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Friday, August 8, 2014

Heat em up!!

    Heat, Brownian motion, atomic or molecular excitement, thermal energy, call it by any of its names, heat is 'just' another form of energy that can be used for work.  Heat, as with any form of energy, can be added or removed from any given system, extracted to do work, stored for later, concentrated and  transferred. 
   Heat is motion.  Heat is the measured wiggle and giggle of the atoms or molecules in a substance.  That motion can be slowed, and the result is a measured drop in temperature; the temperature rising indicates an excitement or increased activity on a molecular level.  

   On the Celsius temperature scale, which is calibrated to 100 degrees when pure water boils at 1 atm or 101.325 kP of pressure and 0 degrees when water freezes to ice, the lowest possible temperature we can create or measure is -273 degrees, what is known as 'absolute zero'.

   Absolute zero is actually zero degrees on the Kelvin scale and 273 degrees is the point at which ice melts to water, 373 where water boils.  Same degree calibration, just 273 degrees slideways down the scale. 

   Because the rest of the world already switched over to the metric system for easier communication, I am not even going to discuss the Fahrenheit scale.  Its outdated, and although there are more fine gradations of temperature built in, the scale is based on sea water, not pure water.  What sea? Dead sea or Caribbean? Big difference.  Purify your water, calibrate your thermometer.

   Temperature, as we know it, is a direct measurement of the kinetic energy of the individual atoms or molecules.  The more agitated the molecules are the higher the observed temperature.  We cannot measure the individual motion of the molecules in our soup say, but we can measure the average energy of the molecules in motion.

   Heat can be transferred two ways by physical means, and by one method  over a distance.  Lets take care of heating via distance first.   When you put your hand up to the sun you are feeling many wavelengths of electromagnetic (EM) radiation that are absorbed by your skin or deeper tissues and converted to heat by adding kinetic energy to the atoms that make up the tissues of your hand.  This is called radiation, or radiative heating.  An object emits infrared, or heat waves, which are a form of EM, as soon as the temperature of said object rises above absolute zero.  If it is warmer than its surroundings, it will emanate EM radiation out, and receive less coming in from cooler objects.  In this way, objects cool via radiative loss and cooler objects in the path of those EM waves heat up via radiative heating.

    Physically we can move heat from one object to another by conduction, or direct contact from the hotter object to the cooler one.  When you place your hand on a hot object, it feels hot to you because you are feeling the movement of heat from the hotter object into your hand.  An object that is cooler than your hand removes heat and feels cold as a result.  Conduction happens through contact or touch.

    The other way heat can move via physical means requires uneven or spot heating of a fluid, a gas or liquid to operate.  This very interesting and complex transfer of heat is called convection.  Most heated fluids lose density and rise upward away from gravity.  Its that simple.  A flame below a pan of water will cause the water to rise away from it.  A convective flow will always occur when you heat a fluid unevenly.  Cooler fluid sinks to the bottom to replace fluid that is rising from the heat being added and a cycle forms in which heat is added at the bottom and circulated up to give that heat off at the top.

  It is convective flow, combined with the Coriolis effect, that causes our weather and the ocean currents on our little planet.  Water in the oceans and the air above are heated more on the equator and less at higher latitudes-  but only during the day.   That heat is moved around and is always uneven because the earth is only ever half lit by the sun.  Half is giving off heat, half is being heated.  Heat moves from hot side to cold side, but never catches up.  Hot air moves, cold air fills in behind it.  It sounds so simple right?  Not even close.  This is the most simplified and basic model that can be devised.  Basic, but real.  The complexity and detail of a reality based model would be staggering.  Weather modeling is done by massive computing arrays... not geeks on blogs.

   Heat is a form of energy, and in the next article I will go into how heat is used in all differing applications from engines and desalination plants to nuclear power plants and even the pumped hydronic heating systems in homes.  Heat removal, transfer and recollection are the hallmarks of our society.  Well ok, that and really cool silicon gizmos.  Thats for other writers to dissect tho...

Til the next round...  If you found this useful, follow on Twitter @MachineWhisprr or MachineWhisperer on Facebook.



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