Saturday, August 6, 2011

Transfer of Heat: The Second Law of Thermodynamics

      Sometimes, it is counterintuitive to think that science does not allow “cold” to transfer. When you hold an ice-cube in your hand, it is most definitely, noticeably, making your hand cold! Yet, you have to wonder what is actually happening as the temperatures in both your hand and the ice-cube change.

      The second law of thermodynamics can be stated several ways and can even apply to more than just heat. We are going to stick with heat (thermal energy) because it makes me happy to stay consistent with “Thermo(heat) Dynamics(flow).”
      Before I jump into the definition of the second law, let me briefly explain the first law of thermodynamics: Energy can change form and travel but the quantity of energy is always the same. This is a brief explanation and is considered the happiest of the laws; should this be the only thermodynamic law, all the world’s energy problems would be solved! We would never have to re-fill our gas tanks, or pay for electricity...but this is for another post and I will bring you down a notch with the second law anyway.
      So, what is the Second Law of Thermodynamics already?! Well, simply put:
“A transformation whose only final result is to transfer heat from a body at a given termperature to a body at a higher temperature is impossible. (Postulate of Clausius)” (Fermi, 30)
      Ahem. Come again? Well, in the genius words of Flanders and Swann (musical duet from before my time):
Heat won't pass from a cooler to a hotter
You can try it if you like but you far better notter
'Cos the cold in the cooler will get hotter as a ruler
'Cos the hotter body's heat will pass to the cooler
Just Listen:

      
AHEM. Okay okay: Heat cannot spontaneously flow from a colder location to a hotter location.
      Thus, the ice-cube (you remember the ice-cube in the first paragraph?) is receiving the heat from your hand, not transferring “cold” to it... And your hand will continue to transfer this heat energy until it and the ice-cube are in “thermal equilibrium.” Which will either be when the ice has melted (in most cases) or when your hand has turned to ice.
      Hey, let’s talk about that second scenario real quick! Also known as frostbite, why on earth would the heat from your hand not melt the ice-cube before all of its thermal energy has been transferred? Well, in most cases, it is due to the speed of transfer. If the ice is so cold (due to size, or make-up, i.e., dry ice or liquid nitrogen) that there is a rush of transfer from your hand too fast to allow your body temperature to catch-up with the transfer, then you end up frostbitten. If that temperature is not raised in time to save the tissue, it causes permanent damage.

      Now, back on topic. There are three ways in which this transfer can take place: Conduction, Convection and Radiation.(Bloomfield, 211)
Conduction: Heat-flow through a stationary material. The atoms and molecules of the material are not “flowing” but the heat is... through vibrations of said atoms.
Convection: This is when heat is transferred through fluid. For instance, fluid air (moving air) can  carry heat from a hotter object to a cooler object.
Radiation: The transfer of heat through electro-magnetic waves (see previous blog post on microwave ovens).
      To imagine these transfers on a molecular and atomic level:
Conduction is a bucket-brigade where the atoms are the brigade and the material in the buckets is heat.

Convection occurs when this bucket brigade is riding on a “train” of fluid.
Radiation is the individuals of the brigade, and their buckets of heat, being carried in photons. J
I know, that last one was pretty much a cop-out analogy, but I might just have to do a completely separate post on radiation...Just know that the sun transfers heat through Radiation.

Now, this is a pretty simplified explanation of the second law. To go deeper, I would get into explanations of friction and other “work into heat” ideas and calories. I would particularly like to dedicate a separate post to calories...note to self...For now, I will leave the second law at this, and remember, “Heat won’t pass from a cooler to a hotter!”
Experiment to be done with adult supervision:
Take two bowls and put a scoop (or two) of ice-cream in each. Have a spoon ready to tasteJ. Pour milk (enough to coat, not so much that it is sopping) onto one of the servings, leave the other alone. Now taste. Pay attention to the temperature of each. What is the difference, if any? Why?

References:
http://en.wikipedia.org/wiki/Convection viewed on 08/6/2011. p. 5.

Bloomfield, Louis A. How Things Work. John Wiley & Sons, Inc. 2006.

Fermi, Enrico. Thermodynamics. Dover Publications, Inc. 1936.

No comments:

Post a Comment