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A couple of days ago I observed a phenomenon which I'm not sure I understand correctly. My brother-in-law has recently bought an ASUS gaming laptop in an aluminum casing. When the power supply is connected and I swipe my finger across the casing on the outside, the motion is not smooth, as one would expect, but "bumpy" as if some force was acting on the finger and interfering with its motion. With the power disconnected the motion is smooth as with any other surface.

I've been thinking about a reasonable explanation and the only thing I've been able to come up with is this: the internal power supply circuit creates a magnetic field and when a conductive finger is moved through this field, circular electrical currents (eddy currents) are flowing through the tissues. This in turn, by Lenz's law, creates opposing magnetic fields and accounts for the "drag force" experienced by the finger. This would be analogous to the eddy current brake systems present in trains and streetcars.

Please, let me know what you think about this explanation. If you find it wanting, please suggest corrections and improvements.

Brandon Enright
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Wojciech Gac
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    My HP laptop had a similar casing and have experienced this phenomenon. Increase your own earthing with the ground (stand on a wooden block maybe), and the feeling disappears/reduces in magnitude of drag.

    In fact, it indicates a problem with your home power supply : poor earthing. With proper Earthing, you wouldn't feel anything.

    I don't think the current flowing though our fingers is sufficient to produce a magnetic field which can in-turn provide us with a noticeable drag.

     – Cheeku Dec 30 '14 at 01:31
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    There is absolutely no way this is a magnetic effect. The effect would be waaaaaay to small to feel. Not only are the magnetic fields at the surface of the laptop very small, but your finger such a good insulator that the eddy current you mentioned are definitely not responsible for the effect. – DanielSank Dec 30 '14 at 01:36
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    Not a magnetic field, for sure, but there may be a strong ultrasound generator inside the laptop. The high voltage power supplies for fluorescent screen backlights were using ultrasonic power transformers. How old is the laptop? Is it using LED backlights? – CuriousOne Dec 30 '14 at 01:54
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    I've experienced this before too. I agree with Cheeku that it's to do with poor earthing, and I believe the 'bumpiness' has to do with the 50 or 60 Hz frequency of the AC mains supply. However, I don't exactly what causes the sensation, and I'm curious, so +1. – N. Virgo Dec 30 '14 at 03:10
  • Great question, always wondered about this. I've also noticed such a phenomenon with plastic-like casing, although I'm not sure it's true plastic, maybe it is a thin layer of insulator on a metal surface or whatnot. My guess is that this may be related to capacitance between the fingers and the case: as you move your finger, it doesn't always have the same pressure on the surface and thus not always the same contact area. And the AC from power supply makes charge-discharge cycles somehow leading to such "bumpy" sensation from nonconstant capacitance. – Ruslan Dec 30 '14 at 07:51
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    Possible duplicates: http://physics.stackexchange.com/q/2824/2451 and links therein. – Qmechanic Dec 30 '14 at 07:59
  • @CuriousOne, interesting remark. It's brand new. If I recall correctly, it has an IPS display with LED backlights. – Wojciech Gac Dec 30 '14 at 10:23
  • Well, it was worth a shot... I guess you will have to take it apart and look for some sort of vibration circuit by circuit. I would start with the power supplies. – CuriousOne Dec 30 '14 at 14:45

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