Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] ?conservation of _internal_ energy



On Jan 14, 2016, at 1:38 PM, Carl Mungan <mungan@usna.edu> wrote:


On Jan 14, 2016, at 2:26 PM, Herbert Schulz <herbs@wideopenwest.com> wrote:

On Jan 14, 2016, at 1:14 PM, John Denker <jsd@av8n.com> wrote:

In the context of:
https://www.av8n.com/physics/thermo/state-func.html#sec-internal-energy

On 01/14/2016 08:23 AM, Herbert Schulz wrote:
Yes, imagine a gas instead of the spring. As the gas expands it
pushes the piston and does work.

I say it does /not/ do work. No work is done by (or on) the blue
system. There is no F·dx: At the place where there is a nonzero
force, there is no displacement. At places where there is a
nonzero displacement, there is no force.

Howdy,

Why certainly work is being done by the `blue' box on the `red' box and the `red' box on the `blue' box via the interactive forces (which must be equal and opposite in direction of course).

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)


You really want to say that?

How about this case: A guy on roller skates pushes off from a rigid wall. System = man. Any work done on the system? Does his internal energy change?

Further, in the real world, the guy will get a bit hotter if he does this several times. Any heat transferred to or from him (on short timescales)?

Howdy,

Of course there has been work done on the guy! He pushes against the wall with a certain force. Therfore (Newton's 2nd Law) the wall MUST push against him with an equal and opposite force so work is being done on Him.

The increase in Temperature of the guy is due to the chemical reactions that take place in his muscles; i.e., they are not 100% efficient.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)