I have a hardware question about CPU and chips in general: what is causing the extra heat when CPUs are busy doing computations ?

I thought that the CPUs would have a way to throttle themselves when they are not busy but it seems that only applies to a few CPUs, so my question is how do CPUs manage to generate less heat when performing less computations and what causes the heat in the hardware when the workload increases ?

EDIT: as per the comment below, it seems that most of modern CPUs can throttle so it is not limited to the mobile editions.

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    @BlueTin - Most if not all Intel and AMD CPUs have the ability to throttle their clockspeed. Your conclusion that this feature is limited to the mobile prouct line is incorrect. – Ramhound Jul 2 '13 at 11:47
  • @Ramhound: I will edit my answer, thanks for correction, I repeated what I read on some random website. – BlueTrin Jul 2 '13 at 12:03

You can think of a modern CPU as a series of hundreds of millions of switches. As the CPU operates, the switches open and close each other in complex patterns.

The heat is caused by electrical current passing through a resistance. When a switch is closed, it has almost no resistance, so almost no heat is generated. When a switch is open, almost no current flows through it, so almost no heat is generated. But if you try to open and close it very quickly, you pass through a state in which current is flowing but there is still resistance. This is what generates the heat.

The more switches that are opening and closing, the faster they try to open or close, and the more often they open and close, the more heat generated.

  • That's basically correct. The heat generated is the waste-product of the (electrical) effort to open/close the switch. – Tonny Jul 2 '13 at 11:26
  • These switches are also made of metal. You flow electrons through metal and heat is generated ( because the collision with protons ). In other words electricity generates heat. This is the reason say water expelled at the Hoover Dam is warmer then the water that goes into the Hoover Dam. Yes...Less conductive a metal is the less heat it would be produced. Of course there would draw backs to a less conductive metal being used. – Ramhound Jul 2 '13 at 12:23
  • not sure if that still correct now, since at very high speed, modern CPU, the logic is not only working at close and open anymore. Here is some info: (en.wikipedia.org/wiki/Logic_family) , but anyway, it is still very outdated information. – user218473 Jul 2 '13 at 15:25
  • @AntonyLee: I debated whether to try to explain gate capacitance. Perhaps a better form of explanation would be saying it takes energy to open and close the switches. – David Schwartz Jul 2 '13 at 18:11

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