# Measuring energy and its change

• I
Hello Forum,

I understand that energy, either kinetic is a relative quantity whose value depends on the frame of reference and its motion. Is that true for potential energy as well?

I read on a physics book that only changes in energy can be measure. What does that exactly mean? Why can we not measure energy directly but only its changes? Energy is apparently the measure of change of a system. What matters are energy differences and energy changes. Is it not correct to state that a system has a certain amount of energy at a particular time? Should we only talk about energy gains and energy losses for a system?

Thanks!

anorlunda
Staff Emeritus
Mechanical potential energy is also relative. Consider the gravitational potential energy of a book on a shelf. We arbitrarily assign one of the shelves as zero.

There are many forms of energy other than mechanical PE and KE. Electrical, chemical, heat, nuclear ...

Thanks anorlunda.

I would say that all forms of energy can be derived from kinetic and potential. For example, temperature is microscopically tied to kinetic energy. Bonds in molecules to potential energy...

I am still not sure why we can only measure energy changes only and not the particular energy of a system...

Thank you. Very interesting.

It still does not seem to address, unless I have missed it, why energy changes is all we can measure and worry about and why energy is a measure of the change in a system while force the agent of change...

PeroK
Homework Helper
Gold Member
2020 Award
Thank you. Very interesting.

It still does not seem to address, unless I have missed it, why energy changes is all we can measure and worry about and why energy is a measure of the change in a system while force the agent of change...
So, what would you say is the absolute energy of, say, the Solar System? Something that is not frame dependent.

You said:

I would say that all forms of energy can be derived from kinetic and potential.
So, what are you going to do to establish an absolute kinetic energy of the Solar System?

fog37
Thanks Perok. I am not sure about the absolute energy of the solar system. I would just say that its value is relative in virtue of these arguments:
• Kinetic energy is relative (depends on the observer's state of motion)
• Potential energy is relative (can always add an arbitrary constant value to it)
• Work is relative since displacement is relative
What is not relative, between different reference frames, is energy changes (differences). Energy differences are the conserved measures of physical changes. So we can measure the energy of a system or compare the energy of one system to the energy of another system but only energy differences have significance.

How do we compare a system with energy ##E_1## and another system with energy ##E_2 <E_1##? The scalar value of their energies is not important. System with energy ##E_1## has a higher ability to change its own dynamical state and/or configuration state OR also a higher ability to change the dynamical state and/or the configuration state of another system...

I feel like all myriad forms of energy are somewhat a form of KE or PE. For example, thermal energy is microscopic PE+KE of the object's constituents. Nuclear energy is some form of potential energy deriving from fusion or fission of the nuclei.

The equation ##E=mc^2## from special relativity then states that mass and energy are intrinsically related. So a hotter object, with more thermal energy, has a higher mass than a cooler object....