```WORK,POWER AND ENTROPY

NORBERT L. ZARUMBA             MARENGO COMMUNITY HIGH SCHOOL
618 E. GRANT HIGHWAY
MARENGO, IL 60152
1-815-568-6511

OBJECTIVES:
1:To demonstrate, explain and allow the student to experience work and power.
2:To allow the student to observe that entropy is a real process that must occur
whenever work occurs.

MATERIALS:
For part B. Meter stick, chain (about 2 meters), double pan balance, stop
watch.
For part A.Jar that can be sealed (gallon condiment jar), 50 ml beaker,
support to hold beaker close to top of jar, distilled water, dehydrating
agent, analytical balance.

STRATEGIES:        ----------------------
|  Display jar with  |
|  beaker and liquids|
|  tell students to  |
|    observe jar.    |
|____________________|
__________|___________
|  Record observations.|
|______________________|
__________|___________
|  Explain that jar is |
|  an engine .         |
|   (pumping water) .  |
|______________________|
__________|____________  ____________________
| Ask for definitions  |__|    Record        |
|     of work .        |  |  definitions.    |
|______________________|  |__________________|
___________|____________
|    Define work.      |
| Work=ForcexDistance. |
|______________________|
_______________|_______________
| Have students do work.      |
|  Build apparatus.(Beaker in |
| jar,record masses,etc.)     |
|______________________________
____|____       ____|____
|        |      |        |
| Part A |      | Part B |
|________|      |________|
|               |
________   ________|______  _______|_________  ___________
|Record |  |   Build     |  | Lift masses,  |  | Record   |
|masses,|__| apparatus   |  | same height,  |__| mass,    |
| time, |  | (beaker in  |  |different times|  |distance, |
|  etc. |  | jar,etc.)   |  | (triplicate)  |  |   times. |
|_______|  |_____________|  |_______________|  |__________|
2
__________    _____|__________ _____|_____    ____________
| Graph:  |__ |     Calculate work.       |___| Graph:    |
|Force vs.|   |___________________________|   | Force vs. |
|distance |   ____|____    _______|_______    | distance  |
|_________|  | Define: |   | Was work same|   |___________|
| Power=  |   | in each case?|
__________   |work/time|   |  HINT-NO     |
| Graph:  |__|_________|   |______________|
| Work vs.|       |         _____|________   _________
| time    |       |         | Define:    |__| Graph:  |
|_________|       |         | Power=     |  | Work vs.|
|         | work/time  |  | time    |
|         |____________|  |_________|
_____|_______________|________
| What caused work to occur  |
|     in each case ?         |
|   (ENTROPY CHANGE)         |
|____________________________|
____________|_______________
| Introduce   entropy.       |
| Disorder has increased.    |
|____________________________|

DEFINITIONS:  WORK=(FORCE)(DISTANCE).  Joule=Newton meter.

POWER=WORK/TIME.         Watt=Joule/second.

ENTROPY = the disorder in a system. Entropy always increases
when work is done.

ADDENDUM: These topics were chosen as a group for two reasons.
A. In common language we confuse work and power; (many times what is
considered "hard Work" or "working harder" is really equal or less work but
greater power). Therefore we attempt to clarify these concepts.
B. To introduce the concept of entropy (work cannot occur without an increase
in entropy).
BACKGROUND: Before introducing this experiment the students usually will have
mastered simple machines (levers, pulleys, inclined planes, etc.). The minimum
knowledge required by the by the student to be at ease with the experiment is:
A. multiplication and division,
B. graphing.
This experiment can then be mastered by students in intermediate grades. The
calculus presentation would be appropriate for better prepared high school
students or college students taking physics for the first time.
TIME REQUIRED: Part B can be completed in a class period (about fifty
minutes).
Part A needs long periods of time to complete; but only a few minutes each
day after the initial set up. In part A the work is caused by diffusion of water
from a lower level to the dehydrating agent which is at a higher level. The beaker
of dehydrating agent plus water need be massed (weighed) occasionally (about every
two days) for at least a month. This massing will take only a few minutes each
time. This part of the experiment is critical to explain entropy. EXPLANATION: In
Part A the student will learn the difference between work and power.
As a phenomenological experiment they will see that something is the same in
each case (work); but that something else (power) varies.
At the intermediate level very good students should be able to deduce the
difference.
At the intermediate level the student should learn:
3
WORK = (FORCE)(DISTANCE);  the unit is, JOULE = (NEWTON)(METER). The
newton (which is a unit of force) can be determined using a newton scale; or by
massing an object on a balance (grams) and then multiplying by the acceleration of
gravity (9.8 meter/second2).
POWER = (WORK/TIME);   the unit is WATT = (JOULE/SECOND).
At the high school level a presentation with problems similar to that in,
"Physics,Principles and Problems", Murphy and Smoot; Charles E. Merrill
Publishing, 1977. Teachers Edition, 1977; pg 139-142; will be convenient.
At a more advanced level "PSSC Physics, Fourth Edition," Haber-Schaim et al,
D.C. Heath and Co. 1976. Pgs 326-328; will be appropriate.
At the calculus level a text like Resnick and Halliday would be used.
CONCLUSION: As Part B progresses it should become apparent, by analogy with
Part A, that work is being done and power is being expended (albeit in very small
amounts). The phenomenon to be observed in B is:
"What causes the work to occur?"
In A it was the muscles of the student, food, etc.
In B the cause is the decrease in concentration of the dehydrating agent.
This decrease in concentration is entropy. The dehydrating agent which was "pure"
is now diluted by water; it is now "impure". The disorder has increased. This
increase in "disorder" (entropy) has caused the work to occur. The work will
continue until the dehydrating agent reaches the same concentration as pure water
(forever). By similar logic "increased disorder" (entropy increase) caused the
work to occur in Part A.
Therefore:
A.whenever work is done entropy is increased,
B.the greater the power the faster entropy is increased.

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