energy. exploring engineering chapter 4, part 1 energy adapted from
TRANSCRIPT
Energy
Exploring Engineering
Chapter 4, Part 1
Energy
Adapted From
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 EnergyPotential Energy
Some other forms of energy:Magnetic energyElectrical energySurface 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)
= ½ mv2/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 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 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 = 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 Energy
Need: 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 temperatureAll 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 workBut 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 UniverseE.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: 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.