Fusion Series Part 2 – Why Fusion is Powerful
Introduction In the previous blog post in this series, we introduced the 4 states of matter ...
We’ve all seen that movie scene: the one where the guy in a space suit has an accident and cracks his visor? He in that moment becomes the vehicle for one of pop cultures most notorious misconceptions. The carefully maintained atmosphere in his visor escapes, space “enters” his helmet and freezes him into a popsicle. Here’s a similar example from the film “Mission to Mars”:
Woody takes off his helmet (in order to save the other two astronauts) and instantly freezes. Lets have a quick look at the Physics that explains why this is totally wrong shall we?
Boiling, freezing, melting, evaporating etc all depend on being able to transfer heat from one place to another. Heat is energy and the objects which transfer heat have the quality of having a temperature. Now temperature should really be taken as a soft measure of kinetic energy density – the amount of kinetic energy contained in a particular area or volume. Matter (literally just means “stuff”) can transfer heat and gain/lose temperature in 3 ways:
Conduction
Conduction describes hot particles vibrating and colliding with nearby 'cooler' particles and sharing the energy through “physical touch”. Essentially, when matter collides, there is a transfer of energy and heat spreads. This heat conduction is expressed via the following formula:
$$ q = -k \nabla T $$
Where \( q \) represents the local heat flux density (effectively the energy available), \( k \) is the intrinsic conductivity of the material (how well it can transfer heat) and \( \nabla T \) refers to the temperature gradient across the object.
Convection
Convection is a lot more relevant to liquids and gases. Convection describes localized heat sources raising the temperature of liquids and gases so that hot liquid/gas rises whilst colder liquid/gas falls – creating currents which over time circulate the heat out towards the entire substance.
The governing expression is similar to that of conduction:
$$ q = h A \Delta T $$
Where \( h \) is the convection heat-transfer coefficient and \( A \) is the exposed surface area of the liquid or the gas.
Radiation
Unlike the previous two transfer processes, Radiation doesn’t require a medium. Matter of any temperature naturally releases energy in the form of electromagnetic radiation. In fact, us humans naturally radiate some of our heat away as Infrared radiation. That is why thermal cameras can pick us up! This electromagnetic radiation can carry energy away from a hot source and deposit in whatever else can absorb them. Again (and this is important), Radiation doesn’t require a medium and can operate just fine in the vacuum of space. If you're very hot, you'll radiate a lot of energy away. If you're very cold, the amount of energy you radiate is of course much smaller in comparison.
Radiative processes are described by the Stefan-Boltzmann law of radiation:
$$ q = \sigma e A T^4 t$$
Where \( \sigma \) is the Stefan-Boltzmann constant ( \( 5.67 \times 10^{-8} Js^{-1} m^{-2} K^{-4} \) ), \( e \) is the emissivity of the object (how good it is at emitting), \( A \) is the surface area of the object, \( T \) is the temperature in kelvin and \( t \) is the time spent emitting. Already we can see that how much energy something emits is heavily dependent on how hot it is. A human being exposed to space is only going to be as hot as room temperature - which isn't very much.
But do not take my word for it. Let's actually calculate how much radiation (in power, so we don't need the time spent emitting) a human being would give out. These are our parameters:
\( \sigma \) = \( 5.67 \times 10^{-8} Js^{-1} m^{-2} K^{-4} \)
\( e \) = \( 0.98 \)
\( A \) = \( 1.75 \, m^2 \)
\( T \) = \( 308 \, K \)
\( P = 875 \, Js^{-1} \)
You may be thinking that this is a lot of energy lost per second. But you'd only freeze if you lost more energy than you received. There are two large celestial objects nearby. The sun and the earth itself. Taking extremely rough values from a cursory glance on google, the sun directs \( 342 \, Js^{-1} m^{-2} \) onto the earth (and therefore a human being in space) whilst the earth itself reflects/emits about \( 100 \, Js^{-1} m^{-2} \) back. A human would essentially be exposed to both sources. So if a human receives about \( 773.5 \, Js^{-1} \) from celestial objects and emits \( 875 \, Js^{-1} \) himself, he is losing \( 100 \, Js^{-1} \). The likelihood is that you'd be burned in space or at least die from hypothermia before you get a chance to freeze - which could take hours.
So if we’re in the vacuum of space (which has almost nothing in it) and temperature is considered to be energy density, what “stuff” (matter) is there that would facilitate this transfer of heat from us to it (such that we freeze)? Because the vacuum of space is more or less empty, there is nothing to take heat from us and freeze us. Things freeze because their temperature falls as heat transfers from them to something else. When you walk into an industrial freezer, you immediately feel cold because the molecules in the air are taking heat from your body in order. Then conversely, when you jump into a hot bath, you feel hot because you are taking heat from the hot water. No such thing happens in the vacuum of space. We also aren't hot enough to radiate our energy away so that we freeze. So, you cannot freeze in space.
Author: Dr. Adetokunbo Ayilaran
