# What ultimately happens to energy?

The law of conservation of energy states that energy can neither be created nor be destroyed, but it can be transformed from one form to another.

So I considered an example of it as follows: water is stored at height with potential energy, the water flows and its potential energy is converted into kinetic energy. It flows to rotate a turbine with mechanical energy. Then the kinetic energy is converted into electrical energy. In our house the electrical energy is converted into luminous energy, heat energy, and so on.

But what happens after that?

• Radiation which escapes into the universe. The Earth is an open system which receives short wavelengths on the day side and radiates long wavelengths on the night side. Dec 28, 2014 at 17:37
• Conservation law of energy does not say that energy is constant in the universe. This is wrong. Enegy conservation does not hold when applies across the entire universe. It says, in an isolated system, the energy remains constant. Aug 22, 2015 at 19:50
• "heat energy" is a difficult phrase. We know what you mean, but the word "heat" has a very specific meaning the does not lend itself to this situation. Using the word incorrectly leads to mis-communication. You want thermal energy here. Sep 23, 2015 at 20:08

Law of conservation of energy states that, Energy in the isolated system remains constant.

Now, for the conversions you have provided, light energy is nothing but collection of electromagnetic waves. These waves carry energy proportional to their frequency. Light gets reflected, transmitted or absorbed. In case of absorption there is conversion of energy. The constituting atoms or molecules vibrate, translate or rotate with greater energy which they get from incident light. This increased energy in object can be stored or given out in some form.

Heat transfer occurs by convection, conduction or radiation. For radiation, same as light. Similarly, transfer of heat energy by convection or conduction is microscopic and depends on object receiving, whether to store or give out this energy in different form.

converted into luminous energy

When hitting a surface this luminous energy, which is just radiation, will be absorbed and thereby converted into heat to raise the internal energy of the surface.

If it doesn't hit anything - maybe it is send out into space - it never changes form.

and heat energy

By heat energy you might mean internal energy.

Energy transport is one thing: Heat up the air in your house (that is, raise the thermal energy), and open the windows, and the warmer air escapes. The warmer air will be mixed with ambient air and heat that up. This is just an exchange between air molecules to reach a common state of internal energy - no energy tranformations here, just transport from one air molecule to another.

Energy conversions is another thing: Any particle with internal energy (a temperature above $0 \:\mathrm{K}$) will emit radiation to some degree. So, at some point the heat of the air will be emitted away from the air into outer space (only to be absorbed by something else, should it ever hit anothor particle, an be reconverted into internal energy by heating that particle).

The circle of life: Of course other conversions will happen also as a consequense. The temperature differences of different layers of the air in our atmosphere causes winds (changes in kinetic energy and gravitational potential energy), causing wind turbines to turn (mechanical energy again and then converted into heat and electrical potential energy).

It goes around, and around, and around. Only when energy leaves the planet the cycle is finished (unless some space object absorbs it and emits it back).

I feel the need to quote Flanders and Swann: the First and Second Law of Thermodynamics (1964... first two minutes are dialog before the song starts):

Heat is work and work's a curse
And all the heat in the Universe
Is gonna cooool down
'cos it can't increase
Then there'll be no more work
and there'll be perfect peace
Really?
Yeah - that's entropy, man!

It's sung light-heartedly, but there is an important point: ultimately, all heat conversions are (somewhat) irreversible, meaning that entropy will increase. This process will continue "indefinitely" - with energy passing from one form to another, and back again, all the while increasing the entropy of the universe. There is no single "end point": any (excited) atom has a probability of emitting photons, and photons have a probability of being absorbed. Mass can have kinetic or (gravitational) potential energy, charge can have potential energy in an electric field, electric and magnetic fields can store energy (on a microscopic or macroscopic level; statically, or dynamically as in electromagnetic waves), and further energy can be stored in the form of mass, and can be converted into other forms of energy through various processes like annihilation, fusion, fission, ... Inside a nucleus, the strong nuclear force and weak nuclear force give rise to potential energy of the nucleons. Note that even an atom in a lattice at 0 K has a certain amount of energy - called the Zero Point Energy - because the uncertainty principle doesn't allow it to be stationary (you would know both the position and momentum at the same time - can't have that). You can't extract the energy - but it's there.