1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Understanding laws of thermodynamics

  1. Mar 31, 2013 #1
    say we have closed system (box) consisting vacum and one mechanichal clock, wound to have (potential) energy (within spring) for say ten revoluntion for second hand in the begining.
    now when watch is allowed to have ten revolution of second hand and then it stops at the same location at it started,
    In the system, we should be having some heat energy released due to friction of mechanism and one dead clock.
    now is the energy of the system at the initial stage and final stage is same?
    has some energy lost in doing mechanical work, as not all energy is converted into heat?
    what is the disorder in system?
  2. jcsd
  3. Mar 31, 2013 #2
    No the at the final stage you don't have the energy that you did at the start in this configuration as the mechanical work has been done and you cannot reverse it so energy has been lost in it.
    Now the heat energy will be there as heat depending on how tight and perfect the box is in a theoretical scenario all the heat energy generated by frictions should remain as heat in the perfectly sealed box.
  4. Mar 31, 2013 #3


    User Avatar
    Gold Member

    Under the assumption of absolutely no energy loss from the box, the system will have the same energy at the end as in the beginning. All of the mechanical energy from the spring will be converted to heat and, for a while, mechanical energy due to vibrations of the clock frame (and these will slowly be converted to heat).

    The 'disorder' in the system comes from its higher temperature. The atoms in the clock will increase their random thermal motion. You should know, though, that 'disorder' is an inexactly defined, subjective term, and the laws of thermodynamics apply to entropy rather than disorder. It is just that the common notion of disorder maps relatively well onto the exact definition of entropy when you take a molecular level view of things, so people talk about disorder in introductory treatments. But keep in mind that it only works reasonably well when you look at the molecular level.

    There are a few sources of loss in a real system like this, though. First, there is thermal radiation, which is a bigger source of heat loss than most people realize. Just like if you heat metal hot enough, it will glow red, things at room temperature are already hot enough to glow infrared (which the human eye cannot see). You could reduce this with mirrors on the inside of the box between the vacuum and the clock. Secondly, if this is on Earth where there is gravity, there will be something connecting the clock to the box and the box to the ground. This could carry vibrations away or conduct heat away.
  5. Apr 1, 2013 #4
    @ crazymechanic
    when potential energy is 'consumed' in mechanical work and apart from heat energy no other energy is conserved can we say part of potential is 'destroied', thus violating first law of thermodynamics.
    if all the potential energy is converted to heat then what made the mechanical work?
    Last edited: Apr 1, 2013
  6. Apr 1, 2013 #5


    User Avatar
    Gold Member

    Be careful. Potential energy is not converted into work. Work is a force times a distance, and you can talk about the amount of work done over a certain time period, but not about the amount of work in the system at a certain point in time. Potential and kinetic energy are things where you can talk about the amount in the system at a certain time.

    The potential energy gets converted first into the kinetic energy in the clock hand moving. Most of that is dissipated as heat when the clock hand stops. Some of it causes the clock to vibrate. The vibration is a combination of potential energy (in the form of the distortion of the clock frame) and kinetic energy (in the motion of the frame).

    I would not have said what crazymechanic said in the first sentence about not having the same energy at the end and at the start due to mechanical work being done.
  7. Apr 1, 2013 #6


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    No. All the energy is converted into heat.

    You can't say the PE was converted to "mechanical work done" AND "friction". The mechanical work done was due to the friction. You have double counted it.
  8. Apr 1, 2013 #7
    don't we need energy to overcome the inertia of components of watch? and is it all that PE converted to heat after mechanical work beeing done?

    one more question (apart from previously discussed one), does energy is required to convert one form of energy to another or is it spontanious.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook