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How come touching heatsinks do not result in electric shock?

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a b à CPUs
July 25, 2010 3:06:01 AM

I've been wonder why touching my heatsinks has not resulted in me getting shocked.

Seeing as there is current running through the CPU, which is touching the metallic heatspreader (does it?), which touches the heatsink (I know there is thermal paste in between but still some parts may not have any paste on it) which means by touching the heatsink I should receive a shock.

So should touching the RAM heatsinks, seeing as the memory chips are in direct contact with the heatspreader, and there is a current running through the RAM chips.

Is there some reason why I'm not shocked or am I forgetting something really obvious or stupid?

On a side note... is it dangerous to touch the PCB, including the side of the PCB while the computer is on? I'm wondering because I want to route some cables which involves it touching the PCB, will the cables be burnt, will something happen?

Thanks.
a b à CPUs
July 25, 2010 3:30:01 AM

I hope your pulling my leg.
The intensity of electric shock you feel is based on Potential difference, which is voltage not amperage. Use to teach a system that had a transformer with an output of 14 Volt AC which "pushed" 35 Amps thru a lamp (6 million cp), Use to demonstrate that it is not the current in a circuit you need to fear by putting my fingers across the transformer. My fingers high resistance - lamp low resistance. Removed lamp and again placed my fingers across terminals. Now no longer have have 35 Amps.

A car battery can supploy several hundred amps, But very few people can feel +12 V. On the other hand 120 VAC that can only supply a small current can Kill.

Normal Maxium voltage on the MB is only 12 V
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a b à CPUs
July 25, 2010 3:41:05 AM

Trust me, I'm not. I'm not exactly good at physics, no thanks to nVidia.

I thought that current is what normally kills because it apparently messes up your heart's natural pacemaker.
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a b à CPUs
July 25, 2010 3:45:33 AM

Current kills, but voltage is what allows current to flow. That's why a 10+kV static shock doesn't hurt you (plenty of voltage, but almost no current), and also why a car battery doesn't hurt you (plenty of current capability, but not enough voltage to hurt you).

Also, the heatspreader of a CPU is not electrically connected to the CPU core or the pins (which is intentional, because if it were, it would be very easy to short out and destroy the CPU). Don't worry about it, though I would be careful about working around the motherboard when it is on - if you accidentally short something out, it could destroy either the board or the components on the board (or both).
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a b à CPUs
July 25, 2010 3:54:41 AM

Best answer selected by Lmeow.
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nAlright thanks for the explanation.
n
nJust wondering then, is it ok for wired components such as SATA and USB cables to touch the PCB while the motherboard is on?
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a b à CPUs
July 25, 2010 3:57:47 AM

The cables? Sure - they're insulated. It would be bad if the bare wires touched, but they are encased in several layers of rubber or similar materials to prevent them from causing any shorts or shocks.
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a b à CPUs
July 25, 2010 3:59:17 AM

They're all insulated with rubber/plastic so as long as they don't knock off a small PCB-mounted component you're fine. I have a power cable running behind my video card that puts reasonable pressure on both the card and the motherboard because of its thickness. It's yet to cause me problems.
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a b à CPUs
July 25, 2010 4:07:02 AM

Alright, thanks guys!
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