# The Naked Scientists Forum

### Author Topic: What is energy?  (Read 9734 times)

#### dlorde

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##### Re: What is energy?
« Reply #50 on: 13/05/2013 20:33:51 »
You wrote:
<<That internal energy can be used to do work; for example, you could use it to turn water into steam to drive a motor or generate electricity >> and this is not  true, unless you have a zero Kelvin heat reservoir (which is impossible to get  ).

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You have advised that you don't imply that "you'll get more out than you put in" but if you say that you can store energy as internal energy, in general, you have to specify when you can do it and with which limitations, because you can't do it if the surround it's not at lower temperature and however you can store just a part, not all of it.
I was using your example. If a box is in thermal equilibrium with its surroundings and you then heat the box by running current through a resistor inside it, you can then use the increased internal energy of that box to do work - by exploiting the temperature difference between it and its surroundings.
« Last Edit: 13/05/2013 20:42:09 by dlorde »

#### dlorde

• Neilep Level Member
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##### Re: What is energy?
« Reply #51 on: 13/05/2013 20:44:15 »
Quote from: dlorde
In a sense, all energy is potential - the potential to do work. 'Potential energy' is just a generic convenience term for energy stored as spatial displacement, just as kinetic energy is the potential to do work due to an object's motion.
Energy can't be defined that way. You'll run into contradictions since there are plenty of examples of energy which can do no work. Zero point energy is an example of energ which can do no work.
OK.

#### lightarrow

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##### Re: What is energy?
« Reply #52 on: 13/05/2013 23:23:57 »
You wrote:
<<That internal energy can be used to do work; for example, you could use it to turn water into steam to drive a motor or generate electricity >> and this is not  true, unless you have a zero Kelvin heat reservoir (which is impossible to get  ).
I mean that you can't convert all that energy.
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You have advised that you don't imply that "you'll get more out than you put in" but if you say that you can store energy as internal energy, in general, you have to specify when you can do it and with which limitations, because you can't do it if the surround it's not at lower temperature and however you can store just a part, not all of it.
I was using your example. If a box is in thermal equilibrium with its surroundings and you then heat the box by running current through a resistor inside it, you can then use the increased internal energy of that box to do work - by exploiting the temperature difference between it and its surroundings.
Ok. Do you agree on the fact you can't convert all internal energy into other forms of energy as could be mechanical energy?
When you wrote "store energy as internal energy" I intended this.
Maybe I was not very clear about it, and if so, it's my fault
« Last Edit: 13/05/2013 23:26:02 by lightarrow »

#### Pmb

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##### Re: What is energy?
« Reply #53 on: 14/05/2013 13:29:51 »
That internal energy can be used to do work;
...
Of course, but see my previous post.
I did. You seem to think that he meant that the only result of using the heat energy is work or that all thermal energy available can do work. He clearly didn't say that or imply it. It appears that you assumed it.

#### dlorde

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##### Re: What is energy?
« Reply #54 on: 14/05/2013 13:57:14 »
I mean that you can't convert all that energy.
Ah, OK. Perhaps you should have just said that.

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Ok. Do you agree on the fact you can't convert all internal energy into other forms of energy...?
Yes, no problem with that.

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Maybe I was not very clear about it, and if so, it's my fault
OK, no worries  ;)
« Last Edit: 14/05/2013 14:12:17 by dlorde »

#### Pmb

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• Posts: 1838
• Physicist
##### Re: What is energy?
« Reply #55 on: 16/05/2013 04:22:31 »
I've been doing a bit more searching on this in my spare time and came across a comment regarding Jacobi's integral in Classical Dynamics, by Donald T. Greenwood, Dover Pub. (1977). On page 73 he defines a natural system as a conservative system which has the additional properties

(1) it is described by the standard holonomic form of Lagrange's equations
(2) the kinetic energy is expressed as a homogeneous quadratic function of the generalized velocities.

He then states that under these circumstances Jacobi's integal, aka the energy function, is the total mechanical energy of the system and is an integral of motion, i.e. constant.

Nice! :)

#### The Naked Scientists Forum

##### Re: What is energy?
« Reply #55 on: 16/05/2013 04:22:31 »