[ErickAloke (OP)]

I took a physics class once and the professor said plainly Energy is not a physical unit: it is a philosophical concept.

The only definition I have seen for energy is capacity to do work. But work is defined as disordering of energy. So energy is capacity to do disordering of energy, a circular definition.

Is there any clarification of this topic?

[Colin2B]

I took a physics class once and the professor said plainly Energy is not a physical unit: it is a philosophical concept.

Well, we can measure energy using physical units, so it is more than a philosophical concept.
Energy is the ability to cause change, which we often phrase as ability to do work ie change the state of things.
If you are thinking of the disorder then you are probably thinking of entropy which can be described as a measure of the unavailability of a system’s energy to do work.

PS it’s worth mentioning that there are many forms of energy, but none of them is ‘pure energy’.

[alancalverd]

The quantity with dimensions  ML²T^-2 is conserved in all interactions. It is called "energy".

[Bored chemist]

I took a physics class once and the professor said plainly Energy is not a physical unit: it is a philosophical concept.

The only definition I have seen for energy is capacity to do work. But work is defined as disordering of energy. So energy is capacity to do disordering of energy, a circular definition.

Is there any clarification of this topic?

This is an interesting enough question, but it's in the wrong place.

[alancalverd]

But work is defined as disordering of energy.

It used to be defined as force x distance, but that was in the bad old days when teaching was supposed to help students understand things. "Disordering of energy" is, I think, meaningless, since the dimensions of energy (and work) do not involve order..

[Swanzy]

I took a physics class once and the professor said plainly Energy is not a physical unit: it is a philosophical concept.

Well, we can measure energy using physical units, so it is more than a philosophical concept.
Energy is the ability to cause change, which we often phrase as ability to do work ie change the state of things.
If you are thinking of the disorder then you are probably thinking of entropy which can be described as a measure of the unavailability of a system’s energy to do work.

PS it’s worth mentioning that there are many forms of energy, but none of them is ‘pure energy’.

sorry to intrude but we do not measure energy . We measure joules etc . Energy is generalised term that we use when not being specific . There is many types of energy . We actually measure output and give it a unit name .

[Colin2B]

sorry to intrude but we do not measure energy . We measure joules etc . Energy is generalised term that we use when not being specific . There is many types of energy . We actually measure output and give it a unit name .

I agree that there are, as I said, many forms of energy. However, Joules is the SI named unit for the quantity of energy. Energy is what we measure, in whatever form it happens to be in eg heat energy, mass energy, etc. Yes, we can also use the term as a general, nonspecific quantity.

[Swanzy]

sorry to intrude but we do not measure energy . We measure joules etc . Energy is generalised term that we use when not being specific . There is many types of energy . We actually measure output and give it a unit name .

I agree that there are, as I said, many forms of energy. However, Joules is the SI named unit for the quantity of energy. Energy is what we measure, in whatever form it happens to be in eg heat energy, mass energy, etc. Yes, we can also use the term as a general, nonspecific quantity.

E=mc^2 , please explain how this relates to thermal energy or similar ?
Doesn't E=mc^2 actually imply the inverse square of light which the energy magnitude  is proportional to the area?

[alancalverd]

When a massive object is converted to energy, the energy output is directly proportional to the mass input. The surprising finding from relativity theory is that the constant of proportionality is the square of the speed of light in vacuo and this is borne out in practice.

In the case of nuclear fission we get a whole bunch of fission fragments ejected with various amounts of kinetic energy, plus a lot of gamma radiation. If you add up all the particle masses you find that there has been a mass loss from the original nucleus, which exactly balances the sum of all the kinetic and gamma energies.  Likewise nuclear fusion.

A much simpler case is pair production, where a gamma ray interacts with a nucleus, and 1.022 MeV of gamma energy reappears as an electron and a positron (1.822 x 10^-32kg plus a tiny bit of kinetic energy), which then annihilate, generating two 511 keV photons.

You can measure photon energy in a reasonably direct manner by converting it to heat but calorimetry is technically difficult to do with great precision (it's done in national standard laboratories and took me 13 years to measure a lethal dose to ± 0.1%) so we tend to measure the wavelength of low-energy photons (E = hc/λ) or the ionisation produced by high-energy photons. 

Mechanical energy is measured by "work done", e.g. the phrase "nineteen to the dozen" refers to the energy efficiency of a Newcomen beam engine pump used to clear Cornish mines. When working maximally the pumps could clear nineteen thousand gallons of water for every twelve bushels of coal, though I can't find a reference to the height of the lift.

Electrical energy is the product of current, voltage and time, so is easy to measure to considerable precision and is usually the preferred reference for calibrating dynamometers or calorimeters.

[alancalverd]

Yes, we can also use the term as a general, nonspecific quantity.

Please don't! That's the scientific equivalent of blasphemy.

[Colin2B]

Yes, we can also use the term as a general, nonspecific quantity.

Please don't! That's the scientific equivalent of blasphemy.

Wow, are there still laws against that?

So, it would be wrong blasphemy to say energy is conserved? 

[alancalverd]

Strength, power, force, work and energy all mean the same to a journalist or a politician. If you devalue the language of science you might as well discuss religion or philosophy.

AFAIK blasphemy is still an offence in Northern Ireland. However I dislike overreaction, so I think the punishment for misusing a scientific term should be limited to one eternity in hell, or a week in Disneyland.

Energy is indeed conserved, thanks to E = mc².

[evan_au]

took me 13 years to measure a lethal dose to ± 0.1%

Were you trying to measure Lethal Dose LD50: ie 50% ± 0.1% ?

Did your 13-year measurements include those who would die after 15 years or 20 years?

You obviously put a lot of energy into this important work...   

[alancalverd]

The object was to measure 1 gray (i.e. 1 joule per kilogram) of ⁶⁰Co γ radiation to ± 0.1%, as the national primary standard for radiotherapy. The LD50/30 is about 5Gy to the whole body, and radiotherapy fractions are mostly in the region 1 - 20 Gy with a total tumor dose of up to 60 Gy. The optimum dose that balances recovery from the primary tumor with the least damage to healthy tissue, has a bandwidth of about 3%, so each fraction needs to be delivered to better than 1% through a chain of secondary standards and field instruments, hence the need for extreme confidence in the primary measurement.

My problem was that 1 Gy raises the temperature of water by about 2.4 mK, so we needed to resolve temperature changes to better than a microdegree. Using graphite as the target gave us a theoretical gain of around 6 but added to the technical problem because of its higher thermal diffusivity, which made the calorimeter more sensitive to ambient fluctuations.

The rule of thumb at the National Physical Laboratory was that it takes about 20 man-years to improve precision by an order of magnitude. Several associates passed through my lab and I spent some time on other projects: the end result was almost exactly the product of 20 m-y.

[Bored chemist]

"Is energy physical or philosophical?"
The philosophers think it's philosophical, the physicists think it's physical.

[alancalverd]

philosophers think it's philosophical

And they are wrong as usual. I think Harold Wilson is credited with saying "If you steal someone else's clothes, beware of walking around with the flies undone."