1. ENGR 10 Lecture on Energy and Power
Ping Hsu
College of Engineering
San Jose State University
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2. 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’.
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3. Examples:
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4. 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:
Note: Energy’s unit is Joule.
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5. And… The work of lifting a mass of m kg by h meter requires
where g= 9.807, the gravitational constant.
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6. State Change (Work) Required Energy (J)
Creation of the Universe
1068
Starting earth moving in orbit
1033
Hiroshima Atomic Bomb Explosion
1014 (energy release)
Pushing a 2006 Honda Accord from 0 to 60pmh
5*105
Hard-hit baseball
103
Lifting an apple by 1 meter 1
Hopping flea (per hop)
10-7
(c) P.Hsu 2007

7. Other Energy Units Energy Unit Equivalent 1 Btu
(British Thermal Unit )
1055 joules
1 calorie
4.184 joules
1 food Calorie
1000 calories
1 kwh
(kilo-watt-hour)
3.6*106 joules
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8. 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
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9. 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.
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.
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10. 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.
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?
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11. Conservation of Energy
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,
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12. IF energy never disappears, why do we have energy crisis?
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.
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13. 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.
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14. 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.
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15. 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.
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18. 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.
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19. What is power?
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20. 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
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21. This electric heater’s power rating is 1500 J/s or watt.
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.
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22. This pump is rated 1500 g/m. Analogy: 1500 gallon per minute
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23. 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)
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24. 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

25. PG&E: Power costs to increase in '08 David R
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 $ Those using 1,000 kilowatt-hours per month would spend $2.70 more on their monthly electric bills, for a total of $
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26. 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)
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27. More Analogy It make sense to say: The tank has 500 gallon of water.
The pump is pumping 10 gallon per minute.
It does not make sense to say:
The tank has 10 gallon per minute of water.
The pump is pumping 500 gallon.
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28. (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
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29. (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.
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30. (Q3) Which equipment will cost you more to run
(Q3) Which equipment will cost you more to run? (A) a 5W equipment (B) a 50W equipment (C) a 100W equipment (D) Insufficient information.
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31. (Q4) John and Peter each has $1000 in his piggybank
(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.
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32. (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