Question

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.

Answer 1

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.