# Is there a fundamental reason why “dynamic energy storage” is lossy?

Consider springs and moving masses - both can be used to store energy, the spring via tension $E=\frac{1}{2}kx^2$ and the mass via kinetic energy $E=\frac{1}{2}mv^2$. But day to day experience tells us that while springs can essentially hold this energy indefinitely, a moving object always slows and stops eventually.

A very similar phenomenon can be observed in electric circuits - capacitors behave like springs, and can hold charge and energy for long times, while inductors hold energy in the form of magnetic field induced by a moving charge - which once again decays pretty fast.

I realize that ideally both methods are lossless, and it is only due to "imperfections" that energy is lost - friction in the case of moving masses and electric resistance in the case of inductors' magnetic field. But is there some fundamental or "philosophical" reason explaining why we might expect it to be so? Why isn't there a mechanism similar to friction/resistance causing a rapid loss of energy in a compressed spring or charged capacitor? Is there some underlying cause explaining the rarity of both frictionless-surfaces and superconductors? Or is it just a coincidence?

## 2 Answers

Here is a possible way to think about it.

To transmit information, some energy has to be spent.

Any detectable change in the environment must carry some information and, therefore, is associated with some energy transfer.

If a mass is moving through air, its movement can be detected by analyzing the changes in the air pressure. The mass has to spend some of its energy to push air molecules.

If a current flows in a regular wire (not a superconductor), it could be detected by the air convection and radiation due to the heating effect of the current. The moving electrons have to spend some energy when they collide with the atoms of the wire.

We can detect a moving object in a vacuum by just looking at it, but, in this case, the energy of the reflected light we detect is coming from somewhere else, i.e., the object does not need to spend any energy to be detected and, therefore, can move indefinitely.

Obviously, an ideal charged capacitor or a compressed spring, don't cause any changes in the environment, so no energy has to be spent.

So summarize (without generalization), if an object causes some detectable changes in the environment, some energy has to be spent. That energy could come either from an external source or from the object itself, in which case, the object will be losing its energy.

• Worth noting that even the light hitting the object should have a (extremely small) effect on the travel of the object as well. You don't get much use out of "Dynamic" unless it's also "Lossy". – JMac Jul 17 '18 at 18:36
• @JMac Yes, but it won't necessarily slow the object down. – V.F. Jul 17 '18 at 18:39

Another possible explanation, is that potential energy is not lost if no "action" is done, while kinetic energy is lost if the "action" is not continued.

In the case of the spring and the capacitor, potential energy is stored, while for a mass and an inductor, it is the motion that generates the kinetic energy.
Since there are a number of things that can interfere with the "motion," kinetic energy is subject to losses.