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Friction and work

by timetoplearn
Tags: conserv of energy, force, friction, physics
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timetoplearn
#1
Sep1-13, 08:32 PM
P: 2
two equally important questions:

If work=forcexdistance, how does a galvanic cell do "work" if there is not force nor any distance travelled by the force.

question 2: Friction is a NC force that increases internal energy. Right?
Doesn't friction also cause heat?

My physics book says:

If no heat: mechanical energy= KE+PE+Internal Energy

If no heat and no frictionME=PE+KE.

How can you have friction without heat?

Also can you tell me a brief description between q, deltaH, and U


by the way if you saw this on another forum, it is because I asked it on another forum, but got a semi-incomplete answer.

this is the answer I got for your reference:
Imagine what happens to space shuttle when it's going back to Earth from orbit: molecules of space ship are colliding with air molecules (denser and denser with lower altitude).
High velocity of vehicle + pretty normal velocity of air molecules (~343 m/s) results in heating space ship to 1650 C..
http://en.wikipedia.org/wiki/Space_S...tection_system
This is pretty extreme example.

Any air molecules colliding with any body that has some velocity is resulting in taking part/all of kinetic energy from that body (or particle), which is resulting as acceleration of air molecules, and deceleration of body. Until both bodies have the same temperature.

And the same happens when we slide some body, on another body like table.

The only difference is scale.
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UltrafastPED
#2
Sep2-13, 02:10 AM
Sci Advisor
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PF Gold
UltrafastPED's Avatar
P: 1,908
Both of your questions require that you look at the microscopic situation.

For friction the work done always opposes the motion, and results in heat at the interface between materials; some goes into the book, some goes into the table.

For a galvanic cell you can look at the traveling ions in the solution. It will reproduce the Joule heating law for a resistive circuit, and will heat the solution which carries the ions.
timetoplearn
#3
Sep2-13, 03:18 AM
P: 2
Why is there two different formulas for ME one with friction but no heat and one with no heat?

UltrafastPED
#4
Sep2-13, 03:28 AM
Sci Advisor
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PF Gold
UltrafastPED's Avatar
P: 1,908
Friction and work

Internal energy could be elastic energy or sound, etc. Heat is a disorganized (random) motion which is usually treated as distinct from other "internal energy".

The reason is that thermal motion is non-reversible (leading to the second law of thermodynamics), whereas most other forms of internal energy are reversible.

Heat still follows the laws of mechanics, but it is usually studied as a separate topic.


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