Megajoule

Megajoule

For the physicist named James Prescott Joule, see James Prescott Joule.

For the electric car, see Optimal Energy Joule.

The joule (written in lower case, pronounced /ˈdʒuːl/ or /ˈdʒaʊl/) (symbol: J) is the SI unit of energy measuring heat, electricity and mechanical work. It was named after English physicist James Prescott Joule.

1 Description
2 Conversions
- 2.1 Practical examples
- 2.2 SI multiples
3 See also
4 References
5 External links

Description

One joule is the work done, or energy expended, by a force of one newton moving one meter along the direction of the force. This quantity is also denoted as a newton meter with the symbol N·m. Note that torque also has the same units as work, but the quantities are not the same. In elementary units:

$\, 1\, \mathrm{J}=1\, \mathrm{N} \cdot \mathrm{m}$

$\, 1\, \mathrm{J}=1\, \mathrm{kg} \cdot \frac{\mathrm{m}^{2}}{\mathrm{s}^{2}}$

One joule is also:

The work required to move an electric charge of one coulomb through an electrical potential difference of one volt; or one coulomb volt, with the symbol C·V.
The work done to produce power of one watt continuously for one second; or one watt second (compare kilowatt hour), with the symbol W·s. Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules
The kinetic energy of a 2 kg mass moving at a velocity of 1 m/s. The energy is linear in the mass but quadratic in the velocity, being given by E = ½mv².
The potential energy of a 1 kg mass at an elevation of 1 m above a reference point in a gravitational field of 1 m/s². Earth's gravity being 9.81 m/s² at sea level, 1 kg at 1 m above the Earth's surface has a potential energy of 9.8 joules relative to that surface. When dropped the potential energy gradually becomes kinetic energy, with the conversion being complete at the instant the mass passes the reference point. Whereas kinetic energy is relative to an inertial frame, in this case that of the earth, potential energy is relative to a position, in this case the Earth's surface.

Conversions

Main article: Conversion of units#Energy

1 joule is exactly 10⁷ ergs.

1 joule is approximately equal to:

6.2415 ×10¹⁸ eV (electronvolts)
0.2390 cal (calorie) (small calories, lower case c)
2.3901 ×10⁻⁴ kilocalorie, Calories (food energy, upper case C)
9.4782 ×10⁻⁴ BTU (British thermal unit)
0.7376 ft·lbf (foot-pound force)
23.7 ft·pdl (foot poundals)
2.7778 ×10⁻⁷ kilowatt hour
2.7778 ×10⁻⁴ watt hour
9.8692 ×10⁻³ litre-atmosphere

Units defined in terms of the joule include:

1 thermochemical calorie = 4.184 J
1 International Table calorie = 4.1868 J
1 watt hour = 3600 J
1 kilowatt hour = 3.6 ×10⁶ J (or 3.6 MJ)
1 ton TNT = 4.184 GJ

Useful to remember:

1 joule = 1 newton meter = 1 watt second

Practical examples

One joule in everyday life is approximately:

the energy required to lift a small apple one meter straight up.
the energy released when that same apple falls one meter to the ground.
the energy released as heat by a quiet person, every hundredth of a second.
the energy required to heat one gram of dry, cool air by 1 degree Celsius.
one hundredth of the energy a person can receive by drinking a drop of beer.
the kinetic energy of an adult human moving a distance of about a handspan every second.

SI multiples

SI multiples for joule (J)

Submultiples			Multiples
Value	Symbol	Name	Value	Symbol	Name
10^–1 J	dJ	decijoule	10¹ J	daJ	decajoule
10^–2 J	cJ	centijoule	10² J	hJ	hectojoule
10^–3 J	mJ	millijoule	10³ J	kJ	kilojoule
10^–6 J	µJ	microjoule	10⁶ J	MJ	megajoule
10^–9 J	nJ	nanojoule	10⁹ J	GJ	gigajoule
10^–12 J	pJ	picojoule	10¹² J	TJ	terajoule
10^–15 J	fJ	femtojoule	10¹⁵ J	PJ	petajoule
10^–18 J	aJ	attojoule	10¹⁸ J	EJ	exajoule
10^–21 J	zJ	zeptojoule	10²¹ J	ZJ	zettajoule
10^–24 J	yJ	yoctojoule	10²⁴ J	YJ	yottajoule
Common multiples are in bold face

This SI unit is named after James Prescott Joule. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (J). When an SI unit is spelled out in English, it should always begin with a lowercase letter (joule), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase.

— Based on The International System of Units, section 5.2.

References

The adoption of joules as units of energy, FAO/WHO Ad Hoc Committee of Experts on Energy and Protein, 1971. A report on the changeover from calories to joules in nutrition.

Contents

Description

Conversions

Practical examples

SI multiples

See also

References

External links