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ENGR 10 Lecture on
Energy and Power
Ping Hsu
College of Engineering
San Jose State University
(c) P.Hsu 2007
What is energy?
Energy is what it takes to change the
physical state of an “object”. In some
cases, energy is released during such a
change. Change of the physical state is
called ‘work’.
(c) P.Hsu 2007
Examples:
0 mph
50 mph
80 mph
65 mph
Energy
Energy
1 ton
1 ton
Gas, air
(oxygen)
Energy
Energy
30ºF
Exhaust gas
78ºF
H
Energy
O2
H2O
Energy
(c) P.Hsu 2007
A mechanical system example:
The “work” of changing the state of a mass of m
kg from standing still to moving at a speed of v
m/sec. requires:
1 2
Energy  mv
2
Note: Energy’s unit is Joule.
(c) P.Hsu 2007
And…
The work of lifting a mass of m kg by h
meter requires
Energy  mgh
where g= 9.807, the gravitational constant.
(c) P.Hsu 2007
State Change (Work)
Required Energy (J)
1068
Creation of the Universe
Starting earth moving in orbit 1033
Hiroshima Atomic Bomb
Explosion
Pushing a 2006 Honda
Accord from 0 to 60pmh
Hard-hit baseball
1014 (energy release)
Lifting an apple by 1 meter
1
Hopping flea (per hop)
10-7
5*105
103
(c) P.Hsu 2007
Other Energy Units
Energy Unit
Equivalent
1 Btu
1055 joules
(British Thermal Unit )
1 calorie
4.184 joules
1 food Calorie
1000 calories
1 kwh
(kilo-watt-hour)
3.6*106 joules
(c) P.Hsu 2007
Where does energy come from and
where does it go?
• Theoretically, the question ‘where does energy come
from?’ is the same question ‘where does the universe
come from?
• In a more practical term, energy is ‘stored’ in various
forms around us.
Energy Content (J)
Gallon of gasoline
1.3 x 108
Pound of coal
1.6 x 107
Candy bar
2 x 105
AA battery
103
A passenger car at 60mph
5 x 105
(c) P.Hsu 2007
Where does Energy go?
Energy ‘used’ in the physical state transformation
process is ‘contained’ in the new physical state of the
objects.
Example:
It takes 5x105J of energy to move a Honda Accord from
0 to 60mph. This amount of energy is contained in this
moving car. This energy is called Kinetic Energy.
Example:
The energy that was used in lifting a weight is ‘stored’ in
the new state (a higher elevation) of the weight. This
energy is called Potential Energy.
(c) P.Hsu 2007
Example:
The energy that was used in heating up a room
(from burning wood, e.g.) is stored in the warn
air in the room.
Example:
The energy that was used for bringing you from
home to school this morning is contained in your
car parked in the garage right now. Correct?
(c) P.Hsu 2007
Can energy be destroyed? No!
Conservation of Energy
Energy never vanishes.
Energy only changes into
different forms.
• ‘Efficiency’ of a energy
conversion is the ratio
between the part of the
energy that caused the
desired effect and the total
energy used, i.e., in this
example,
=
Gas,
air
Exhaust
gas
Heated
engine
Stirred up
air
0 mph
Kinetic energy of the car
Total energy released
from gas combustion (c) P.Hsu 2007
50 mph
IF energy never disappears, why
do we have energy crisis?
Energy Conservation Car
Never Needs Gas!
Some form of energy is difficult or even impossible
to transform efficiently to a re-useable form. The
energy in the exhaust gas from a car engine or
heating of the brake are such form of energy. When
energy is converted into such a form, it is essentially
lost, from a practical point of view.
(c) P.Hsu 2007
How difficult is it to convert energy from
one form to another (a more useful) form?
Very Easy:
Burning wood, coal, fossil fuel, potential energy stored in
the water in a reservoir, nuclear reaction, energy stored
in a battery, etc.
Not difficult but cost more:
Solar, wind (kinetic energy of the air mass), ocean
current (kinetic energy of water).
Impossible to transform efficiently to a ‘useful’ form:
Energy in the exhaust gas from a car engine.
Energy in the warm ocean water in a tropical region.
(c) P.Hsu 2007
Energy Conversion Machines
Some energy conversion can be done by
a natural process such as burning wood
for heat.
To better suit our need, we build machines
to facilitate, manage, and control the
energy conversion.
(c) P.Hsu 2007
For example, in a car’s engine:
– The spark plugs initiate the combustion which
release energy form the gasoline.
