Physics of X Episode 10: Shields Revisited—What to Do About Heat?

In the blog post for Episode 4 (or video if you prefer), we addressed how Harry’s shield bracelet might work. Of course, Harry discovers in a battle with vampires that his original design does not work great against fire, and he suffers a severely burned hand for this oversight. What is the physics of this? And how would he ultimately make a shield bracelet that dealt with fire or heat? (Check out the YouTube video below on this for a quick peek!)

The physics of fire, heat, and temperature is rather interesting. From a physics perspective, there are three terms that describe very specific things that may or may not match your intuition of each term. Fire is perhaps the closest to our intuition. If you Google the definition of fire, you get “combustion or burning, in which substances combine chemically with oxygen from the air and typically give out bright light, heat, and smoke.” This highlights some very important physics concepts we have discussed.

First, fire is fundamentally a chemical reaction between oxygen and whatever is burning. We already discussed how energy provides a measure of the state of something. In this case, the burning causes the combined energy of the oxygen and the object it is reacting with to be lower. Conservation of energy requires that the excess energy shows up somewhere else, in this case as light and increased temperature of the air around the object. The light is what we call the flame. But, the “increased temperature of the air” is where language gets tricky.

In general, everyday terms, we say the energy generated by oxygen reacting with whatever is burning is released as “light and heat.” And we use the word “heat” to mean “something that is hot!” However, this is a bit different from the physics usage of the word “heat.”

In physics, how hot something is is associated with its temperature. Temperature is a measure of the kinetic energy of motion of molecules, and as with any energy, it describes the state of a system. The higher the temperature, the more the molecules are vibrating and moving. When the temperature (vibration energy) is just right, they will even emit light! As every blacksmith knows, as you heat metal to work with it, it glows with different colors.

Therefore, from a physics point of view, burning things results in light (the flame) and an increase in temperature (the hotness) and both of these are measures of the energy being released during the reaction of the oxygen and the materials. So, if this is NOT heat, what is heat from a physics perspective?

As we have said, energy describes the state of a system. But we often think of two systems that interact with each other, and in the process of this interaction, energy will be transferred from one system to another. For example, when I push something to make it go faster, it has more kinetic energy, and I have used up chemical energy in my muscles to generate the force. This is an example of adding energy to a system by doing work. Another way to transfer energy between two systems is to put systems that are at different temperatures in contact with each other. In this case, the energy in the system at a higher temperature is transferred to the system with the lower temperature. This decreases the temperature of the system at a higher temperature and increases the temperature of the system that was at a lower temperature. This exchange of energy is the physics definition of heat.

But what does all this have to do with Harry’s shield bracelet and defending against fire? We have established that there are effectively three elements to fire: the chemical reaction that releases energy, the energy that is released as light, and the energy that is associated with the increased temperature of the air and/or the material that is burning.

From Harry’s perspective, the light doesn’t really matter. The type of electromagnetic radiation emitted by an object is roughly related to its temperature. For most fires, the bulk of the radiation is in the infrared range (and visible since we can see the flames). These wavelengths do not significantly interact with our skin. If the fire got significantly hotter, it would eventually emit ultraviolet radiation, which does burn. After all, that is the source of sunburn!

The chemical reaction causing the fire is not a direct danger to Harry either. However, stopping that reaction is one thing that can help protect him, so we will return to that. The real danger is the high temperatures being generated. These can cause your skin to burn and can damage your lungs from breathing the hot smoke/air. So, from a defensive perspective, what Harry is trying to stop is the high temperatures, or gas that has high kinetic energy, from reaching him.

If you recall our earlier discussion of shields, we focused on ways to stop the kinetic energy of “normal” sized objects, such as bullets, rocks, etc. The flame/high-temperature situation is a bit different. There are two things going on. First, the flame coming at you means that the chemical reaction is heading your way. This usually requires a physical fuel to be aimed at you and oxygen in the room for it to react with. There are a few different strategies you could use to stop this. Depending on the fuel, you could use something like the kinetic energy rings from Episode 2 to send a pressure wave that deflects the fuel. This would act like a shield as the fire would avoid you. But you might still have to contend with the second issue, the high temperature coming at you.

This is where the physics version of heat becomes relevant. Essentially, heat is the flow of energy from high temperature to low. So, if the hot air from the fire is close enough, it will continue to heat the air in your direction even if you deflect the flame, and you could still burn even without a visible fire. In this case, you do not necessarily even need molecules to physically be moving at you. Basically, the really high-temperature gas bounces around hitting the gas molecules right next to it, heating up those to a higher temperature and so on. So there is basically nothing physical coming at you. What you have to figure out is how to block the flow of temperature — and this is basically what insulation does.

One way that insulation works is to have a very low density of molecules so it is very hard for the high-temperature material to interact with any of the molecules in the insulation. This is why having windows with an air gap or a thermos with a vacuum layer works so well. In fact, a perfect vacuum is a great insulator because only radiation can cross it, not molecular vibrations! This type of insulation utilizes conduction, which refers to heat transfer that occurs through direct contact; in this case, the molecules bumping into each other. If Harry was able to insulate himself, he could create a barrier by constraining conduction within the insulation layer. The best way to do this is with a layer of vacuum. But thinking about ways for Harry to create a vacuum around himself is difficult.

Instead of going for insulation and trying to reduce conduction, Harry can use another way heat transfer occurs — convection. This is when you actually move stuff around to transfer hot and cold stuff to different areas. In reality, some of the hot air is most likely moving at Harry, so it is convecting. A great counter to this is for Harry to create a fan to blow the high-temperature air away, sucking in all the cool air that is behind Harry. Exchange enough cool air for hot air and you are safe.

One can actually take this up one notch by including evaporation as a means of dissipating the energy associated with the hot air. Basically, the hot air put in contact with water will make the water hotter and hotter until it reaches the point where it wants to turn into water vapor. It takes a lot of energy to turn liquid water into vapor, and that loss of energy can significantly lower the temperature of the hot air around you. This is why sweating is so effective. As your sweat evaporates, it uses the energy of your skin and the air around you and lowers the temperature in the process! Combine some evaporation with wind, and you get even better protection.

In the end, whether I am trying to deflect bullets or stop fire, I am basically manipulating the density, pressure, and speed of the air around me. Though hard to imagine how exactly Harry does this with just a bracelet, these are not crazy things to imagine. After all, watches that monitored our biological functions were a thing of science fiction not that long ago, so who can say bracelets that generate wind and pressure waves are that far off?

Originally posted on Medium.