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CHEMISTRY
Plenty of ways of doing work using chemical reactions
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PHYSICS & CHEMISTRY: RADIOACTIVE ELEMENTS IN THE EARTH
The Earth is around 4,500,000,000 years of age. Lord Kelvin & others tried to calculate the age of the Earth based on (their) contemporary physics & got results which were well short of the truth. One reason for this [but, not the only reason] was that radioactivity was unknown to them:
http://en.wikipedia.org/wiki/Age_of_the_Earth
Without radioactivity I suppose that Earth would no longer have a molten core, tectonic plates, earthquakes & volcanoes. The mountains would have worn down & there would be no mechanism to mountain-build. Also no magnetic field & the atmosphere of the Earth would be very different to today. Also I think the carbon cycle would be less active...
We tap the energy of these radioactive elements fairly directly by mining them & making them do work via a nuclear reactor. We also do so indirectly by using geothermal machines of various types such as harnessing the heat of hot springs. When we burn coal/oil/gas not all of that is stored solar energy - some % by a circuitous route is due to Earth radioactivity [I'll leave you to figure this out - the clues are in the previous paragraph]
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LUNAR TIDAL ACTION
The gravitational action of the Earth/Moon system creates movements in the Earth like squeezing an exercise ball. This generates heat inside the Earth & to a lesser extent in the moon. This same tidal action also influences water of course. We can tap the energy of the tides in the seas & oceans
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MISCELLANEOUS [miniscule amounts of energy]
Cosmic rays, electromagnetic spectrum from non-Sol space, meteors, gravitational tides with the other planets must generate some heat despite the distance-squared law
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What have I forgotten ?
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Michael
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