MOD 3   LECTURE 5                                    HEAT

In this lecture you will learn about:

THE DIFFERENCE BETWEEN THERMAL HEAT AND INFRARED
Heat is a term used "loosely" in that even when scientists use the term they may be talking about two different things unless they specify "thermal heat" or "infrared heat." 
 
THERMAL HEAT

... thermal heat is the kinetic energy of matter due to the movement of electrons
... it always moves from an area of high energy to low energy, or hot to cold
.... it CANNOT travel thru space, it is a property of matter 
... you have read the lecture on thermodynamics and when energy is transformed from one form to another the loss of energy is generally in the form of thermal heat. 

INFRARED or RADIANT HEAT

... IR is a wave of the EMS (which is discussed in mod 5)
... it can travel thru space and doesnt need matter for transmission
... it readily transfers its energy to matter 
.. FOR EXAMPLE:  the heat of the rays of the sun, fire or heat lamps is felt as the IR rays are absorbed by our skin and clothes

Because it cannot travel thru space THERMOS bottles with a vacuum between the inside and outside keep the heat inside the container.  "Specially designed coatings, often based on metallic oxides...reflect radiant infrared energy, thus tending to keep radiant heat on the same side of the glass from which it originated. ...radiant heat originating from indoors is reflected back inside, thus keeping heat inside in the winter, and infrared radiation from the sun is reflected away, keeping it cooler inside in the summer.
http://en.wikipedia.org/wiki/Low-emissivity
(see explanation of a vacuum flask below)

 
TEMPERATURE - HOW TO MEASURE THE KINETIC ENERGY OF MATTER

- The standard references are called temperature scales. Like measuring in feet, toes or puppy dog tails, temperature scales are arbitrary.

- The Fahrenheit scale SETS 32o as the temp of freezing water and 212o the temp of boiling water.

- The Celsius scale sets 0o as the temp of freezing water and 100o the temp of boiling water (which you did). 
- to change from Celsius centigrade(C) to Fahrenheit (F) is relatively simple.

1. find a site online that will do the calculations for you.....
2. Each degree C is about 2 degrees F. The F scale is shifted up 32o compared to the Celsius scale. To go from F--> C scale, subtract 32 and divide by 2. To go from C--> F scale, multiply by 2 and add 32.

-- KELVIN is another scale. (- 273oC)= (0 K) is absolute zero. It is the temperature at which all movement of atoms stop. It is considered to be unreachable.


 
THERMOMETERS 

- glass thermometers are based on the EXPANSION of a liquid (mercury or alcohol) up a small glass bore tube.

- Metal thermometers (like those stuck into a roast or turkey) are based the expansion of metal. A furnace thermometer uses the expansion of bimetallic strips (two metallic strips bonded together), where the metals heat at different rates causing one to bend.

-- Digital thermometers use a thermocouple. A thermistor uses a wire in a glass. As the temp changes, the resistance of the wire to electrical conduction changes.

-- the human sensing of hot and cold is odd. Very hot and very cold are both sensed as "hot" or burning. 


 
 
  
  Fahrenheit Celsius
water boils 212 100 
body temp 98.6  37
room temp 75 25
water freezes 32
 

 
THE MOVEMENT OF THERMAL HEAT 
CONDUCTION

-- Is the transfer of  thermal energy between molecules in a substance due to a temperature gradient. 
 
"It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences. " wikepedia

-- "Conduction needs matter and does not require any bulk motion of matter. Conduction takes place in all forms of matter, viz. solids, liquids, gases and plasmas.  In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons. In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion." http://en.wikipedia.org/wiki/Heat_conduction

Example:  conduction is responsible burning one's hand when picking up hot objects, for rapidly losing heat when falling into ice cold water

- thermal breaks between high conducting materials like metal with like low conducting materials like rubber

- insulation is low conducting material that is spun into fibers that TRAP AIR which is a good insulator

- gap between windows is good insulation, but vacuum is even better 

CONVECTION

- the actual movement of molecules of liquids and AIR when they are heated

-- a good video showing the convection of air click HERE

... heating decreases the density of liquids and gases so the heated, less dense molecules begin to move upwards 

Example:  in a boiling pan, rising air on hot asphalt, the movement of thermal cells over land masses, responsible for "weather" and "climate"

-- Of course, when the molecules move they also bump into other molecules and transfer some of that energy by conduction. 
 

In zero gravity, convection does not carry the hot combustion products away from the fuel source, resulting in a spherical flame front. (Click on the picture for more information.)

-- rising thermal air masses makes hang gliding possible


 

 
HEATING THE EARTH, INFRARED, CONVECTION AND CONDUCTION

 
 
 
 
 
 
 
 
 
 
 
 
 

Obeying the laws of thermodynamics we get our energy from the Sun in the form of infrared AND from the core of the Earth from decaying isotopes like Uranium contained within the mantle. The heat from inside the planet causes convection currents in the mantle (which sometimes springs a leak called a volcano) and heat moves by conduction outwards towards the surface. 

http://www.uwgb.edu/dutchs/PLANETS/earth.htm



http://www.explainthatstuff.com/vacuumflasks.html