Summary of the ‘Intro to Energy’ Lecture, EES 199 2007

Energy can be described as “the ability to do work” (it does not “define” energy).

We distinguished various forms of energy (kinetic, thermal) and relate those to the fundamental forces in nature.

‘Mass’ energy - E=mc², conversion of mass to energy through nuclear processes (fission, fusion, r.a. decay), all of which deliver heat (or mechanical energy in the form of explosions when things go wrong!). Reactions within the atom nucleus.

‘Chemical’ energy – exothermic chemical reaction release thermal energy, with the most familiar oxidation reactions (burning of carbon to make CO₂ + heat). Other reactions can deliver heat as well (e.g., mixing of water and sulfuric acid), not just oxidation. The release of heat in a chemical reaction comes from energy ‘stored’ in that particular compound. Reactions of electrons surrounding the nucleus of an atom.

‘Potential’ energy – ability to generate kinetic energy or heat because of elevated position in the gravitational field (e.g., rock rolling down, hydropower, also tidal power). This is a form of energy related to the force of gravity.

‘Electro-magnetic’ energy – electric motors (kinetic energy) or electric heaters (thermal energy). Flow of electrons (remember the Greek word for amber is ‘electron’ - ήλεκτρον).

Units of energy

SI unit - Joule (energy needed to move an object 1m in the direction of a force with a magnitude of 1 Newton) or to lift a sizeable apple 1 meter above the ground.

calorie – amount of heat needed to raise the temperature of one gram of water by 1 ^o C.

1 cal = 4.18 Joule

BTU - amount of heat needed to raise the temperature of one pound of water by 1 ^o F.

1 BTU = 1055 Joules

Quad – quadrillion BTU’s (10¹⁵) – common energy unit in global studies, roughly equivalent to an exaJoule (10¹⁸).

Watt – this is a unit of POWER, which is energy per time unit, or J/sec.

In electrical power studies the unit kilowatt-hour is commonly used, which is a contortion of (J/s)*hour and thus is simply an amount of Joules (3.6 MegaJoules). Electricity demand for countries is sometimes phrased as kilowatt-hours per year, which really gets our head spinning because now we are back at power units (Watts).

Energy conversion factors between the various sources of fossil fuel energy get really messy: barrel of oil, versus an American ton of coal (short ton or 2000 lbs) versus a metric ton of coal (1000 kg) versus a trillion cubic feet of gas and so on.

A common unit is the “toe” or ton of oil equivalent, which of course should be phrased in Joules, but nobody does that. We will deal with this when the time is there.

The human energy intake is of interest and our daily recommended diet is 2500 kcal, which is equivalent to 120 W. A pound of bodyfat is about 4200 kcalories (1.68 days of fasting will get rid of it), a run of a mile in 8 minutes burns away about 32 kcal or about 1.28 % of a daily diet.

Energy demand per capita in the USA is about 12000 W, in Europe 6000 W and in India 500 W. This demand can be translated into the use of per capita “energy-slaves” if that power had to be generated by human effort (USA: 100 slaves, Europe 50 and India 4).

Total energy received from the sun on earth is about 1.7 10¹⁷ W, amount used for photosynthesis is 4 10¹³ W, terrestrial heat flow is 3 10¹³ W and tidal energy is about 3 10¹² W. The energy demand in the USA alone is about 3 10¹² W, or 10% of the global terrestrial heat flux, or similar to all the energy locked up in living plants. Clearly, biofuels can never satisfy the world energy demand completely, and the only source large enough to cover our needs in the long run is the sun.

SI multiples of a Joule

Multiple	Name	Symbol	Multiple	Name	Symbol
10^0	joule	J
10^1	decajoule	daJ	10^–1	decijoule	dJ
10^2	hectojoule	hJ	10^–2	centijoule	cJ
10^3	kilojoule	kJ	10^–3	millijoule	mJ
10^6	megajoule	MJ	10^–6	microjoule	µJ
10^9	gigajoule	GJ	10^–9	nanojoule	nJ
10^12	terajoule	TJ	10^–12	picojoule	pJ
10^15	petajoule	PJ	10^–15	femtojoule	fJ
10^18	exajoule	EJ	10^–18	attojoule	aJ
10^21	zettajoule	ZJ	10^–21	zeptojoule	zJ
10^24	yottajoule	YJ	10^–24	yoctojoule	yJ

energy conversion and equivalence websites:

http://jumk.de/calc/energy.shtml

http://hyperphysics.phy-astr.gsu.edu/hbase/egex.html#eu

http://hyperphysics.phy-astr.gsu.edu/hbase/gaseng.html#ge

Human energy cycle and food website:

http://hyperphysics.phy-astr.gsu.edu/hbase/humeng.html#em