Let say we have some DC voltage source which produce some voltage which I will label as V. Let also say that we have some real bulb which is supposed to work when voltage V is applied across its terminals. That bulb has some internal resistance R_internal. I will model real bulb as ideal bulb in series with its internal resistance R_internal. We know that for current to flow we need closed path between source and load so if we connect our voltage source and our bulb like on picture below, we will see light from bulb because current will flow: I draw two equivalent schematics for this situation.
Let us now see another situation. We know that in AC wall outlet there are two "holes". