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I am a little confused with the terminology with what's a passive and active heat sink.

First of all I want to know where the motherboard comes with this heat sink pre-installed as shown in the following picture?

enter image description here

if, yes, then it is already a passive heat sink which doesn't require you to install anything?

In some cases as here shown in the following:

enter image description here

Am I seeing a fan mounted over the passive heat sink or is it something different altogether called active heat sink? Why is the fan necessary if it is already a passive heat sink? Would you call the last one active-passive heat sink?

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I think you may be over-thinking it.

A passive heat sink is one with no moving parts. An active heat sink has moving parts. thus the second image is an active heat sink.

The fan is necessary because the volume of heat is to much to dissipate without forcing air through it. (If you think about it, there would be little point in bolting a fan directly onto a cpu as there would be no way for the air to flow through it - thus any active heat sink must have passive parts)

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    To be more precise, an active component (be it a heat sink or something else) requires additional power when a passive component does not. Peltier junction heat sinks can have no moving parts, but are still active since they require additional power to operate. – MBraedley Feb 19 '16 at 19:48
  • So, a passive heat sink + fan = an active heat sink? which is the case with second image? – user10232 Feb 19 '16 at 19:51
  • @user10232 precisely. – MBraedley Feb 19 '16 at 19:58
  • If in case the fan fails, would it still be good won't it because underneath it the passive heat sink would still be working? Considering if the fan was just optional to turn into active sink. – user10232 Feb 19 '16 at 20:22
  • You can sometimes run an active heatsink without the fan, at the risk of forcing the CPU to underclock when under load. I would not say that the fan is optional as without it you are forcing the system to work outside its design specs. – davidgo Feb 19 '16 at 20:33

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