Energy

The Physics Hypertextbook™
© 1998-2008 by Glenn Elert -- A Work in Progress
All Rights Reserved -- Fair Use Encouraged

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Discussion

A system possesses energy if it has the ability to do work.

Work shifts energy from one system to another.

Energy is …

• a scalar quantity,
• abstract and cannot always be perceived,
• given meaning through calculation,
• a central concept in science.

Energy can exist in many different forms. All forms of energy are either kinetic or potential. The energy associated with motion is called kinetic energy. The energy associated with position is called potential energy. Potential energy is not "stored energy". Energy can be stored in motion just as well as it can be stored in position. Is kinetic energy "used up energy"?

• kinetic energy -- motion
  • mechanical energy -- motion of macroscopic systems
    • machines
    • wind energy
    • wave energy
    • sound (sonic, acoustic) energy
  • thermal energy-- motion of particles of matter
    • geothermal energy
  • electrical energy -- motion of charges
    • household current
    • lightning
  • electromagnetic radiation -- disturbance of electric and magnetic fields (classical physics) or the motion of photons (quantum physics)
    • radio, microwaves, infrared, light, ultraviolet, x-rays, gamma rays
    • solar energy
• potential energy -- position
  • gravitational potential energy
    • roller coaster
    • waterwheel
    • hydroelectric power
  • electromagnetic potential energy
    • electric potential energy
    • magnetic potential energy
    • chemical potential energy
    • elastic potential energy
  • strong nuclear potential energy
    • nuclear power
    • nuclear weapons
  • weak nuclear potential energy
    • radioactive decay

Another scheme (economic)

• solar
  • sunshine
  • wind
  • ocean currents
  • ocean thermal temperature gradients
  • biomass
    • food
    • wood/charcoal
    • dung
  • fossil fuels
    • coal
    • petroleum
    • natural gas
• everything else
  • geothermal
  • tidal
  • nuclear

units

For those who want some proof that physicists are human, the proof is is the idiocy of all the different units which they use for measuring energy. -- Richard Feynman.

Energy Units Discussed in This Book
unit	joule equivalent			other equivalent		reference
joule	1	J			N·m	work
erg	0.1	μJ			dyne·cm	"
foot pound	1.35582	J	(approximate)			"
watt second	1	J				power
watt hour	3.6	kJ			3600 W·s	"
kilowatt hour	3.6	MJ			1000 W·h	"
thermochemical calorie	4.184	J	(by definition)			heat
International Table calorie	4.1868	J	(by definition)			"
kilocalorie*					1000 calorie	"
thermochemical Btu**	1.054350	kJ	(approximate)			"
International Table Btu**	1.05505585262	kJ	(by definition)			"
therm					100,000 Btu	"
quad***	1.055	EJ	(approximate)		1015 Btu	"
cubic meter natural gas	37 - 39	MJ	(variable)			chemical
ton of oil equivalent	41 - 45	GJ	(variable)			"
ton of coal equivalent	29.3	GJ	(approximate)			"
ton of TNT	4.184	GJ	(by definition)			"

*	The kilocalorie is also known as the kilogram calorie, dietetic calorie, food calorie, and Calorie (with an uppercase "C").
**	Btu is the abbreviation for "British thermal unit".
***	Quad is the shortened form of "quadrillion Btu".

 

Energy Equivalents from Atomic and Nuclear Physics
unit	symbol	joule	equivalent
atomic mass unit	u	muc2	= 1.492 × 10−10 J
electron volt	eV	e	= 1.602 × 10−19 J
hartree	Eh	2R∞hc	= 4.35974381 × 10−18 J (exact)
inverse meter	m−1	hc	= 1.986 × 10−25 J
inverse second	s−1	h	= 6.626 × 10−34 J
kelvin	K	k	= 1.381 × 10−23 J
kilogram	kg	c2	= 89,875,517,873,681,764 J (exact)

Where:	c	=	speed of light in a vacuum	k	=	Boltzmann constant
	e	=	elementary charge	mu	=	atomic mass unit
	h	=	Planck constant	R∞	=	Rydberg constant

 

Summary

• bullet

Problems

practice

1. Write something.
   • Answer it.
2. Write something else.
   • Answer it.
3. Write something different.
   • Answer it.
4. Write something completely different.
   • Answer it.

worksheets

1. energy-transformations.pdf
   Identify a device, phenomenon, process, or event whereby one form of energy is transformed into another.

Resources

• general
  • Momentum, Work and Energy, Michael Fowler, University of Virginia
• renewable/sustainable energy
  • FPL Energy (Florida Power and Light), largest wind-electric utility in the US
  • National Renewable Energy Laboratory (NREL)
  • Seabased Energy AB, using oceanwaves to generate electricity
• statistics
  • Annual Energy Review, Energy Information Administration (USDOE)
  • Data and Story Library (DSTL)
    • Nuclear Power Story & Nuclear Plants Datafile
    • Oil production Story & Oil production Datafile
  • Energy in the United States 1635-2000, Energy Information Administration
  • Energy Statistics, International Energy Agency (IEA)

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