On Wednesday, June 11, 2014 1:11:12 PM UTC+5:30, Gregory Ewing wrote: > Steven D'Aprano wrote: > > Everything *eventually* gets converted to heat, but not immediately. > > There's a big difference between a car that gets 100 miles to the gallon, > > and one that gets 1 mile to the gallon.

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> With a car, the engine converts some of its energy to > kinetic energy, which is subsequently dissipated as heat, > so it makes sense to talk about the ratio of kinetic > energy produced to energy wasted directly as heat. > But when you flip a bit, there's no intermediate form > of energy -- the bit changes state, and heat is produced. > So all of the heat is waste heat. Actually the car-drive and the bit-flip are much more identical than different. Its just that the time-scales are minutes/hours in one case and nanoseconds or less in the other so our powers of visualization are a bit taxed. In more detail: One drives a car from A to B for an hour (assume no change in height above sea level so no potential difference). All the energy that was there as petrol has been dissipated as heat. A bit flips from zero to one. Pictorially (this needs to be fixed-pitch font!): +------------- | | | -----------+ However in reality that 'square' wave is always actually sloped +---------- / / / -----------+ Now for say CMOS technology, one may assume no currents in both zero and one states (thats the C in CMOS). However when its neither zero nor one (the sloping part) there will be current and therefore heat. So just as the car burns energy in going from A to B, the flipflop burns it in going from 0 to 1 Steven D'Aprano wrote: > Not the point. There's a minimum amount of energy required to flip a bit. > Everything beyond that is, in a sense, just wasted. You mentioned this > yourself in your previous post. It's a *really* tiny amount of energy: > about 17 meV at room temperature. That's 17 milli electron-volt, or > 2.7×10^-21 joules. In comparison, Intel CMOS transistors have a gate > charging energy of about 62500 eV (1×10^-14 J), around 3.7 million times > greater. > > Broadly speaking, if the fundamental thermodynamic minimum amount of > energy needed to flip a bit takes the equivalent of a single grain of > white rice, then our current computing technology uses the equivalent of > 175 Big Macs. Well thats in the same realm as saying that by E=mc² a one gram stone can yield 21 billion calories energy. [Ive forgotten how the units stack up, so as usual relyin on google instead of first principles: http://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#Practical_examples :-) ] ie. from a a pragmatic/engineering pov we know as much how to use Einstein's energy-mass-equivalence to generate energy as we know how to use Landauer's principle to optimally flip bits. -- https://mail.python.org/mailman/listinfo/python-list