chapter 4 energy conversion

8/11/2019 Chapter 4 Energy Conversion

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Exploring EngineeringChapter 4

Energy Conversion


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Energy

Energy is the capability to do work

Work = force x distance

Where distance is the distance over

which the force is applied

Energy Units:

SI: joules

English: ft lbf foot pound force

Exploring Engineering


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Power

Power is defined as time rate of doingwork or time rate of change of energyWork = force x distance

Power = work/time

Where time is the time over which thework occurs

Power Units:SI: watts (joules/sec)

English: Horsepower (550 ft lbf/s)



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Power Example

A person takes 2.0 seconds to lift a 1.0 kg book a

height of 1.0 meter above the surface of earth.

Calculate the power expended by the person.Need: Power

Know: mass = 1.0 kg, distance = 1.0 m, time = 2.0 s

How:work = force distance, and power = work/time

Solve:Work = (m a)/gc (distance)

= (1.0 kg)(9.8 m/s2)/1 (1.0 m)

= 9.8 kg(m2/s2) = 9.8 joules

Then, Power = (9.8 joules)/(2.0 seconds) = 4.9 J/s = 4.9 W



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Kinds of Energy

Kinetic Energy

Potential Energy

Other forms of energy:

Magnetic energy

Electrical energy

Surface energyInternal energy etc.


Mechanical Energy


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Kinetic Energy

Also known as Translational Kinetic Energy (TKE)

TKE = mv2/gc(SI units)

Where m= mass, v= speed, gc= 1 (dimensionless)

OR

TKE = mv2/gc(English units)

Where m= mass, v= speed, gc= 32.2 lbmft/lbfs2


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


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Kinetic Energy Example

What is the translational kinetic energy of an

automobile with a mass of 1.00 103kg traveling at

a speed of 65.0 miles per hour (29.0 m/sec)?

Need: TKE of the vehicle

Know: Mass: 1.00 103kg, velocity: 29.0 m/sec

How: TKE= mv2

(SI units)

Solve: TKE = 4.23 105J



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Gravitational Potential EnergyGPE is the energy acquired by an object by virtueof its position in a gravitational field-- typically by

being raised above the surface of the Earth.In SI, GPE = mghin units of joules

In Engineering English units,

GPE = mgh/gc in units of ftlbf



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Gravitational Potential EnergyMt. Everest is 29, 035 ft high. If a climber has

to haul him/herself weighing 200. lbm

(including equipment) to the top, what ishis/her potential energy above sea level when

on the summit. Give your answer in both in

joules and in ft lbf.



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Gravitational Potential Energy

Need: GPE in English and SI units

Know: m= 200. lbm = 90.7 kg; h= 29, 035 ft = 8850.

m; g= 32.2 ft/s2

= 9.81 m/s2

and gc= 32.2 lbmft/lbfs2

(English) and gc= 1 [0] (SI)

How: GPE = mgh/gc



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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 106ft.lbf (3 significant figures)

SI, GPE = mgh/gc= 90.7 9.81 8850./1 = 7.87 106J

A check direct from the units converter:

5.81 106ft lbf = 7.88 106J OK



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Potential Energy (PE)

GPE is NOT the only form of PE.Chemical, nuclear and electromagnetic are other

forms of PEFor us, chemical and electrical energy are soimportant that we will reserve extra chapters and

lectures to them for later presentation.



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Thermal Energy

Thermal energy, often referred to as heat,is a veryspecial form of kinetic energy because it is therandommotion of trillions and trillions of atoms andmolecules that leads to the perception oftemperature

All higher forms of energy dissipate thermal thermalenergy, the ultimate energy sink

The laws of thermodynamics state 1) all energy is

conserved and 2) that the thermal energy in the universealways increases



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Energy

We have defined energy is the capability to dowork

But energy comes in different forms

Potential, translational kinetic, rotational kinetic, thermal and

othersAnd energy can be converted from one form toanother

The energy in the Universeis conserved

A control volume is a subset of the Universe you construct to

isolate the problem of interest. It exchanges energy with therest of the Universe



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Energy Conservation

Energy = Fdistance is the

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

System

The Universe

: Energy exchanges

System energy changesUniverse energy changes = 0

0Universe energy changes = 0



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Energy Conservation

This is an example of aControl Volume (CV)

The energy in the room isconstant unless we allowexchange with the outside

(e.g., the Universe)E.g., a person could walkthrough the door and add orsubtract energy

A heating duct could also

add thermal energyOn a winter day, a windowcould break and the c.v.would lose thermal energy

Door

Control volumeexample

C.V. boundary

Insulated walls

Your class room



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Energy Conservation

Energy exchanges between a speeding car and therest of the universe.



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Application of Control Volumes

In the last slide, we have TKE of the vehicle,

RKE of the wheels, electrical energy in the

lights, thermal energy from the radiator, etc.We deduce that all these energies are exactly

equalto the loss in chemical (potential) energy

in the fuel that is driving the vehicle.



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Summary

We specifically identified kinetic, gravitational,potential, and thermal energy

We learned that energy is conserved in the universe,but not necessarily within a control volume.

Deficiencies within a control volume mean thatsomewhere energy in leaking in or out of the controlvolume at an exactly compensating amount.


chapter 4 energy conversion

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