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Today we had a weird discussion over lunch: What exactly causes power consumption in a computer, particularly in the CPU? (ETA: For obvious reasons I don't need an explanation why a hard drive, display or fans consume power – the effect there is pretty obvious.)

Figures you usually see indicate that only a percentage (albeit a large one) of the power consumption ends up in heat. However, what exactly does happen with the rest? A CPU isn't (anymore) a device that mechanically moves parts, emits light or uses other ways of transforming energy. Conservation of energy dictates that all energy going in has to go out somewhere and for something like a CPU I seriously can't imagine that output being anything but heat.

Us being computer science instead of electrical engineering students certainly didn't help in accurately answering the question.

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very good physics question :) –  kokbira Apr 27 '11 at 17:39
    
Hei, my answer id deleted. It's the only correct answer. I know physics. C'mon. I am an electrical engineer here. –  Jim Thio Jul 25 '12 at 10:41
    
"only a percentage (albeit a large one) of the power consumption ends up in heat." is a bit of an understatement. For the computer enclosure (or for the CPU for that matter) it is 100% for all practical purposes. There is a tiny 'rest' amount in the form of radiation emanating from the device. For an LCD the monitor it is only slightly less because it emits light. –  Jan Doggen Feb 7 at 15:58

9 Answers 9

up vote 25 down vote accepted

Electrons are being pushed around, that requires work. And the electrons experience "friction" as they move around, needing more energy.

If you want to push electrons into a PNP junction in order to turn it on, that requires energy. The electrons don't want to move, and they don't want to move closer together; you have to overcome their mutual repulsion.

Take the simplest cpu, a single, lone, transistor:

alt text

Electrons lose energy as they bump around, generating heat. And overcoming the electric fields of attraction and repulsion requires energy.

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so, to make it short: mainly heat :) –  akira Jun 2 '10 at 15:41
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This still doesn't explain where the rest of the energy goes, ie. what isn't lost to heat. The work you mention eventually turns in to heat and the work/energy you put in to overcome e-e repulsion doesn't disappear; it could be reused, like when you release a spring. Maybe it isn't reused - the spring is released into thin air? –  trolle3000 Jun 2 '10 at 15:43
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Energy = heat, light (RF radiation), noise (vibration) and the output signal that eventually becomes stored, transmitted and/or displayed. However, I think you underestimate the amount of heat given off by a PC. –  Chris Nava Jun 2 '10 at 16:26
    
@Chris Nava: and i think by a huge percentage :) –  akira Jun 2 '10 at 19:13
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The energy isn't regained by you, or the power supply. Yes it's converted back into electrical energy, it's not in a position to be used by you. –  Ian Boyd Jun 6 '10 at 11:40

My understanding is that the vast majority of the energy use by a CPU is output as heat. To do work a physical system converts or moves energy - the CPU does work by converting electrical energy into heat, changing it's internal state a large number of times along the way (so some of the energy is effectively stored for a time that way).

Caveat: my practical electronics and physics training stopped around age 20 over a decade ago, unless you count reading New Scientist, so a passing physicist may be about to tell me I'm completely wrong!

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Heat energy is almost always a waste product: if we could move electrons around inside a CPU without creating heat, we'd do it in a second. –  Satanicpuppy Jun 2 '10 at 13:59

A lot of it also goes to moving your hard drive and fans, and lighting up your monitor.

Some of it goes to transmitting data over the network. Think about how much power a large radio station needs for this. The computer is doing the same thing with network data, even if it's on a much smaller scale over an ethernet line or wifi antenna.

Moreover, paths within the cpu and motherboard work pretty much the same way as the network transmissions. It takes energy to move electrons down those paths. An electron may not have much mass, but you're moving billions of them, and doing it billions of times per second.

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There is also energy used in in turning memory bits on and off, plus the CPU memory must continue to use power to maintain the current memory even when nothing else is being processed. I was unable to find figures, but you have me interested now so if I do find something I will add it.

