Download Lecture 5 - HCC Learning Web

Chapter 4
Energy
4th Edition
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 (J)
English: foot pound force (ft∙lbf)
Exploring Engineering
Power
Power is defined as time rate of doing work
or time rate of change of energy Work =
force x distance
Power = work/time
Where “time” is the time over which the work
occurs
Power Units:
SI: watts (1 W = 1 J/s)
English: Horsepower (1 hp = 550 ft∙lbf/s)
Exploring Engineering
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 = [(ma)/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
Exploring Engineering
Kinds of Energy
Kinetic Energy
Potential Energy
Mechanical Energy
Other forms of energy:
Magnetic energy
Electrical energy
Surface energy
Internal energy etc.
Exploring Engineering
Kinetic Energy
Also known as “Translational Kinetic Energy” (TKE)
TKE = ½mv2 /gc (SI units)
Where m = mass, v = speed, gc = 1 (dimensionless in SI)
OR
TKE = ½mv2/gc (English units)
Where m = mass, v = speed, gc = 32.2 lbm∙ft/lbf∙s2
Anything that has mass and is moving in a line has TKE.
Exploring Engineering
Kinetic Energy Example
What is the translational kinetic energy of an
automobile with a mass of 1.00 × 103 kg traveling at
a speed of 65.0 miles per hour (29.0 m/sec)?
Need: TKE of the vehicle
Know: Mass: 1.00 × 103 kg, velocity: 29.0 m/sec
How: TKE= ½mv2 (SI units)
Solve: TKE = 4.23 × 105 J
Exploring Engineering
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
Exploring Engineering
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.
Exploring Engineering
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 lbm∙ft/lbf∙s2
(English) and gc = 1 [0] (SI)
How: GPE = mgh/gc
Exploring Engineering
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 (to 3 significant figures)
In SI, GPE = mgh/gc
= 90.7  9.81  8850./1 = 7.87  106 J
A check direct from the units converter:
5.81  106 ft lbf = 7.88  106 J …OK
Exploring Engineering
Potential Energy (PE)
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.
Exploring Engineering
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 thermal 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
always increases
Exploring Engineering
Energy
We have defined energy is the capability to do
work
But energy comes in different forms
Potential, translational kinetic, rotational kinetic, thermal and
others
And 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
Exploring Engineering
Energy Conservation
Energy = F  distance 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
Exploring Engineering
: Energy exchanges
“The Universe”
System
System energy changes  0
Universe energy changes = 0
Energy Conservation
This is an example of a
“Control Volume” (CV)
The energy in the room is
constant unless we allow
exchange with the outside
(e.g., the Universe)
E.g., a person could walk
through the door and add or
subtract 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
Exploring Engineering
Your class room
C.V. boundary
Insulated walls
Door
Control Volume
Example
Energy Conservation
Energy exchanges between a speeding car and the
rest of the universe.
Exploring Engineering
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
equal to the loss in chemical (potential) energy
in the fuel that is driving the vehicle.
Exploring Engineering
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 that
somewhere energy in leaking in or out of the control
volume at an exactly compensating amount.
Exploring Engineering