# Why can we only measure energy (enthelpy) change?

fog37
Hello,
I asked this similar questions on a different thread but it may be more appropriate for the chemistry forum.

What is the reason we can only measure energy "changes" and not the actual energy value associated with a system? Absolute energies and enthalpy cannot be determined but I am not sure why...

I guess we can still say that a system has more or less energy than another system but we cannot know the actual energies of each system, only their difference...

However, when a car moves at a certain speed, we can certainly express the car's kinetic energy without a problem...

Thanks!

Gold Member
I guess we can still say that a system has more or less energy than another system but we cannot know the actual energies of each system, only their difference...

Yes, that's the essence of the problem. We can measure the energy difference between different states, but there's not really a way to define the absolute energy of the system.

However, when a car moves at a certain speed, we can certainly express the car's kinetic energy without a problem...

Are you sure? It makes sense for a car at rest on the side of the road to have a kinetic energy of zero, but is it really motionless, say, relative to the sun?

Mentor
This is probably a dumb question, but isn't the absolute energy of a mass given by Einstein's equation?

• jim hardy
Gold Member
I guess the difficulty of measuring absolute energy versus relative energies applies most to potential energy, which is defined by integrating across a conservative force field: $$U(x) =\text{ } –\int_{x_o}^x F(x) \, dx$$
which requires arbitrary definition of some reference position from which to calculate energy differences.

• Chestermiller
fog37
Thanks. I am still confused. I see how kinetic energy ##KE## and potential energy ##PE## are relative quantities.

Does that means we can measure the total energy of a system but that value is frame dependent? For instance, if we said that something has zero energy, it would mean ##KE+PE=0##...

I see how potential energy is about differences: we don't know the initial and final potential energies but we know their difference...

In chemistry, only enthalpy changes are measurable. A calorimeter is the instrument used. I guess, experimentally, the only possible result happens to be the energy difference and the minuend and the subtrahend are unknown...

Mentor
Would knowing the answer to this question in any way change the way that you would analyze a practical thermodynamics problem? If the answer is no, why even bother worrying about it? I think that your time is much more valuable than that.

Comeback City
Does that means we can measure the total energy of a system but that value is frame dependent?
I would say yes... it’s been mentioned already that energy can only be measured relatively, whether it be potential or kinetic, therefore there shouldn’t be a way to measure an absolute energy. It can only be measured per whatever frame you measure it within.

DrStupid
This is probably a dumb question, but isn't the absolute energy of a mass given by Einstein's equation?

The unknown amount of internal energy comes from classical mechanics. In relativity we can indeed use m·c² as a measure of the internal energy if we define U(m=0):=0 but the practical use is limited.