The Physics Philes, lesson 123: Conduction, Convection, Radiation, Oh My!
You know, we’ve been talking a lot lately about energy transfer, aka heat. But we haven’t talked much about how that happens. Let’s fix that now. There are three ways heat can move from one place to another: conduction, convection, and radiation.
First, let’s start with conduction. Say you heat one end of a metal rod. Soon, the cold part you are holding will start to heat up. You’re feeling conduction.
Here’s what’s going on: Atoms in the hotter region of the rod have more kinetic energy than the atoms at the cooler end. The more energetic atoms will jostle and bump the atoms next to them, and those will jostle and bump the ones them, and so on and so on. Like falling dominoes, soon the entire rod is hot and you’ve burned your hand. Better get some ice for that.
In this process, the atoms themselves don’t move from one region to the other, just their energy moves. That’s not always quite how it works, though. Most metals have these things called “free electrons,” or electrons not bound too closely to a nucleus. These electrons can move rapidly from place to place, carrying energy as they go. This is also what causes metals to be good conductors of electricity.
Transfers of heat will only occur between regions that have different temperatures and the heat will always flow from regions of higher temperatures to regions of lower temperatures. The rate at which the heat flows is called the heat current, and experiments have shown that the heat current is proportional to the cross sectional area of the rode and the difference in temperature (in either Celsius or Kelvin), and it’s inversely proportional to the length of the rod. Multiply all of that by a material’s thermal conductivity constant k and you get heat current in watts, or joule/second:
The magnitude of the quantity ΔT/L called the temperature gradient. The thermal conductivity constant k depends on the material you’re dealing with. Large values of k tend to indicate that the material is a good conductor of heat, while small values of k indicate that the material is more of an insulator or at least a poor conductor.
So that’s conduction in a nutshell. But there are two more heat transfer mechanisms to go! Next let’s talk about convection.
Convection is a big more complicated than conduction. There’s no simple equation that describes it. However, it’s very important for life.
Convection is the transfer of heat by the motion of fluid from one place to another. There are two types of convection, forced convection and natural or free convection.
Forced convection is basically what it sounds like. A fluid is artificially circulated with a pump or a blower. An example of this actually your blood pumping though your body. Your body needs to maintain a constant temperature, and your heart and blood help with that. Your heart acts as a pump and moves your blood all around your body.
Natural or free convection is also what it sounds like. Free convection occurs when a flow is caused by density differences due to thermal expansion. We see this with molten rock in the Earth’s crust, air currents that have a major role in daily weather, and in ocean currents, which is important in global heat transfer. In addition, if you’ve ever seen a hawk or some other bird kind of spiraling up higher and higher into the air, you’re witnesses the effect of free convection currents.
Convection is much more complicated than conduction, and there isn’t a nice equation that we can use to describe it. However, there are some experimentally verified facts we can use to analyze the phenomenon. First, the convection heat current is directly proportional to the surface area. Second, the viscosity of the fluid effects free convection; it slows it near a stationary surface. This gives the surface a film on a vertical surface that will insulate about as well as 1.3 cm of plywood. Forced convection will decrease the thickness of the film, which will increase the rate of heat transfer. And third, convection heat current is approximately proportional to the temperature difference between the surface and the main body of the fluid to the 5/4 power.
Not as easy as conduction. But they have something in common: The both need matter to get the heat to travel from place to place. Not so with the third heat transfer mechanism, radiation. Radiation is the transfer of heat by electromagnetic waves. Visible light, infrared, and ultraviolet are all examples of EM waves. Heat can move through a vacuum via radiation.
What’s cool about radiation is that everything does it. Everybody does it. You, me, the table, my computer, it all emits EM radiation. At normal temperatures, though, we can’t see it. The energy is carried away by infrared waves, so we can’t see the with our eyes. However, if we heat things up they will start to glow. At about 800°C, the wavelengths will shift and emit more visible light and become self-luminous and appear “red hot.” But even at this temperature, most of the energy is being carried away by waves in the infrared. If we amp that up to 3000°C – the temperature of the filament of an incandescent light bulb – the radiation will contain enough visible light to be seen as “white hot.”
Like conduction and convection, radiation is also proportional to the surface area of the emitter. For radiation, the rate of heat transfer increases with temperature; it depends on the temperature in Kelvin taken to the fourth power. In addition, the rate also depends on something called emissivity ε, which depends on the material and to a lesser extent the temperature. Emissivity is a dimensionless quantity between zero and one, and it represents the ratio of the rate of radiation from a particular surface to the rate of radiation from an equal area of an ideal radiation surface at the same temperature. If the emissivity is one, that means we have an ideal emitter (which is also an idea absorber) called a blackbody. A blackbody absorbs all radiation that hits it. Unlike convection, we have a simple equation to describe radiation:
That weird squiggle between ε and T is the the Stefan-Boltzmann constant σ and this relationship is called the Stefan-Boltzmann law. The physical constant is
Well! We’ve made it through the chapter on temperature and heat! But don’t worry, we’re not done studying how the transfer of heat interacts with the world around it. Stay tuned!
Featured image credit: Wikipedia