What Makes A CPU Get Hot?

This is a discussion on What Makes A CPU Get Hot? within the Tech Board forums, part of the Community Boards category; Hello, This little thought just crossed my mind. During day to day Windows usage, my CPU fan stays relatively quiet, ...

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    What Makes A CPU Get Hot?

    Hello,

    This little thought just crossed my mind. During day to day Windows usage, my CPU fan stays relatively quiet, yet it is supposedly still working at 2GHz. Then if I run a program that performs some complex calculations and doesn't voluntarily yield its timeslice, the CPU fan starts to sound like a vacuum cleaner.

    But what is actually happening that's creating excess heat? The CPU is still running at 2GHz, yet the fact that it's not in an OS's idle loop (which still has to be executed AFAIK) means the temperature shoots up. Maybe it's elementary electronics or something, but I don't get it.

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    Reverse Engineer maxorator's Avatar
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    Well, I have "AMD Cool'n'Quiet" installed, which means that the processor just "sleeps" when it has nothing to do.

    My computer's fan is always quiet. I've never heard it sounding like a vacuum cleaner.
    "The Internet treats censorship as damage and routes around it." - John Gilmore

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    Obviously the actual audible noise is dependant on hardware; this is a 2002 Dell desktop so its insides have seen better days.

    But surely (assuming you're not using a water pump) if you ran a complex program for a couple hours you'd hear something?

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    The superhaterodyne twomers's Avatar
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    It may be running at 2 GHz, but that doesn't mean that it's retaining memory, or changing memory as much when it's idling to when it's playing games or something. The latter requires power, which as a result creates heat.

    >> CPU fan starts to sound like a vacuum cleaner.
    Attach a hose to it and see what it does

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    Registered User Jaqui's Avatar
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    why does a copper wire get hotwhen you pump electricity through it?
    why does it get hotter when you increase the amount going through it?

    it's simple, FRICTION. electrons passing through copper, or silicone, fight friction, which generates heat.
    an idling computer doesn't push anywhere near as much current as one working on processing any application.
    Quote Originally Posted by Jeff Henager
    If the average user can put a CD in and boot the system and follow the prompts, he can install and use Linux. If he can't do that simple task, he doesn't need to be around technology.

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    The superhaterodyne twomers's Avatar
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    I wouldn't say it's friction, just electrical power lost to the surroundings as heat. This is modeled as friction in mechanics (as a parallel), for moving objects, but in electrical systems, it's resistance. Friction makes your hands warm when you rub them together (my hands are cold ATM ). Some reading.

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    Now if windows didn't have tourettes then laptops would run longer on a single battery.

    Back on topic, processors get hot due to the resistance as twomers said. The fan kicks up when the heat goes up, it is like self defence .

    If your computer has ever just kicked its fan on high when idling that is due to the above mentioned tourettes. Windows does stuff now and then, and jacks the processor up to 80%+ for a second and sometimes that triggers the fan.

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    Registered User Jaqui's Avatar
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    resistance is friction, under another name
    it's the same principle at least, and the easiest term for people to grasp the why for the heat generation.
    Quote Originally Posted by Jeff Henager
    If the average user can put a CD in and boot the system and follow the prompts, he can install and use Linux. If he can't do that simple task, he doesn't need to be around technology.

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    {Jaxom,Imriel,Liam}'s Dad Kennedy's Avatar
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    Quote Originally Posted by Wiki
    In metals

    A metal consists of a lattice of atoms, each with a shell of electrons. This can also be known as positive ionic lattice. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.

    In a metal the thermal motion of ions is the primary source of scattering of electrons (due to destructive interference of free electron wave on non-correlating potentials of ions) - thus the prime cause of metal resistance. Imperfections of lattice also contribute into resistance, although their contribution in pure metals is negligible.

    The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance.
    R != friction.

    (Can't help myself) -- What makes a CPU get hot? -- Looking at appliances? [not my original, stolen from Red Dwarf].

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    Reverse Engineer maxorator's Avatar
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    Quote Originally Posted by twomers
    I wouldn't say it's friction, just electrical power lost to the surroundings as heat. This is modeled as friction in mechanics (as a parallel), for moving objects, but in electrical systems, it's resistance. Friction makes your hands warm when you rub them together (my hands are cold ATM ). Some reading.
    If electrical powers moves through wires, there is always friction. Do you know how light bulb works?
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    and the hat of int overfl Salem's Avatar
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    Of course you can always combine technologies and get this
    http://www.rabidhardware.net/index.p...92b24964436117
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    Hmmm, I don't buy it, Ken.

    I'm just trying to understand... supposing an OS's idle routine looked like:-
    Code:
    while (1)
    {
        // poll drivers/apps for requests
    }
    What makes that "drive less current" than say 10 mins of continuous timeslices of one program that never yields?

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    and the hat of int overfl Salem's Avatar
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    > What makes that "drive less current"
    Code:
    HLT - Halt CPU
    
            Usage:   HLT
            Modifies flags: None
    
            Halts CPU until RESET line is activated, NMI or maskable interrupt
            received.  The CPU becomes dormant but retains the current CS:IP
            for later restart.
    Power is drawn when all those transistors in the CPU change state - which they do a lot of when working. There's always a regular clock interrupt to wake the CPU after it's little cat-nap.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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