Why do some people talk of energy like it is some tangible thing and others simply describe it as "the ability to do work"?

[Achilles]

Conservation of energy, energy can be neither create or destroyed talks about energy as if it is something. Where as talking about energy in classical mechanics is usually where "the ability to do work" is used. Even e=mc2 has ambiguous use of Energy equating mass with energy that means mass = the ability to do work rather than some tangible quantity that actually exists in some way.

[DrP]

47 minutes ago, Achilles said:

 Even e=mc2 has ambiguous use of Energy equating mass with energy that means mass = the ability to do work

It isn't ambiguous, it is the absolute energy you can get from a mass. The energy is stored or bound up in the mass.  The energy in this equation is what is released if the mass is broken to it's absolute most basics of individual protons and neutrons once their binding energies have been overcome and their stored energy has been released. With this 'ability to do work' as you state, I think you are referring to the potential energy a mass has at a certain height from the ground. The energy from the E=MC^2 equations is released as kinetic energy in the speed of the particles that are released. I guess these released particles 'have the ability to do work' in the classical sense due to the energy they have.

Sorry - I thought I was going to be able to explain this better when I started typing but now I read it back I am not sure I have. I am sure someone will explain it to you.  

 

 

 

[geordief]

It is a mathematical equation (so it is "equals" ,not "is"). The numbers move from one column to another  but it doesn't help to define energy as a "thing"

 

Well prepared to be shown my error ,but that is my present understanding.

[DrP]

55 minutes ago, geordief said:

 but it doesn't help to define energy as a "thing"

-  yea - like 'temperature'   -  it is a measure of a number of things that are happening to a substance, not an actual 'thing' by itself.

[studiot]

To appreciate energy in Science you need to understand the terms object, process and property.

Energy is a property of object or process.
The objects may substantial or insubstantial (ie theoretical or virtual).

 

If the OP ever comes back we can develop this further.

[swansont]

We do even worse when we talk about heat. To be fair, it's not all that difficult to fall into the trap. Energy is that which is conserved because of time translation symmetry. That sort of makes it sound like a substance. When we are actually working problems, we write down an energy balance. Energy is a property of the particles and systems in that equation, but one could easily mistake it as a stand-alone description of a substance.

E = mc^2 is simply saying mass is a form of energy. IOW, a mass at rest does not have zero energy.

I think one reason confusion arises (in both cases) is because many people are simply reading about physics and not actually doing physics, so they have a superficial knowledge, at best, of the issues.

[Achilles]

Can you guys personally describe to me what energy is, because I ask on quora and other sites like yahoo and they have different opinions. But I take "energy is the ability to do work" as defining energy... personally

[DrP]

2 hours ago, Achilles said:

Can you guys personally describe to me what energy is, because I ask on quora and other sites like yahoo and they have different opinions. But I take "energy is the ability to do work" as defining energy... personally

It isn't anything. It is the measure of how much potential something has to be able to do work, move, heat something etc..  

 

[studiot]

3 hours ago, Achilles said:

Can you guys personally describe to me what energy is, because I ask on quora and other sites like yahoo and they have different opinions. But I take "energy is the ability to do work" as defining energy... personally

 

That's a good start, but not all energy can do work.

Further Energy is frame dependant. That is it's value depends upon who measures it.

I observed that energy is a property.

Perhaps you would like to comment so I know where to start?

Edited August 3, 2018 by studiot

[DrP]

8 minutes ago, studiot said:

Further Energy is frame dependant. That is it's value depends upon who measures it.

I think you might confuse him  -  he isn't even grasping that energy isn't a physical thing yet. It is a property. Once/if he gets that then maybe take it a bit further.

 

[studiot]

So let's clear up about properties.

 

 

[geordief]

2 minutes ago, studiot said:

So let's clear up about properties.

 

 

Do you also need  to define "objects" and "processes" ? (unless it is obvious)

 

"Properties" means types of measurements that can be taken of the above?

[Sensei]

On 2.08.2018 at 9:47 AM, Achilles said:

Why do some people talk of energy like it is some tangible thing and others simply describe it as "the ability to do work"?

In high energy quantum physics, particle with high kinetic energy colliding with other particle, can create pair of particle-antiparticle.

Which kind of pair will be created depends on velocity/kinetic energy of incoming particle.

e.g. if kinetic energy exceeds 1.022 MeV there can be created pair of electron-positron.

 

You started with two particles at the beginning, accelerated one of them to relativistic velocity, but ended up with four particles at the end of experiment.

Pair production of proton-antiproton:

[math]p^+ + p^+ \rightarrow p^+ + p^+ + p^+ + p^-[/math]

 

Particle with low kinetic energy colliding with other particle is creating photon.

Particle with kinetic energy exceeding 2.22 MeV colliding with nucleus of Deuterium can destroy it, i.e. free proton and free neutron will be created in reaction.

[math]D^+ + \gamma + 2.22 MeV \rightarrow p^+ + n^0[/math]

(photodisintegration of Deuterium)

 

Gamma photon with high energy can be absorbed, scattered, emitted, repeat it millions of times, and instead of one single gamma photon at the beginning you end up with millions of photons, with low energies.

 

[swansont]

5 hours ago, Achilles said:

Can you guys personally describe to me what energy is, because I ask on quora and other sites like yahoo and they have different opinions. But I take "energy is the ability to do work" as defining energy... personally

The basic definition of mechanics is the capacity to do work. And that's fine, as far as it goes. Other parts of physics try to maintain that concept. 

As I said above, it's that which is invariant under time translation.

[MathGeek]

The same issues arise often in physics when defining a quantity.

I first learned that mass is the "quantity of matter" in early science courses.  I later learned of distinctions between inertial mass, gravitational mass, and relativistic mass.  Which is more fundamental?

When I think like an experimentalist, I focus on how a quantity is measured.  Can it be measured directly, or is it usually computed from other quantities that are measured more directly?

When I think like a theorist, I focus more on how a quantity is used in calculations to predict the outcome of some experiment.

But there are often alternate approaches and more than one way of defining a quantity in physics.  It helps to pay attention.  Which equation is the _definition_ and which equations are descriptions or physical laws that follow from the original definition?

[studiot]

Good stuff, mathgeek. +1

Edited August 3, 2018 by studiot