I love playing with fire
And I don't think I'll ever learn.

("I Love Playin' with Fire," by J.Jett of the Runaways, 1976)

NOTE TO ALL PTOA READERS AND STUDENTS:  The "A-OK hand signal" was developed by NASA in the 1960s and used by astronauts to visually indicate that the progress of the space mission was where it was intended to be during launch and recovery. Since this PTOA Segment was written, this useful signal has been appropriated by nefarious hate-inspired individuals which Your Mentor and the PTOA do not support.

PTOA Readers and Students will recognize that the above photo featuring playing with fire also exemplifies heat transfer by conduction.

Is there a physical barrier?

Square piece of material about an inch thick = Physical Barrier ... CHECK!

Does the barrier separate a hot area from a cold area?

Blow torch with active flame directed onto the cross sectional area of square barrier = Hot Area ... CHECK!

Human cheek at 98.6 °F = Cold Area ... CHECK!

Is the heat trying to migrate through the barrier in an attempt to equalize temperatures?

Yes! Nobody can change the rules of the Universe ... CHECK!

So why is the woman in the photo at the top of the page smiling to let the viewer know that she is she is  A-OK?

The phrase "A-OK" (with a thumbs up) was made popular by USA astronauts as an indication that all systems were go.

The heat from the blow torch is not able to migrate through the barrier because the barrier is made of the same space age material as the tiles that lined the re-entry surface of USA Space Shuttles.

Upon re-entry into the earth's atmosphere, the heat of friction increased the temperature of the shuttle's re-entry surface to 3000 °F (1649 °C).

The highly insulating material prevented the heat of friction from migrating through the tiles to the aluminum skin of the shuttle.

(Sadly, several of the space shuttle Columbia's tiles were dislodged during a January 2003 launch and that shuttle did not survive re-entry).

 

SIMPLE TECHNOLOGY CAN ALSO SAVE THE  DAY

PTOA Readers and Students were introduced to heat transfer by conduction in the previous PTOA Segment and recalled the learning experiences of burning hands and mouths because of misunderstanding how heat is conducted through metals.

Simple technology ... not space age technology ... is sufficient to solve household conductive heat transfer problems.

Potholders were logically invented (and named) to make it comfortable to hold the hot handles of cookware pots.

Plastic and wooden spoons can stir hot cocoa and immediately be licked clean without fear of searing the inside of the mouth.

But why does a pot holder and a plastic spoon solve these problems of undesirable heat transfer?

Afterall, PTOA Readers and Students already know that the Universe commands that heat shall be transferred when a Delta T exists.

And a Delta T definitely exists between the hot pan handle and the cooler potholder by which the handle is being picked up.

And a Delta T definitely exists between the hot cocoa and the room temperature plastic spoon that is stirring the delicious beverage.

 

THE OPPOSITE OF CONDUCTION IS "INSULATION"

Recap of Conductive Heat Transfer

PTOA Readers and Students will recognize the above graphic as an illustration of heat transfer via conduction.

The  cross sectional area exposed to the heat source is labelled "A." The width of the barrier through which heat is conducted is labelled "L" (and sometimes "L" is labelled and called "d").

What the barrier is made out of determines whether or not:

• heat will be conducted through the barrier (a conducting material)
•  heat will be prevented from being conducted through the barrier (an insulating material).

 

Recap of Conduction Heat Transfer Definition 

PTOA Readers and Students will also recognize the mathematical expression to the right that defines heat transfer via conduction ("Q" expressed in BTUs or Joules).

PTOA Readers and Students also know that when both sides of the expression are divided by "t," heat rate is defined as "Q/t" and is expressed in BTU/hr or J/hr.

K in the expression characterizes the thermal conductivity capability of the material that the barrier is made of.

Doesn't it make sense that heat transfer via conduction cannot be the same for all the materials?

So... no kidding ....

Several people spent their entire lives figuring out the thermal conductivity of various materials and developed handy-dandy tables the rest of us can just look up and insert into the expression.

The below link gives a list of k for a variety of materials:

Thermal Conductivity Factors

If the link is not working, open up another window and copy this URL into your browser:

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

According to the table:

k for a stainless steel spoon or pan is 16.

k for a potholder made of cotton is 0.04

Since 16/0.04 = 400, the rate of heat transfer through stainless steel is 400 times faster than through a cotton potholder.

That means the molecules of cotton just take longer to get excited by conduction heat transfer. So the hand touching the cold side of the potholder has plenty of time to handle the pan without being burned.

k for a spoon made out of plastic is 0.03.

Since 16/0.03= 546, the rate of heat transfer through stainless steel spoon is 546 times faster than for a spoon made out of plastic.

The molecules of plastic take such a long time to get excited by heat that there is plenty of time to stir the cocoa without heating the plastic spoon to a temperature that would burn the mouth.

In summary:

k = Thermal Conductivity Factor
expressed in [Watt / (meters-deg K)]
which is equal to 0.5779 [BTU / (foot-hr- deg F).

The units of k are weird because they cancel out all other units in the heat transfer via Conduction expression so that the final units are BTU/hr and J/hr.

The weird units also provide the perfect segue into the next topic!

 

VERIFYING THE UNITS OF A MATHEMATICAL EXPRESSION

PTOA Readers and Students have focussed on the definitions of Velocity and Conduction Heat Transfer that have been defined by mathematical expressions.

The below expression defined Conduction Heat Transfer Rate and by now PTOA Readers and Students should understand what all the alphabet letters mean:

Conductive Heat Transfer is defined:

Q/t = [k* A* (Delta T)] / d

The final step in understanding a mathematical expression that defines an entity is to make certain that the units on both sides of the expression are consistent using the following procedure:

1. The units of measure for each factor in the expression are listed as they would show up in the expression.

2. Then the units of the mathematical expression are crossed out and reduced to just the units that remain.

3. The units of the defined entity on the left hand side should match the remaining units of the mathematical expression on the right hand side.

Q/t      =             k                   A    Delta T   /  d

BTU/hr =[(BTU/ft*deg F*hr)*(ft2)*(deg F)]  /  ft  = BTU/hr

The expression for Conduction Heat Transfer Rate models what is observed in the Universe AND  passes the Unit Check!

Next PTOA Readers and Students will focus on Heat Transfer via Convection.

 

TAKE HOME MESSAGES: The rate of heat transfer via conduction is greatly dependent upon material of construction used to construct the barrier that separates the hot area from the cold area.

The expression that defines heat transfer via conduction includes a Thermal Conductivity Factor, k.

The Thermal Conductivity Factor relates how capable the physical barrier is at conducting heat through it.

k is a big number for good conductors and a small number for good insulators.

A link to a list of Thermal Conductivity Factors was included in this PTOA Segment.

The mathematical expression used to define heat transfer by conduction passes the unit verification test: BTU/hr = BTU/hr.

2015 PTOA Segment 00063
PTOA Heat Transfer Focus Study Area

You need to login or register to bookmark/favorite this content.