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I have a mid-2012 Macbook Pro Retina, and over the years I notice that when I wipe my finger along the surface of the chassis (like when wiping off dust), I sometimes feel a vibrating effect, a more subtle version of what I would feel rubbing my finger across a squeaky-clean piece of cookware.

This only happens sometimes, and eventually I figured out that it only happens when the laptop's power cable is connected. When the laptop is not plugged in, wiping the finger along the chassis does not produce the vibrating effect. The laptop has functioned normally for almost 10 years, with a battery replacement about 5 years ago.

Playing around, I do feel the effect to some degree when the power cable is not quite connected (MagSafe connector attached magnetically, but green charging indicator light is not on).

What causes this? I guess I'm curious about the specifics (is something internal to the laptop wrong / improperly grounded) as well as whether there is a general physical phenomenon at play.

andyras
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    This is likely related to capacitive coupling, which is described in [this question/answer pair](https://electronics.stackexchange.com/a/478626/9612). – nanofarad Mar 17 '22 at 15:07
  • andyras - Hi, This is quite a common question here (with some different descriptions and affected devices). I've linked your question to a previous one that specifically mentioned the *apparent* vibration that you mentioned. *nanofarad* has kindly mentioned another one. – SamGibson Mar 17 '22 at 15:16
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    Not properly grounded. –  Mar 17 '22 at 16:19

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The given comment already says that the ungrounded metal case is capacitively connected to the mains AC line in the power supply. If the metal is anodized aluminium you do not actually touch the metal, but you are connected to it capacitively through a big capacitor formed by the aluminium, the oxide and your a little wet finger.

When you move your finger along the surface new areas of the surface get polarized under your finger and the electric field causes a force to your finger. The vibration frequency is 2 x the mains AC frequency, it doesn't depend on the speed how fast your finger slides along the surface.

The vibration stops if you stop your finger. Why? I guess the Al oxide starts to leak quite soon if you keep the finger in one place.

Totally different is the case when you touch clean conductive metal (=no insulation layer on the surface). If you happen to have even a slight contact to the ground through your shoes or otherwise and you touch the metal very gently (= small contact area) you may well have in the contact so high current density (=Amperes per square millimeter) due the capacitive leakage that the small contact area gets a local electric shock. I have even smelled burned skin in my hands after such incident. The haptic effect was like like a pinbrick by a needle. Touching the metal firmly caused much less notable effect.