Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Mpemba effect - just how a freezer works?

  1. Jan 25, 2015 #1
    I read the Wiki page, then read Nikola Bregović's paper which Wiki mentions was selected as the best idea of about 22,000 paper submissions called by the Royal Society of Chemistry in 2012...

    I'm not seeing any solutions that indicate they take into account how a freezer works... a freezer responds to hot water differently than cool water because it has a thermostat..

    Mpemba's original observation was from using a freezer; and a freezer was also what Nikola Bregović described as using in his paper for his experiments.

    The point is, a freezer uses a thermostat to power the cooling function as required. The thermostat is measuring the air temperature in the freezer compartment (not the temperature of the water)...

    Simply put, elevated air temperature in the freezer compartment makes the thermostat kick on the cooler. Imagine two identical freezers with identical thermostat settings. Put cool water into one and hot water into the other. Now measure frequency and durations with which the thermostats engage the cooling function for both freezers...

    Keeping in mind that the cooling function is designed to present a temperature to the air well below the freezing point,.and well below the subfreezing setting for the thermostat; which freezer is going to be subject to a greater warming of the air in the compartment, and which freezer will therefore be more aggressively cooling the air temperature (more frequent and/or longer durations of running the cooling function) in its compartment?

    Seems to me that the Mpemba paradox arises from the assumption that both the hot and cool water are subject to the same conditions (a "static" subfreezing air environment), but freezers don't present a static environment; they actively engage their cooling function in response to the temperature of the air inside.

    I did notice that some who do this experiment get the result and others don't... maybe the former are using a single freezer for separate trials or two freezers one for each sample, whereas maybe the latter are using one freezer to hold both samples for each trial. That would be consistent with how freezers use their thermostats and cooling functions.

    Any thoughts?
     
  2. jcsd
  3. Jan 25, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    How can have that an effect if you put both pots of water in together? Yes it works with a single freezer.
     
  4. Jan 26, 2015 #3
    Hmm... well that's interesting. :)
     
  5. Jan 26, 2015 #4

    russ_watters

    User Avatar

    Staff: Mentor

    Most fridges/freezers turn on the compressor instantly when you open the door, so I doubt the heat loss from the water would make much difference.

    I did a fairly rigorous experiment on this a few years ago and the thread is around here somewhere. My conclusion is that if the effect is real at all, it only happens under very specific/contrived circumstances. It is not generally true.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Mpemba effect - just how a freezer works?
  1. Mpemba Effect (Replies: 3)

  2. Hot Freezers (Replies: 1)

Loading...