Transcript Accounting for Energy

1
THE ABILITY TO CAUSE CHANGE
2
Radiant energy
mechanical energy
3
o
o
o
o
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types of energy
Thermal energy - from the motion of
molecules
Chemical Energy – mostly among chemical
bonds
Nuclear energy - from the nuclei of an atoms
Electrical and Magnetic energies – E&M
interactions
Radiant energy -- carried by electromagnetic
waves
Mechanical energy – usually by large bodies
4
Energy
• Energy is the capacity (ability)
to do work.
• Kinetic Energy: Associated with
an object in motion
• Potential Energy: Is the
interaction energy of an object
wit another
Accounting for Energy - Part 1
Chapter 22
Objectives


Understand what energy is
Understand path energies

Work (specific classification; see next)


mechanical, shaft, hydraulic, electrical, photon…
Heat


conduction
blackbody radiation
What is energy?
Let’s define in the most general
way Work first
Work is a change in the universe
Energy – is the source or driving
force able to perform such a work
Thus, work done on x changes the x
energy
Energy


Make an analogy for energy: money is a “unit
of exchange”,
Example



1 car = $20 k
1 house = $100 k
5 cars = 1 house
=
Energy Equivalents

1 kg coal =
1 kg uranium =

1 kg uranium =

42,000,000 joules
82,000,000,000,000 joules
(82x1012)
2,000,000 kg coal!!
Energy is a conserved quantity

Generally, energy is a conserved quantity,
i.e., generation and consumption are zero.
In fusion and fission systems, energy can be
generated (reactors, sun, stars, radioactive
atoms, …
 In particle accelerators, energy can be
consumed, also everywhere in the universe
cosmic rays became particles
These are not common engineering systems


For most applications:
Accumulation = Net Input
Types of Energy
Accumulation = Net Input
State Energies
•Kinetic
•Potential
• Internal
(Independent of Path)
Path Energies
•Work
•Heat
(Depend on Path)
Path Energies


Work is an energy flow across a boundary
from a driving force
 mechanical + shaft + hydraulic +
electrical + chemical + laser
Heat is an energy flow resulting from a
temperature driving force.
 conduction + blackbody radiation
Mechanical Work

Results from a force applied over a
linear distance
W   F dx
 F  dx, for constantF
 Fx
Shaft Work

Results from a twisting force (torque, T)
applied over a circular distance
W   F dx
distance traveled(x) is rθ and r is constant ,thus
dx  rdθ, so
W  Fr  dθ, for constantF
 Frθ
 Tθ, where T  Fr is T orque
θ
Pairs Exercise #1

A lawn mower engine is started by
pulling a cord wrapped around a
sheave. The radius of the sheave is 8.0
cm and the cord is wrapped around the
sheave twice. If a constant force of 90
N is applied to the cord, what work is
done?
Shaft Power


Power = work/time
Units are usually horsepower (hp)
shaft work
shaft power 
time
T

t
T

t
 T , where is therotationalvelocity(rad/s)
Pairs Exercise #2

How much power (in hp) is produced if
your automobile engine is producing
250 ft-lb of torque at 4000 rpm?
Hydraulic Work
W = F x
F = A(P2-P1)
A (area of piston face)
F
W = A(P2-P1)x
Volume of fluid that flows:
V=A x
W = V (P2-P1)
W = Qt (P2-P1)
x
P2
P1
Q = V/t (flow rate)
Pairs Exercise #3
Atmospheric-pressure water is pumped at rate
of 15 gal/min into a 60-psig storage tank.
A)
How much work (in ft-lb) is done by the
pump in one day assuming it pumps
constantly for the 24-h period?
B)
What “size” pump (i.e., power requirements
in hp) is required for this job?
Electrical Work
Welectric  F dx  Fx


In electricity, the driving force is a voltage potential
difference.
 Electrons move from lower to higher potential;
i.e., they flow uphill, just opposite to positive
charges
The force required to move a charge, q, a distance
x from voltage V1 to V2 is
F = q(V2-V1)/x
Electrical Work


Current [Amperes] is charge/time, thus
i = q/t, or q = it
Therefore electrical work is:
q(V2  V1 )
Welectric  Fx 
x
x
 q(V2  V1 )  it (V2  V1 )
Pairs Exercise #4


You buy a light bulb that uses 0.545 A
(current) for your porch light (110 V). You
leave your light on all weekend (48 hours).
The electric company charges you 5.75
cents per kW-h (kilowatt-hour).
How much does it cost to leave your light
on all weekend?
Lasers and Photons
LASER: Light Amplification by Stimulated
Emission of Radiation
The energy (E) of a photon (quantum of light)
with frequency n is
E = hn Joules
n = c/l
E = hc/l Joules
h = Planck’s constant = 6.62 x 10-34 J s
c = speed of light = 3.0 x 108 m/s
Heat


Heat is energy flow driven by temperature
differences
Two major types of heat flow:


Conduction
Blackbody radiation
Conduction

Conduction equation
k
A
Q/t
Q
T
 kA
t
x
k = thermal conductivity
x
T1 T2
Rate of heat conduction
Pairs Exercise #5

A 4-cm-thick insulator (k=2x10-4 cal
cm/(s cm2 oC) has an area of 1000 cm2.
If the temperature on one side is 170
oC and the temperature on the other
side is 50 oC, what is the heat transfer
by conduction?
Black Body and Blackbody Radiation



A black body adsorbs all incident electromagnetic radiation, regardless of frequency or
angle of incidence.
A black body in thermal equilibrium (that is, at
a constant temperature) emits electromagnetic
radiation called black-body radiation with a
spectrum that is determined by the
temperature alone, not by the body's shape or
composition.
Heat transfer between blackbodies:
Q
4
4
 A (T2  T1 )
t