Like, i know it’s the ability to do work and all that, but this isn’t what energy is, it’s what energy does. What IS energy tho?
In: Physics
#Not ELI5, but…
Like you said… it’s a quantity describing how much work a system is capable of producing. Work is a “rate” quantity defined as an amount of energy being used up per second. That’s black and white definitions of things.
In practice you care about the “work” quantity a lot more often because you are usually trying to design something that can output “energy” *at a specific rate.*
Energy, then, **is just a quantitative property that can describe the state of an object or system.** A red hot piece of iron has high (thermal) energy State. A bullet speeding through the air has high (kinetic) energy state. An anvil sitting atop a 500-meter tall cliff has a high (gravitational potential) energy state.
If that anvil gets kicked off the cliff it will begin LOSING (potential) energy while instantly GAINING that exact amount of energy in another form (kinetic AND thermal due to air drag). This re-balancing of energy will continue until the anvil hits the lowest ground it can reach (or hits a coyote), at which point it is said to have 0 potential energy. The instant before impact, the anvil would have maximum kinetic energy and minimum potential, as it reaches top speed before a quick stop.
The re-balancing is known as “conservation of energy” and follows what we observe in real life. Energy is always conserved. The anvil started with a high state of potential energy and it ends with a high state of potential energy, even if we can’t see it. The air drag heated up the anvil slightly during its travel. Sound waves were emitted as the anvil falls through the medium of air. All the potential energy was replaced by kinetic during free-fall. Once it has landed, gravity still acts on it. If we could create a new deeper cliff, the anvil would fall some more. What this means is that **in the long term, objects and systems tend to exist in discrete states of energy.** The anvil can EITHER be on the cliff OR at ground level. So there are two possible energy states for that system.
It should be no surprise that the physics of electromagnetism are very similar. 12V car battery has a 12V “potential difference.” That means a single electric charge within the battery “feels” 12V of “force” pulling it towards the direction of opposite charge. This is analogous to how the anvil “feels” the potential energy of gravity pulling it off the cliff from high energy to lower energy. It’s pure luck, our previous understanding, and the beauty of nature to thank for this striking resemblance between Newtonian mechanics and electromagnetism.
It might help to imagine energy as a “system property” where you are given a bar graph of the various types of energy we know of like grav potential, kinetic, thermal, acoustic, electric potential, etc. When the system is dormant, everything is FIXED and the bars each show a different value. When we force the system to accept or perform work… aka the system undergoes an event…the energy levels will change by type. At the end of the event, the system will have the EXACT SAME amount of energy, just distributed to the categories a bit differently.
Now, you might think giving off heat and sound would be a net energy loss, thus breaking the rules. That just means you haven’t properly considered the system at hand, which includes the atmosphere that the sound and heat has dissolved into. When the “system” is properly selected, we can take advantage of the “conservation rule” and know for a fact that the net CHANGE in energy will be 0. This can help us safely assume things we otherwise couldn’t measure or know about.
In practical matters, if you know the mass of a bullet as well as the mass AND speed of the gun’s recoil. You could determine the bullet speed as it leaves the barrel. This is one example where energy methods can help out a lot with something that would be tough to measure or calculate.
Latest Answers