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Energy

- 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”

Energy

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

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

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

Kinetic Energy

- Also known as “Translational Kinetic Energy” (TKE)
TKE = ½ mv2 (SI units)

= ½mv2/gc (English units)

m = mass, v = speed, gc = 32.2 lbm.ft/lbf.s2

Units: ???

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= ½mv2
- SOLVE: TKE = 4.2 x 105 J

Anything that has mass and is moving in a line has TKE.

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

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 = mgx 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

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.

Gravitational Potential Energy

- Need: GPE in English and SI units
- Know:
- m = 200. lbm = 90.7 kg (“Convert”); h = 29, 035 ft. = 8850. 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

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 8850. = 7.87 106 J

- A check direct from the units converter: 5.81 106 ft.lbf = 7.88 106 J …OK

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.

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.

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

- But energy comes in different guises

: Energy exchanges

“The Universe”

“The Universe”

System

System

¹

¹

System energy changes

System energy changes

0

0

Universe energy changes = 0

Universe energy changes = 0

Energy Conservation- Energy = F distance is generic equation for energy
- Energy is conserved (although it may change form)

Example of a book lying on a table and then falling on ground

C.V. boundary

This class room

This class room

Insulated walls

Insulated walls

Door

Door

Control volume

Control volume

example

example

Energy Conservation- 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

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.

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.

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