Please explain this Physics sentence from EK physics.. (Force/Work)

[IndianVercetti]

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page 45

If the total energy transfer is due to forces and none to heat, the work done on an object is also given by:
W = deltaK + deltaU + deltaEi (if no heat)

If there is neither heat nor friction:
W = deltaK + deltaU (if no friction, no heat)

I'm pretty good with these concepts, but this explanation/variables used is confusing to me. What is delta Ei first of all? Internal energy? And why is this variable only present when there is no heat?? Shouldn't it be there when heat is present..?

Also, why does the presence of a friction force completely eliminate this Ei variable?

Thanks

 

[raab32]

So the underlying concept here to branch out from is that energy transfer can be mediated by two things: 1) forces and 2) heat. Forces include any of the multitude of forces you know about minus the frictional force and heat is just energy lost due to thermal contact.

Energy transfer by forces is work. The equations that you posted are attempting to quantify work, but not heat. This is why heat is not incorporated in the equations. If you look at the first paragraph on that EK page, it states that work and heat are separate things.

You're right, Ei is the internal energy. Internal energy is essentially the energy in molecules. This term is necessary when you're accounting for energy transfer in instances where friction is involved. This is because friction affects the internal energy of molecules and not just the mechanical energy. Think about what friction is... if we consider a wooden block on a cement ramp, it's the interaction between the molecules of wood and molecules that constitute cement. Because friction messes with stuff at the molecular level, it causes changes in internal energy, which again is the energy of molecules.

However, if you have a case where there is no friction and energy transfer is only mediated by forces that affect mechanical energy, then it follows that you only need to account for the mechanical energy (kinetic and potential), which is why the internal energy term can drop out.

 

[LostInStudy]

Yea, this is the reason I stopped reading EK for physics.

Anyway, basically what it's saying is that W done in the absence of friction is just potential + kinetic + internal energy (which is just the inherent energy of the molecules due to vibrational, translational, rotational, etc motion). Usually for objects the internal energy is so small that it is negligible.

You don't really need to know the difference because you will only need to know the 2nd equation which is that in the absence of friction: potential + kinetic equal total change in energy aka work.


Hope this helped,

-LIS

 

[dingyibvs]

I wouldn't read the EK physics book if they teach concepts like that. While it's technically accurate, it's something that you'll never have to face on the actual MCAT(i.e. the Ei part of the equation).

 

[IndianVercetti]

Alright, thanks, I'm pretty familiar with this concept, but the internal energy just kind of threw me off. Appreciate the input.