Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

When my wife uses her laptop, if I touch her skin, I can feel a buzz. She doesn't feel the buzz, but she can hear it if I touch her ear.

So I'm guessing it's a faulty laptop, and she's conducting an electrical current.

But why would she not feel anything, and what would it be that she would be hearing when I touch her ear?

More info:

The effect is only intermitent - it's pretty reliable in a single session on the laptop, but some sessions it won't happen and others it will.

I had the same sensation with a desk lamp that I had several years ago, with no moving parts (as far as I could tell)

The effect only occurs when I move my finger - if I'm stationary, I don't notice anything.

I was playing with my son, and noticed the same buzz. First I thought he was touching the laptop. Then I realised he had skin-to-skin contact with my wife who was using the laptop.

share|cite|improve this question
I'm not sure that physics really has anything to say about this question, unless someone happens to know of a physical effect that explains the phenomenon... let's see what others have to say. – David Z Jan 14 '11 at 0:45
Won't it just be the vibrations of the laptop? Namely the hard drive. try putting something thin which isn't an electrical conductor (Eg paper) between her and the laptop) – Jonathan. Jan 14 '11 at 1:43
Maybe the question could be restated in a manner that doesn't reference any particular individual, as in - "If a person uses a laptop in certain way, would that be responsible for a buzzing sensation on their skin?" – user346 Jan 14 '11 at 4:30
the buzz is permanent until she breaks contact with the laptop – Tim Gradwell Jan 15 '11 at 9:18
Are that the so called "Good Vibrations"? – Georg Jan 31 '11 at 14:24
up vote 9 down vote accepted

I felt this and even drew sparks from it on a product I was working on, and was very worried, but after a lot of research and the standard government safety testing, it was deemed normal.

A small amount of touch current is allowed because it is not considered harmful. It happens with devices which have isolated transformers but the primary side is not grounded. There is a small amount of capacitance across the transformer isolation (especially in switch-mode power supplies, which require extra capacitance here to prevent EMI), which means the chassis will be at line voltage relative to Earth if you measure it with a voltmeter, but the capacitance is small so no significant current can flow.

If you concentrate the current to a very small point (by brushing two metal surfaces together, so that microscopic spikes touch each other momentarily), the metal will melt and make sparks. If you press the metal tightly together, nothing will happen.

If you ground yourself and then brush against the metal very lightly with a finger, especially on a corner or point, you will feel it, because it is high voltage and stimulating a few nerve cells at the tiny point of contact, but if you press hard you will not, because even though more nerve cells are in contact, your body's resistance/capacitance is grounding out the object and the voltage drops below the threshold of sensation. I also noticed that if I rubbed a shielded object against my skin (the shield of a cable), the friction sensation felt different with the power on and power off.

share|cite|improve this answer
While I am agreeing with you, do you have a link to a science paper that has actually analyzed this in detail? I do not mean the displacement currents from the power supply. I can measure those myself and have. What I mean is the part about the sensation. Is it proven beyond doubt that it's direct electrical stimulation of nerve endings in the skin? – CuriousOne Jun 20 at 18:40
@CuriousOne No I don't, sorry – endolith Jun 20 at 19:08
Too bad, I think somebody should really analyze this in detail. I've come across this effect a lot, but I have never seen an explanation that was rooted in an actual investigation. – CuriousOne Jun 20 at 20:43
@CuriousOne Well it's the same cause as any other electric shock. You can model the circuit as 120 VAC in series with a ~1 nF capacitor. As the load resistance decreases, the voltage also decreases. – endolith Jun 20 at 21:39
I know how to model all of this, but none of that proves that what you are feeling are actually electric shocks, for that one would have to do some serious electrophysiology research. – CuriousOne Jun 20 at 21:44

This effect occurs very often when touching electronic devices that are connected to the power mains.

You can verify that it is the connect to the power mains: unplug the power adapter and all other connections to other devices connected to the power mains and try again. The effect will be gone away.

It can best be felt also by the person touched if you touch with one finger only and move the finger (you can feel the difference on conducting surface areas of the device also).

What you are feeling is inducted power mains frequency so its probably 100 Hz (if you have a 50 Hz power mains or 120 Hz if you have 60 Hz power mains).

You can see this inducted voltage if you hold single-endedly the tip of a oscilloscope.

share|cite|improve this answer
It sounds likely to me, but unfortunately we can't check the laptop without power mains because the battery no longer works. Next time I have access to an oscilloscope I will try this out. – Tim Gradwell Jan 15 '11 at 9:23
I found this phenomen touching aluminium surfaces of my external hard drive and some laptops. It goes if you pull the main plug and use battery. – Marcel Feb 2 '11 at 7:57

Its due to residual transductance of the live AC current into the shielding of the device. Get an earthed plug and it will disappear. I've experienced the issue in dozens of shielded but unearthed electrical appliances, not just laptops. It feels exactly like a vibration (when you move the finger over the surface) but it's not, my guess is that the mechanoreceptive nerve-endings in the skin react to the small current.

share|cite|improve this answer
Having an "earthed plug" is not enough. There has to be a connection from the PE of mains througout the mains adaptor to the laptop case. This is not installed easily in an existing devioce. Sometimes it helps to rotate the mains plug for 180 degrees. – Georg Nov 18 '11 at 9:49
good luck rotating a uk 3 pin plug 180 degrees – David McGowan Apr 25 at 0:46

1 - Use a voltage detector pen in USB connector GND to see if there is a bad isolation between AC ground and DC ground (this is first option not because probability, but because it could be dangerous). It's normal a little fugue current between primary and secondary of transformer but the AC plug have 3 conectors, 2 for voltage and the third is for GND (ground), perhaps your home installation haven't a good grounding, check that! is very important.

New Zeland gov picture

2 - Beware of what is on your wife's laptop screen when her skin buzz =P

picture source

share|cite|improve this answer
I will check this - thank you Hernan! – Tim Gradwell Jan 15 '11 at 9:20

I agree with jonathon that it is vibration, although it is far more likely to be the laptop fan, rather than the hard drive

Why she doesn't feel it? The human body has vibration receptors, and I'm sure she notices the laptop vibrating when she turns it on. However, like with all constant stimuli, but particularly touch, our brains are great at blocking them out. I suspect that the fact it happens to her whole body makes the effect even greater, as there is no sensory discrimination from different nerves

You touch her, and two things are different. For you, it is a new sensation, and it is occurring in your hand alone. If you held her for a while, while focussing on something else, you would probably stop noticing

I would argue that this is a physical effect, biophysical to be precise.

share|cite|improve this answer

So I've been feeling the same thing with my girlfriend. I believe it has more to do with what Peer is talking about. Both my girlfriend and I are on our laptops, both plugged into power sources. She has a dell, I have a mac. At first I thought I could be that our comps are spinning at different rates, until I read this blog and experimented.

I touched her arm while we both had our comps on our laps and plugged into power source. -vibration

I took my comp off my lap while she still had hers on her lap, then touched her arm. -no vibration

I put my comp back on my lap and unplugged it from power source, she had hers on her lap still plugged into power source and I touched her arm. -no vibration

share|cite|improve this answer

protected by David Z Apr 14 '12 at 17:00

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.