead of about 3.5% nitrogen.and 96.5% CO2. So, I believe greenhouse gases can be a real thing.
Summary: Thermal radiation is not as well known by the general public as thermal conduction and convection. But it is real and easily observed if it is pointed out to you. Thermal radiation becomes the dominate heat transfer mechanism when long distances are involved. Thermal radiation exists at different frequencies. Substances have different absorption, reflective, and transmission characteristics at the different frequencies. Because of these different characteristics at different frequencies, it is possible to create one-way “heat valves”. Cooling paint and “greenhouse gases” are possible and exist because of these thermal dynamic “valves” utilizing thermal radiation.
* I won’t directly cover transfer of energy by phase changes or transfer of mass of differing chemical species.
** I’m not going to address the case of a zero-gravity environment.
*** Yes, I know, at temperatures between 0 and 4 C this is not true. Let’s not complicate things. But it is interesting to note this anomaly is why ice generally forms on the top of a body of water rather than on the bottom then floating to the surface.