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# Energy - PowerPoint PPT Presentation

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

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### Exploring Engineering

Chapter 4, Part 1

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”

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

• 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

• 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

• 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: ???

• 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.

• 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

• 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

• 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.

• 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

• 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

• 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, 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.

• 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

: 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

• 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.

• 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.