Power Consumption vs Power Dissipation

[Nominal Animal]

So when someone tells you that some gadget consumes energy, they are either lying, mislead, or they use some kind of made-up definition.

Or just using stupid, illogical language – just like people who say "twice more" when they actually mean "twice as much" (which includes basically all reporters writing in Finnish, even though the language actually supports the logically correct expressions).

In this particular case, that stupid language stems from the simplistic view that the device "eats" energy to produce an useful effect.  Which is not true, because devices just convert energy in some form to some other different forms, one of the resulting forms always being heat.  If the purpose is not to produce heat, then that heat is usually called "waste heat".

To translate from this stupid language, "power consumption" is the amount of energy one needs to supply to the device, and "power dissipation" is the amount of waste heat it produces.

In laptop, desktop and server processors, TDP (thermal design power) is a related term.  The instantaneous heat produces varies a lot, because the same processor package contains also connectivity logic, cache RAM, and so on.  TDP indicates the amount of waste heat any cooling system should be able to dissipate, so that the semiconductor junction temperatures in the package do not exceed their operating limits.  It is nowhere near exact, as it depends on what the processor and its subsystems are doing at any point in time, but it is a widely accepted term to approximate the "power dissipation" in a laptop/desktop/server processor.

Almost all of the input power in computers is "dissipated", turned into waste heat.  As discussed above, a little bit of it goes to producing light, sound, or electromagnetic waves (radio, WiFi, LTE).  When driving external buses most of the energy is turned into waste heat in the transceivers and transmission lines.  The majority of it becomes waste heat.

(If you live in cold climate, and often use electricity to heat your house, placement and configuration of the computer matters a lot.  Indeed, I've helped build one machine that had inverted airflow –– intake at the top, exhaust at the bottom –– just because it happened to make sense for its use case.)

If the circuit OP discussed does anything useful other than heat up, then indeed not all of the "consumed power" is "dissipated" as waste heat.  Sometimes, the difference may be tiny, but of vast importance to humans.  Think of things like LCDs: changing the crystal orientation with an electric field takes very little energy, but to do so in an useful way requires quite a bit of processing, maybe even a radio receiver (for a radio-synchronized clock).  Or better yet, an e-ink reader!  At steady state, it really requires no energy; it only takes energy to change its state.  To make the new state meaningful, the energy conversions need to occur in rather interesting patterns; thus, the change can take quite a lot of energy.  In such cases, you would be technically correct to say that basically all energy "consumed" by the device is "dissipated" as waste heat; but, in human terms, the fact that the tiny bit that was not, produces the desired and useful effects (the new picture we can read or look at), means that that tiny bit is worth all the waste heat!

With that in mind, I do suspect that the underlying cause of the disagreement between tarun172 and their colleagues is that they did not specify clearly enough the domain in which they consider the circuit or system.  Very little energy is needed to produce many human-useful and desired effects, compared to the amount of energy converted to waste heat.  However, humans definitely see the waste heat as worth it, to get the desired effect.  So, do you and your colleagues consider that tiny bit of non-heat energy as significant or not?  One answer equates e-readers and big resistors connected to a Li-ion battery, the other distinguishes clearly between them.  Both answers/viewpoints are correct, just in different domains.

[Nominal Animal]

TL;DR: tarun172, ask your colleagues whether they are talking in quantitative or qualitative terms.

The tiny fraction of input energy that is not turned into waste heat in electronic circuits, matters a lot to us humans (and is thus very important in qualitative terms), because our devices are horribly energy-inefficient.  In quantitative terms, indeed basically all input energy is turned into waste heat in electronic circuits, unless the circuit itself provides energy to other devices or circuits.

[magic]

By that token, talking about food consumption also makes no sense. You just transform it into... some other things

[Nominal Animal]

By that token, talking about food consumption also makes no sense. You just transform it into... some other things

If we look at Wiktionary for consume,

• To use up
• To eat
• To destroy completely
• To waste away slowly
• To trade money for good or services as an individual
• To absorb information, especially through the mass media

then I have to disagree.  The definition works for food and fuels (tangible matter), but not so much for electrical energy.

Then again, it is used for electrical energy in common speech as well, so I must admit my opinion is the minority one.

Languages can be a pain in the butt, so to speak. 

[Zero999]

Probably less than 5% of the electrical input ends up being "stored" in the potential energy of the pressure. And even this just slowly dissipates, it is never let to do any work (that's not the purpose).

You totally can deliberately store energy in compressed air, but that requires something to convert the energy back to some useful form of energy. Tire is not example of such energy storage, though.

Not to derail the thread, but there is an interesting paradox here.

The pressure energy stored in volume of gas is given by pressure x volume (e.g. N/m2 x m3 => Nm => J).

But compressed air more or less follows the ideal gas law, and the ideal gas law (Boyle's law) says that at a given temperature, pressure x volume remains constant (double the pressure, halve the volume).

The conclusion is that compressed air contains no more pressure energy than uncompressed air.

How to explain this entirely unintuitive conclusion? Why do pressure vessels explode with great force?

The answer is that the energy of expanding/exploding compressed air comes from the temperature of the air, not the pressure as such. When the air is compressed its free energy increases, which means the ability to access the thermal energy contained within it increases. For a given temperature the thermal energy is the same in the compressed and uncompressed state, but in the compressed state the stored internal (thermal) energy is more available to do work.

It's also why decompressing a gas causes its temperature to drop: the heat within it becomes distributed over a much larger volume. If the same heat energy in a larger vs smaller volume, the larger volume will be cooler.

[IanB]

It's also why decompressing a gas causes its temperature to drop: the heat within it becomes distributed over a much larger volume. If the same heat energy in a larger vs smaller volume, the larger volume will be cooler.

The temperature does drop when decompressing a gas, but not for that reason. The internal (heat) energy of an ideal gas is a function only of temperature. Therefore the heat energy contained within the gas is independent of the volume it occupies, and for the same heat energy in a larger vs smaller volume, both volumes will have the same temperature.

The reason a gas cools down when it expands is that the gas has to do work against the surroundings as it expands (the gas is pushing out and the surroundings are pushing back). The energy to do this work comes from the internal heat energy of the gas: this energy is depleted and the gas cools down.

This process is neatly illustrated in a heat engine like an internal combustion engine. The heat energy from the burning fuel enables the combustion gases to do work pushing down the piston in the cylinder, which through the transmission is transferred to the wheels of the vehicle. Thus the heat energy released by burning gasoline is transferred to kinetic energy in forward motion of the car.