[Physics] experiencing vibration when in contact with the desk computer and a rubbing a friend’s arm


I was with a friend sitting next to my desk computer, I put my hand on the computer and the other one on my friend who is in contact with the floor. The thing is that when I rub my hand on his arm we feel a vibration, why do we only feel it when there is motion?

Any hints will be much appreciated

Best Answer

I apologise for the heuristic nature of this answer.

What you are feeling is essentially a periodic variation in friction as your hand moves. You will feel the same thing if you move your hand close to any reasonably well-earthed smooth surface (see below).

The reason you feel this is that your body is 'floating' (electrical engineering term) at some high AC voltage with a frequency of 50 or 60Hz (depending on what the mains frequency is where you are) and there is thus a varying electrostatic attraction between your hand and something which is sitting at earth potential. You don't notice it unless you move your hand because it's very slight.

If you are actually touching an earthed object -- in good electrical contact with it -- you won't feel the effect because it will earth you and you will no longer be floating. You need to be very close to something earthed but not actually touching it: clothes achieve this when touching someone.

So, the final question is: why are you floating at some high voltage and why isn't this killing you when you touch earthed things? The answer is your computer, and its power-supply arrangements. Modern power supplies often have no earth: there is only a two-wire connection to the mains. Neither of those wires can be assumed to be safe to touch because either of them can be at a high voltage. So the low-voltage side of the system is very carefully isolated from the high-voltage side -- in the UK such a supply is called 'double insulated'.

So your computer is electrically isolated from the earth, and from the mains. But inside it there are bits of mains cable with 50 or 60Hz current in them. And it can 'hear' this through at least two mechanisms:

  • metal parts of the computer can get currents induced in them, essentially as the secondary of a transformer, and thus voltages can appear across resistances between parts;
  • although the metal parts of the computer are isolated from the mains, they may be less isolated from it than they are from the earth: the resistance between the case and the mains might be millions of ohms, but between the case and earth it might be hundreds of millions, for instance.

Both of these effects (I don't know which is more important in practice but I suspect the second) can cause metal parts of the computer to float at hundreds of volts (AC) relative to earth: there is nothing holding them at earth potential, because there is no earth connection. And if you are also not earthed and you touch the case, you are also floating at hundreds of volts.

The reason that this is not a problem is because there are stupidly high resistances involved: there are effectively millions (may be tens or hundreds of millions) of ohms of effective resistance in the circuit: the moment there is any reasonably small resistance between it and earth the whole thing drops down to earth potential.