October 1, 2011

Units for Energy

The Sami of the Scandinavian polar circle region, also known as the Lapps, have hundreds of words for that which is so important in their lives, snow. Similarly, energy, which is so important to all humans, comes in a bewildering array of flavours, depending on its use. But unlike snow, energy is energy, and to go from one unit to the other just take the right translation.

Don't miss the fascinating list of energies, covering such things as the kinetic energy of a flying mosquito (10-7J); the kinetic energy of a person jumping as high as they can (390 J); the energy to accelerate a 4-ton truck to highway speed (0.9 MJ); the food energy in a Mars Bar (1 MJ) whoa!; the annual electricity consumption in the US in 2005 (1.37 x 1019J); the total solar energy striking the face of the earth daily (1.5 x 1022J).


1 cal = 4.184 J
It all started when a Frenchman, Nicolas Clément, defined the calorie (cal) as the energy it takes to heat a gram of water by one degree Celsius.

1 Cal = 4184 J = 4.184 kJ
The food calories we think about a lot is actually the kilocalorie (Cal), the energy it takes to heat a kilogram of water by one degree Celsius. So Cal = 1000 cal.

1 BTU = 1055 J
Not to be outdone by the French, the rivals across the Channel introduced their own version, the British Thermal Unit (BTU), which is the energy it takes to heat a pound (0.454kg) of water by one degree Fahrenheit. BTU is how energy is measured in the world of heating and air conditioning in the US. For instance, the burning of a US gallon of gasoline releases 115,000 BTU of heat.

1 therm = 105.5 MJ
Consistently eclectic (and to avoid having to use the continental prefix kilo), the British defined 1 therm = 100,000 BTU, perhaps taking a hint from the Lakh of the Indian subcontinent. You know the therm from your monthly gas bill, for one therm is about the energy released at the burning of 100 cubic feet of natural gas.

1 J = 1 J
The metric unit of energy, the Joule, is the amount of energy, or work, done by applying a power of one Watt over a time of one second:
1 J = 1 W.s . Boring, but functional.

1 kWh = 3.6 MJ = 3.6 x 106J
A kiloWatt-hour is a thousand Watts of power applied for an hour. Running your 1200 W hairdrier for half an hour makes your electricity meter go forward by 0.6 kWh. Turning on a 100W lightbulb for 3 hours is good for 0.3 kWh.

1 ton TNT = 4.184 GJ = 4.184 x 109J
Exploding one gram of tri-nitro-toluene releases an energy of 1000cal (or 1Cal), so a ton of TNT releases a million times that amount of energy.

1 eV = 1.6 x 10-19 J
One electron-Volt is the kinetic energy acquired by a free electron that is accelerated through an electric potential difference of one Volt. Electric car enthousiasts: a Nissan Leaf's fuel efficiency is 0.25 kWh/mi = 5.6 x 1024 eV /mi. You gotta have 5.6 yotta-eV to make one EV go one mile.

1 boe = 6.1 GJ = 1.7 MWh
One Barrel of Oil Equivalent is the energy released by burning a barrel (42 US gallons) of oil.

Gasoline gallon equivalent
This is complicated, and the subject of another post.

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