# B What is energy?

#### sophiecentaur

Gold Member
Can all sorts/types of energy translate into some other kind? Heat for example.
Is there a hierarchy in this "translation"? Is heat the lowest form or state of energy?
We could turn this thread into a very practical discussion about Energy - which is quite valid, I think.
Heat Engines (the general term for a device to obtain mechanical work from the flow of heat) work best (higher efficiency) between large temperature differences. Once the work has been 'extracted' from a mass of (say) steam and the steam has condensed, you can sill get some work out of it by using a cold sink at an even lower temperature than 100C BUT NOT MUCH. So you could say that the energy in the condensed steam is a 'lower grade' than that of the superheated steam that is supplied by the original boiler.
The hot water would still be 'useful' for heating but not so much for doing work.
It's basically down hill all the way, though.
Your suggested "hierarchy" is ok as an arm waving term and if you want to talk in terms of relative usefulness. Considerations like possibilities of storing for the various forms of energy are very relevant sometimes. You could perhaps put Mechanical Potential Energy at the top because you can store large amounts of water at height or use a wound spring to store useful energy.

#### Merlin3189

Homework Helper
Gold Member
If you think of energy as an accounting term, then all types of enery are equivalent. The energy simply tells you the conversion value, not whether and how a conversion process happens.
You suggest heat may be a low form of energy, but in a power station we use it to generate electricity, a more noble form. We can use that to drive motors and raise objects to give them potential energy, to charge batteries and give them chemical potential energy, produce light and other electromagnetic energy. I don't know if we can put nuclear energy back in the bottle, but I guess if we could, it might involve using large amounts of electrical energy in particle accelerators.

The thing about heat is that it is the kinetic energy of random motions of molecules, lots of them. If two molecules collide, it is normally perfectly elastic, energy is conserved and energy can be transferred from the slower to the faster or vice versa. But it is more likely for energy to be transferred from the faster to the slower. Statistically, when there are large numbers of molecules, energy gets shared out. Energy could not be concentrated in a small number of very fast molecules, because they would very quickly collide with the large number of very slow ones and lose some energy. These ideas lead us down the road of entropy and thermodynamics. Heat doesn't travel from a colder to a hotter (by conduction.) That is probably what you have in mind.

"What is energy?"

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving