Last week we took a look at how a regular fire starts and spreads.
This week we will go into a little more depth, investigating different types of fire and how we have utilised and harnessed their unique properties.
But first, let’s have a little history.
Fire is a prime example of the resourcefulness and ingenuity of humans, and it’s origin is steeped in myth and legend – each civilisation has its own tale of discovery.
The Greeks believed that Prometheus stole fire from the gods while a Cherokee legend tells of Grandmother Spider who stole fire from the sun, hid it in a clay pot, and gave it to the people. It has always held a mystique but, of course, this is all nonsense, the discovery of fire was far less exciting and fantastical.
However, it was only after humans became able to harness fire that were were able to evolve. The earliest evidence for controlled use of fire is at the Lower Paleolithic site of Gesher Benot Ya’aqov in Israel, dated 790,000 years ago.
Humans taught themselves to easily manipulate other substances and began to build tools and other implements and our early development was accelerated greatly as a result.
Our use of fire rapidly increased and we began to discover better and more varied ways of creating it. Slower burning flames for instance were used to create light and the invention of the candle allowed use to efficiently create light and allowed us to expand our range beyond that of the sun lit hours.
But what about now? What have we learnt and how do we use it’s power now?
How Fires Differ
We now know that Fire is self perpetuating – once the fuel reaches its ignition temperature it will maintain and continue to burn as long as it has fuel and oxygen to feed on.
The fuel’s composition is the main determinant factor in the way the fire burns. Different elements combine more easily with oxygen or have a lower burning temperature and so release their energy at a quicker rate. The most flammable compounds contain carbon and hydrogen, both of which react with oxygen relatively easily.
The fuel’s physical size and density also affects the burn rate – diesel burns a lot slower than gasoline for instance, because it is much more dense. In a more tangible example, something long and thin will burn quicker than something fat and dense because it has a larger surface area. This larger surface area emits more heat and more of it is able to react with the oxygen in the air. A large fuel source can also absorb a lot more heat before reaching its ignition temperature.
How Do We Use Fire?
The energy released when a fuel is burnt can be converted into many different types of energy – light, heat and through complex processes electricity. The below diagram shows how we use fire to convert inert chemical energy into useful electrical power.
This electrical power has been used to turn our society into a digital one, once again accelerating our development and allowing a multitude of developments we now deem essential to our modern lives.
In the chemical reaction, compounds break down to form various gases. The reactants (the original chemical compounds) have a lot of energy stored up as chemical bonds between different atoms. When the compound molecules break apart the products (the resulting gases,) may use some of this energy to form new bonds, but not all of it. Most of the “leftover” energy takes the form of extreme heat.
The concentrated gases are under very high pressure, so they expand rapidly. The heat speeds up the individual gas particles, boosting the pressure even higher. In a high explosive, the gas pressure is strong enough to destroy structures, injure and kill people. If the gas expands faster than the speed of sound, it generates a powerful shock wave. The pressure can also push pieces of solid material outward at incredible speed.
Cyclotrimethylene-Trinitramine (C4) is a good example of how man has manipulated and enhanced a natural reaction to create a tool that is perfectly fit for purpose. Most explosives, by their nature are unpredictable and unsafe. C4 however, has been mixed with a binding agent (di(2-ethylhexyl) sebacate). This binding agent coats the explosive making it less volatile and less likely to explode when you don’t want it to. It also makes it malleable expanding its usage and making it one of the most useful explosives we have.
So these are just a few modern day uses of fire. Next week, we’ll look at how fire reacts with something much more fragile, the human body.