– The cylinders and pistons transform the
expansion force of the combusted fuel/air to
mechanical rotational force (torque).
– The cooling systems takes the heat energy ( a
byproduct) away from the engine.
(c) P.Hsu 2007
Gas,
air
Exhaust
gas
50 mph
0 mph
25ºC
Break
200ºC
Break
Heated
engine
Stirred up
air
0 mph
50 mph
(c) P.Hsu 2007
Fast
moving air
(wind)
30ºF
Slower
moving air
78ºF
(c) P.Hsu 2007
Key Concepts
• Energy is what it takes to (or releases from) change
the physical state of an “object”.
• Energy cannot be ‘created’ or ‘destroyed’.
• Efficiency is the ratio between the part of output
energy that is beneficial to us and the total used
energy
• We build machines to ‘manage’ energy conversion.
• While energy cannot be destroyed, once it is
transformed into a certain form (heat, often the
case), it is basically lost.
(c) P.Hsu 2007
What is power?
(c) P.Hsu 2007
It takes time to convert energy from
one form to another!
‘Power’ is a measure of how fast energy
is converted. The unit of power is WATT.
1 watt = 1 Joule/second
(c) P.Hsu 2007
Example:
1500J of
electrical
energy per
second
1500J heat
energy per
section
This heater converts 1500J of electrical
energy into heat energy per second.
This electric heater’s power rating is
1500 J/s or watt. (c) P.Hsu 2007
Analogy:
1500 gallon
per minute
This pump is
rated 1500 g/m.
1500 gallon
per minute
(c) P.Hsu 2007
Other Units of Power
horse power (HP)
1HP = 746 Watts
(Used mostly for mechanical systems)
Kilo-Watt (KW)
1KW = 1000 watts
BTU per Hour (BTUH)
1BTUH = ? watts
(Used mostly for thermal systems)
(c) P.Hsu 2007
Back to Energy for a moment
Kilo-Watt-Hour (kwh) is a unit of energy.
Since 1w=1Joule/second,
1 kwh=1000 (J/s) * 3600 (s) = 3.6*106 joules
You are charged by PG&E monthly by the amount
of energy in kwh. You are paying about 25
cents for 1kwh of energy these days.
(c) P.Hsu 2007
PG&E: Power costs to increase in '08
David R. Baker, Chronicle Staff Writer
9/1/07
According to PG&E's estimates, homeowners
using 560 kilowatt-hours per month would
see their electricity bills rise by 28 cents, to
$74.54. Those using 1,000 kilowatt-hours per
month would spend $2.70 more on their
monthly electric bills, for a total of $199.83.
(c) P.Hsu 2007
Energy vs. Power
The information about a certain
amount of energy (J) does not
involve a sense of time.
(Analogy: 100 gallon tank )
Power (J/s=w), on the other hand,
tells how fast the energy being
converted.
(Analogy: 2 gallon/sec. pump)
(c) P.Hsu 2007
More Analogy
It make sense to say:
Tank
•The tank has 500 gallon of water.
•The pump is pumping 10 gallon per
Pump
minute.
It does not make sense to say:
•The tank has 10 gallon per minute
of water.
•The pump is pumping 500 gallon.
(c) P.Hsu 2007
(Q1) Which of the following heaters
can heat up a gallon of water to 90oC.
(A) 5W heater
(B) 90W heater
(C) 100W heater
(D) All of the above
(c) P.Hsu 2007
(Q2) Which of the following heaters can
heat up a gallon of water from 10oC to
90oC faster?
(A) 5W heater
(B) 90W heater
(C) 100W heater
(D) All heaters above will take the same
amount of time.
(c) P.Hsu 2007
(Q3) Which equipment will cost you
more to run?
(A) a 5W equipment
(B) a 50W equipment
(C) a 100W equipment
(D) Insufficient information.
(c) P.Hsu 2007
(Q4) John and Peter each has $1000 in his
piggybank. They decided to use the money to
buy music CDs. Each CD cost exactly $5.
John took $20 out of the piggybank each week
and Peter, $50. Who will have the most CDs?
(A) John
(B) Peter
(C) Same
(D) Insufficient information.
(c) P.Hsu 2007
(Q5) Both your and your neighbor's basement
are filled with water from the storm. You are
given a 5HP gasoline power pump and your
neighbor, a 10HP pump. Each of you is given
only 1 gallon of gasoline. If both pumps have
the same efficiency, which one can pump more
water out of the basement ?
(A) You
(B) Your neighbor
(C) Same
(D) Insufficient information.
(c) P.Hsu 2007