1. Energy

2. Exploring Engineering
Adapted From
Exploring Engineering
Chapter 4, Part 1
Energy

3. Energy Energy Units: Energy is the capability to do work
Work = force x distance
Distance over which the force is applied
Energy Units:
SI: joules
Mixed SI units: Watt-hours (= 3.6 kJ)
English: ft-lbf “foot pound force”

4. Energy
Mixed SI units: Watt-hours (= 3.6 kJ)

5. Power
How fast work is done or how rapidly the amount of energy possessed by an object changed
“Power is defined as time rate of doing work or time rate of change of energy”
Power = work/time
Power Units:
SI: watts (joules/sec)
English: Horsepower

6. Kinds of Energy Kinetic Energy Potential Energy
Some other forms of energy:
Magnetic energy
Electrical energy
Surface energy
Chemical energy (a form of potential energy)
Internal energy etc.
Often mechanical energy

7. Kinetic Energy Also known as “Translational Kinetic Energy” (TKE)
TKE = ½ mv2 (SI units)
= ½ mv2/gc (English units)
m = mass, v = speed, gc = 32.2 lbm.ft/lbf.s2
Units: ???

8. Kinetic Energy: Example
What is the translational kinetic energy of an automobile with a mass of 1X103 kg traveling at a speed of 65 miles per hour (29 m/sec)?
Need: TKE of the vehicle
Know: Mass: 1X103 kg, speed: 29 m/sec
How: TKE= ½ mv2
SOLVE: TKE = 4.2 x 105 J
Anything that has mass and is moving in a line has TKE.

9. Gravitational Potential Energy
GPE is the energy acquired by an object by virtue of its position in a gravitational field-- typically by being raised above the surface of the Earth.
In SI, GPE = mgh   in units of joules
In Engineering English units,
GPE = mgh/gc  in units of ft.lbf
In English units, GPE = [lbm] [ft/s2][ ft][lbf s2/lbm ft] = ft lbf

10. GPE & Power: Example
A person takes 2.0 seconds to lift a 1. kg book a height of 1. meter above the surface of Earth. Calculate the power expended by that person or calculate the energy spent by the person per unit time.
Work done = Force x distance = mg x h = 1. x 1. x 9.81 [kg][m/s2][m] = 9.81 [J][m] = 1. x 101 J
Power expended = Work done/time = 1. x 101/2.0 [J/s] = 5 Watts

11. Gravitational Potential Energy
Mt. Everest is 29, 035 ft high. If a climber has to haul him/herself weighing 200. lbm (including equipment) to the top, what is his/her potential energy above sea level when on the summit. Give your answer in both in joules and in ft.lbf.

12. Gravitational Potential Energy
Need: GPE in English and SI units
Know:
m = 200. lbm = 90.7 kg (“Convert”); h = 29, 035 ft. = m (“Convert”); g = 32.2 ft/s2 = 9.81 m/s2 & gc = 32.2 lbm ft/s2 lbf (English) and gc = 1 [0] in SI
How: GPE = mgh/gc  English
GPE = mgh  SI
Note: I have rounded up h from to one more sig. figure than I will need.

13. Gravitational Potential Energy
Solve: English … GPE = mgh/gc
= 200.  32.2  29,035/32.2 [lbm][ft/s2][ft][lbf.s2 /lbm.ft]
= 5.81  106 ft.lbf (3 significant figures)
SI … GPE = mgh
= 90.7  9.81  = 7.87  106 J
A check direct from the units converter: 5.81  106 ft.lbf = 7.88  106 J …OK
Why to 3 significant figures? That’s how well we know m.
Convert has some roundoff error since we had already rounded the English answer

14. GPE is NOT the only form of PE.
Potential Energy
GPE is NOT the only form of PE.
Chemical, nuclear and electromagnetic are other forms of PE
For us, chemical and electrical energy are so important that we will reserve extra chapters and lectures to them for later presentation.

15. Thermal Energy
Thermal energy, often referred to as heat, is a very special form of kinetic energy because it is the random motion of trillions and trillions of atoms and molecules that leads to the perception of temperature
All higher forms of energy dissipate to thermal energy, the ultimate energy sink.
The laws of thermodynamics state 1) all energy is conserved and 2) that the thermal energy in the universe, corrected for temperature, always increases.
The two laws are, respectively, the 1st and 2nd laws of Thermodynamics. Possibly worth mentioning.

16. Energy We have defined energy is the capability to do work
But energy comes in different guises
Potential, translational kinetic, rotational kinetic, thermal and others
Energy can be converted from one form to another
The energy in the Universe is conserved
A “control volume” is a subset of the Universe you construct to isolate the problem of interest. It exchanges energy with the rest of the Universe
The conversion of one kind of energy to another, say chemical to mechanical, thermal to mechanical, or say mechanical to electrical, is a very large part of what several types of engineers do.

17. Energy Conservation
System
“The Universe”
: Energy exchanges
System energy changes
Universe energy changes = 0
Energy = F  distance is generic equation for energy
Energy is conserved (although it may change form)
In theory, its problematic what the Universe in expanding against…
Example of a book lying on a table and then falling on ground

18. C.V. boundary This class room Insulated walls Door Control volume
Energy Conservation
Insulated walls
This class room
Door
Control volume
example
C.V. boundary
Example of a control volume
The energy in the room is constant unless we allow exchange with the Universe
E.g., a person could walk through the door and add energy
A heating duct could also add thermal energy
On a winter day, a window could break and the c.v. would lose thermal energy
Use of cubes each labeled a different kind of energy is useful
- cubes can be KE. PE, internal energy and thermal energy.
Most useful is one marked “Potential” on one face and “thermal” on a hidden face.
Show “Potential” . Define the class room as the control volume.
Drop the cube and rotate to a blank face.
ASK where the PE went? Finally rotate the cube and show”thermal”

19. Application of Control Volumes
The TKE of the vehicle, RKE of the wheels, electrical energy in the lights, thermal energy lost from the radiator, etc.
We deduce that the source of all these energies is exactly equal to the loss in chemical (potential) energy in the fuel.

20. Summary: Energy
We specifically identified gravitational, potential, and thermal energy
We learned that energy is conserved in the Universe, but not necessarily in a control volume.
Deficiencies within a control volume mean that energy in leaking in or out of the control volume at an exactly compensating amount.