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!

U values W/M2/K

  1. Apr 29, 2009 #1
    u values ....W/M2/K

    I am getting confused by the definition of the u-value .
    I want to do some calculations for heat loss but I do not understand the timescale for the U- value calcs
    I understand that a U value describes the energy in watts that is lost per Sq Mtr for each degree (K) diferential , but is that per hour , day ,year ?
    I may have missed something obvious but any advice would be gratefully accepted
    Kev
     
  2. jcsd
  3. Apr 29, 2009 #2

    russ_watters

    User Avatar

    Staff: Mentor

    Re: u values ....W/M2/K

    A watt is a joule per second.
     
  4. Apr 30, 2009 #3

    Andy Resnick

    User Avatar
    Science Advisor
    Education Advisor
    2016 Award

    Re: u values ....W/M2/K

    If the 'U-value' is the concept as the 'R-value' for insulation, then there is no time dependence.

    http://en.wikipedia.org/wiki/R-value_(insulation [Broken])

    Heat transfer (as a thermodynamic application) has no time dependence. The amount of heat flow depends only on the temperature differential.
     
    Last edited by a moderator: May 4, 2017
  5. May 5, 2009 #4
    Re: u values ....W/M2/K

    Thanks for the help
    I have sussed now that the figure relates to the rate of energy transfer and that the amount of energy transfered is calculated by the the timescale .
    I probably was being thick, but at the time I just couldn't see it .
    Kev
     
  6. May 5, 2009 #5

    russ_watters

    User Avatar

    Staff: Mentor

    Re: u values ....W/M2/K

    They are inverses of each other.
    What? That's like saying velocity has no time dependence!? Quite obviously, for a given piece of insulation, "the amount of heat flow" in an hour is 60 times larger than "the amount of heat flow" in a minute.
     
    Last edited by a moderator: May 4, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: U values W/M2/K
Loading...