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There's an interesting article on wikipedia about Landauer's principle wich states that (quote):

"any logically irreversible manipulation of information, such as the erasure of a bit or the merging of two computation paths, must be accompanied by a corresponding entropy increase in non-information bearing degrees of freedom of the information processing apparatus or its environment"

This means that (quote):

Specifically, each bit of lost information will lead to the release of an amount kT ln 2 of heat, where k is the Boltzmann constant and T is the absolute temperature of the circuit.

Still quoting:

For, if the number of possible logical states of a computation were to decrease as the computation proceeded forward (logical irreversibility), this would constitute a forbidden decrease of entropy, unless the number of possible physical states corresponding to each logical state were to simultaneously increase by at least a compensating amount, so that the total number of possible physical states was no smaller than originally (total entropy has not decreased).

So, as a consequence of the Second Law of Thermodynamics (and Landauer), some types of computations cannot be done without generating a minimum amount of heat, and this heat is not a consequence of internal CPU resistance.

Cheers!

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I believe that every kind of computation can be transformed to reversible computation: "Although achieving this goal presents a significant challenge for the design, manufacturing, and characterization of ultra-precise new physical mechanisms for computing, there is at present no fundamental reason to think that this goal cannot eventually be accomplished, allowing us to someday build computers that generate much less than 1 bit's worth of physical entropy..." en.wikipedia.org/wiki/Reversible_computing –  Infragile Nov 15 '13 at 8:25

To add to the other excellent answers:

Figures you usually see indicate that only a percentage (albeit a large one) of the power consumption ends up in heat. However, what exactly does happen with the rest?

Actually, almost everything ends up in heat. By the law of Conservation of energy, all the energy (which is power multiplied by time) has to end up somewhere. Almost all processes inside a computer end up turning the energy into heat, directly or indirectly. For example, the fan will turn energy into moving air (=kinetic energy), however the moving air will be stopped by friction with the surrounding air, which will turn its kinetic energy into heat.

The same goes for things like radiation (light from the monitor, EM radiation from all electrical components) and sound (noises, sound from loudspeakers) a computer produces: They too will be absorbed and transformed into heat.

If you read of a "percentage" that ends up in heat, that may have referred to the power supply alone. The power supply should indeed turn a large percentage of its input into electrical power, not into heat (though it does produce some heat as well). This energy will then be turned into heat by the rest of the computer :-).

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An eariler respondent indicated almost everything ends up in heat. That's almost correct. In fact, all the power input ends up as heat eventually. The fan was a good example. The fan will turn energy into moving air (=kinetic energy), however the moving air will be stopped by friction with the surrounding air, which will turn its kinetic energy into heat. The same concept applies to light from the monitor etc. If you put a computer system drawing 250 watts of power into a sealed room, the net result is the same as putting a 250 watt heater in the room.

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Yes yes, a CPU converts a lot of the electricity it absorbs into heat. We all know that; that’s why we have such crazy cooling devices attached to the CPU now.

However you’re missing the most basic principle of electronics.

Your debate makes it sound like when electricity enters a light or motor, all of it is converted to light or kinetic energy, which is not the case. Think about a simple electric circuit: a device (any device) attached to a battery. Where does the electricity go? It doesn’t stop at the device; some of it is used to do whatever it is the device does, but the rest continues through the wire, back to the battery (hence the closed circuit).

A computer is no different. Charge carriers come in through the mains, enter the PSU, then to the CPU where they do their work, create heat in the process, then the rest comes out, back to the PSU, and back out to the mains.

Ian Boyd had a good start by pointing at a transistor, but did not follow it up with a tangible explanation of what exactly the electricity is used for (the “payoff” of the device, specifically as an analogy to the movement of a fan or light of an LED). You can do a little research into how a transistor works to really understand it, but suffice it to say that the electricity is used to physically alter the atomic arrangement of part of the transistor to allow or block electron flow. Granted its “action” is not nearly as clear or obvious as movement or light, but the energy is still used to do something (and as Ian mentioned, a bunch of heat is created when you push atoms around). I’ve seen some SEM photos of a CPU gate in action which really helps to visualize things; if I can find one, I’ll add it.

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Computation is heat. While of course not all heat is computation. So the only logical answer to; How much is lost to heat? The answer is all of it.

Computation is organized heat. In the form of data. What we consider to be waste heat is just disorganized data and not used for computation.

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