Exploring Engineering
Chapter 4, Part 1
Energy
Energy Energy is the capability to do work
Work = force x distanceDistance over which the force is applied
Energy Units: SI: joules Mixed SI units: Watt-hours (= 3.6 kJ)English: ft-lbf “foot pound force”
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 EnergyAlso 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: ???
Kinetic Energy: ExampleWhat 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 vehicleKnow: Mass: 1X103 kg, speed: 29 m/secHow: TKE= ½ mv2
SOLVE: TKE = 4.2 x 105 J
Anything that has mass and is moving in a line has TKE.
Gravitational Potential EnergyGPE 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 = 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
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 EnergyNeed: GPE in English and SI unitsKnow:
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 PEFor us, chemical and electrical energy
are so important that we will reserve extra chapters and lectures to them for later presentation.
Thermal EnergyThermal 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
Energy Conservation
Energy = F distance is generic equation for energy
Energy is conserved (although it may change form)
System
“The Universe”
System
“The Universe”
: Energy exchanges: Energy exchanges
System energy changes 0Universe energy changes = 0System energy changes 0Universe energy changes = 0
Example of a book lying on a table and then falling on ground
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
Insulated walls
This class room
Door
Control volume example
C.V. boundary
Insulated walls
This class room
Door
Control volume example
C.V. boundary
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: EnergyWe specifically identified gravitational,
potential, and thermal energyWe 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.