Why is the outlet ground voltage different from grounded Macbook voltage

Question

Measuring on A/C with a digital voltmeter:

Measurement One I connect the ground lead to earth on a 3-prong plug, And hold the other lead with my thumb and forefinger, And observe that I register approximately 53 +/ 2 mV. In this configuration I conclude that my body electrical potential compared to ground is about 53 mV.

Measurement Two Same as above, but now my body is also grounded to earth. I now observe that I register 1 +/ 1 mV. In this configuration I conclude that my body electrical potential is about the same as earth.

Measurement Three I measure my body vs. ground with a hand on my Macbook Pro with aluminum case and connected to power through a 3 prong outlet. I observe 38 +/ 2 mV.

Measurement Four Macbook case vs. ground yields 5 +/- 1 mV.

Hypothesis: I had thought that by touching my MacBook Pro, my body would have the earth voltage (basically something like 2-5 mV).

Instead, even though the case is sitting at 5 mV, when I touch it, my body voltage (53 mV) drops to only 38 mV instead of the hypothesized 2-5 mV.

Why is this happening?

And is my experiment any good to begin with? I had hypothesized that working on my Macbook was equivalent to touching the grounding strip at removing voltage across my body. This seems to not be true.

Thank you in advance for any insight and education.

Answer 1

Your body has the bigger surface area (higher capacitance) and therefore picks up a larger extraneous signal compared with the MacBook case.

The MacBook case has a smaller capacitance to ground/environment and as such picks up a smaller signal. In fact, when it is plugged in to the charger, it will have more capacitance to ground than to the extraneous signals from the "environment". However, that capacitance to ground might only be about the same order as your body has to the "environment".

You cannot conclude that the Mac case is grounded. It is highly likely that there is no galvanic connection to ground - there will be capacitance to ground through the charger isolation transformer but this is not a galvanic connection and, as such, cannot be at all relied upon as a "hard" ground.

On this basis, you could expect a reduction in body signal when touching the Mac but there's no reason to believe it will take 53 mV all the way down to 5 mV. In fact by touching the Mac case you will cause the Mac case to rise in voltage.

Did you forget to measure this?

Answer 2

The human body and the computer chassis are like antennas. My body voltage meter shows that the combined emf fields in my living room give me a body voltage of at least 1.2 volts most of the time. This does not tell me which frequency has a particular power density. I doubt that all this voltage can be explained by Comcast modems.