Why does a CPU get hot in a computer?

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asked Dec 2, 2021 in Hardware by detresh8 (1,080 points)
Why does a CPU get hot in a computer?

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answered Dec 2, 2021 by Shawn (99,990 points)

The CPU in a computer gets hot because of electrical resistance.

Temperature is function of electrical resistance.

As electrons move through wires, it produces heat.

On a small computer chip, the area to dissipate heat isn’t very much and is concentrated.

Thus, it heats up to high temperature.

A CPU uses (thus generates) as much heat as 100W incandescent light bulb.

And that light bulb got pretty hot as well!

CPU chips dissipate more power per volume than the core of the sun.

A CPU chip is about 100 w/cc, or about a hundred million watts per cubic meter. (It is a good thing they are so small!)

The sun’s core operates at about 277 watts/cubic meter, about the same as a good compost heap.

Actually the power density of a human is higher than that of the core of the sun too, which really helps point out how huge the sun is.

As to why, mostly it is because they are fast.

CPU circuits don’t really have any resistors in them which use power deliberately. Instead, the innards of a CPU are made of CMOS logic, which is compose of switches which are fully open (high resistance) or fully closed (low resistance). Unfortunately, the wiring and transistors themselves have a little bit of capacitance, which is the ability to store electric charge.

Every time a tiny wire inside the chip switches from low to high, it has to be charged up.

Every time a tiny wire switches from high to low, it has to be discharged.

These charge and discharge cycles transport charge - an electric current - and the passage of that current through wires - even with low resistance - shows up as heat.

Every charge cycle of a capacitance C uses an energy 1/2 C V**2.

When multiplied by the clock frequency of the CPU you get the actual power consumption of the chip: 1/2 CV**2 f, where C is the total capacitance of the logic that is switching.

So running a CPU at a low voltage helps reduce power, and running it at a lower clock frequency will reduce power.

Designers know these things, and they use “clock gating” to stop the clock entirely on parts of the CPU that are not in use at any given moment, and the circuit and semiconductor engineers work hard to reduce the voltage needed for reliable operation.

Nevertheless, as you cram more logic on chip, and more cores, and larger caches, and run at higher frequencies, you get to rather high power numbers.

There is an interesting connection between voltage and frequency though.

If you slow down a CPU clock, it will work at a lower voltage.

Two cores at half speed use less power than one core at full speed!

To the extent that we can figure out how to parallelize our programs, we can save power.

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