1. Systems, Energy, & Efficiency
Work & Energy - Part 1: Heat
Systems, Energy, & Efficiency
EGR 1301: Introduction to Engineering

2. Source: Professor Thomas
Systems
System
A particular subset of the universe specified in time and space by a boundary (Ch 17, p. 484)
System boundary
tinitial = start time
tfinal = stop time
Source: Professor Thomas

3. System Definition Rules the engineer must follow:
Once a system is specified, it cannot be changed midway through a calculation.
The system boundary can be any shape, but it must be a closed surface. It must also be closed (or bounded) in time.
The system boundary can be rigid (defining a volume of space) or it can be flexible (defining an object).

4. Importance of System Definition
Lecture 30 - Work & Energy - Part 1
Importance of System Definition
Questions that an engineer may ask:
What pressure is required to lift the automobile?
How much work does the pump do when lifting the auto?
What volume of hydraulic fluid must be pumped to lift the auto a given height?
Engineer must specify a system in order to analyze the problem.
Each of these systems will yield the same answer to the problem, but some systems will be easier to analyze than others.
4 (A-D) are rigid and define a volume of space
1 (E) has a flexible boundary that encompasses just the hydraulic fluid.
Source: Foundations of Engineering, Holtzapple & Reece, 2003

5. Intensive vs. Extensive
Extensive quantities
Change with size of the system
Intensive quantities
Remain constant, regardless of size
Quantity
Intensive
Extensive
Volume
Mass
Density
Temperature X X X X

6. Lecture 30 - Work & Energy - Part 1
What is Energy?
“The capacity for doing work” OR
Unit of exchange (Ch 22, p. 572)
Examples:
Electricity  light or heat
Chemical energy in gasoline  torque in car or heat
Natural gas  electricity or hot water
Source: Webster’s New Collegiate Dictionary
Dictionary’s definition is circular because work is defined as a type of energy – Sheesh!
Another definition of work, from Newton’s perspective
Work  The transference of energy that is produced by the motion of the point of application of a force and is measured by multiplying the force and the displacement of its point of application in the line of action.
Unit of exchange - much like money is a unit of exchange
Energy is used to relate physical phenomena

7. Lecture 30 - Work & Energy - Part 1
Units of Energy
So, just as we have exchange tables for different systems of money, we have exchanges tables for different ways to account for energy.
Table on pg 693 in text.
Appendix A – unit conversion tables – starting on pg 683.
Source: Foundations of Engineering, Holtzapple & Reece, 2003

8. 1st Law of Thermodynamics
Lecture 30 - Work & Energy - Part 1
1st Law of Thermodynamics
“Law of Conservation of Energy”
Energy can neither be created nor destroyed
Therefore, energy must be conserved
Energy can only be transformed
Work can be converted into another form of work
Work can be converted into heat
Need to keep track of, or “account” for, these changes
After the transformation, the final amount of energy is the same as the final amount.

9. Money Accounting Can “account” for the money in your bank: Ex:
Start with $1000
Pay you $500 for coming to class
Spend $800 on new laptop
How much do you have (i.e. final balance)?
Final balance – Initial balance = Deposits - Withdrawals
Accumulation = Net input
Final balance = Initial balance + Deposits - Withdrawals

10. Energy Accounting For any system, the same relationship is true:
Final energy – Initial energy = Input - Output
Accumulation = Net input
State quantities = Path quantities
System Boundary
Energy in/out
(Path Quantities)
Accumulated Energy
(State Quantities)

11. State Quantities Kinetic Energy Potential Energy Internal Energy
Energy associated with motion
Potential Energy
Energy associated with position, either against a field (e.g. gravity or electric field), compressed spring, or stretched rubber band
Internal Energy
Energy associated with atoms, such as temperature, phase changes, or chemical reactions

12. Lecture 30 - Work & Energy - Part 1
Path Quantities
Work
Energy flow due to a driving force other than temperature: mechanical (shaft, hydraulic), electrical, photonic (laser, solar PV)
Heat
Energy flow due to temperature: conduction, blackbody radiation
Mass
Energy flow due to mass crossing the boundary: fuel
Mass isn’t in the Holtzapple book

13. Universal Accounting Equation
Mathematical version of the accounting equation:
All have the form:
Change in kinetic energy
Change in potential energy
Change in internal energy
Change = Energy at tfinal - Energy at tinitial

14. Universal Accounting Equation
Mathematical version of the accounting equation:
Heat and Mass have the form:
Work input = work done on the system from its surroundings
Work output = work done by the system to its surroundings
Energy added to system – Energy removed from system

15. Lecture 30 - Work & Energy - Part 1
Joule’s Experiment
System boundary
tinitial = mass is raised
tfinal = after mass falls and propeller and water stop moving
Assume perfect insulation.
How are variables related?
This process can be 100% efficient
Known amount of work input  Gravity exerts a force F on the mass as it travels distance delta x.
As the mass travels, the stirrer churns the water, causing its temperature to rise.
Where does the energy go?
Macroscopic energy (mechanical work) is added.
This energy increases the microscopic energy (internal energy) of the water.
In this formula,
Q = Amount of heat added
C = heat capacity
M = mass of the water
Delta T = temperature change of the water
Source: Foundations of Engineering, Holtzapple & Reece, 2003

16. 2nd Law of Thermodynamics
Lecture 30 - Work & Energy - Part 1
2nd Law of Thermodynamics
Naturally occurring processes are directional
Closely tied to idea of reversibility
Reversible processes have no directionality
Entropy
Ex: balloon, car, office
Examples of 2nd law of thermodynamics:
Which is most likely to happen
A balloon explodes and balloon fragments fly in different directions
Balloon fragments spontaneously reassemble themselves
Gasoline in an automobile used to move a car up a hill will not reappear in the tank when the car comes down the hill
My office will become spontaneously cleaner
My office will become increasingly messy as the semester continues
UNLESS I put some energy into cleaning my office
Work can be converted into heat with 100% efficiency, but

17. Energy Conversion A system converts energy from one form to another
The process is not always perfect
Energy Conversion Device (System)
Energy In
Energy Out
Wasted Energy (often heat)

18. Efficiency Measure of how well a system can convert energy
Greek letter eta, η

19. Example
If a system outputs 70,000 J and η = 0.7, what is the input energy?
How much was wasted? J
30,000 J

20. Cascaded Conversion
Can connect multiple systems together and do several conversions
Natural gas
Rotating shaft
Electricity
Light
Gas turbine
Generator
Light bulb E2 E3 η1 η2 η3 E1 E4
Waste 1
(heat)
Waste 2
(heat)
Waste 3
(heat)

21. Overall Efficiency Treat multiple conversions as a single processη1 η2 η3 E1 E4
Total waste
(heat)

22. Recap Systems – boundary (time & space) Energy – unit of exchange
Intensive vs. Extensive Quantities
State vs. Path Quantities
Universal Accounting Equation
Efficiency
Cascaded systems
Next: examples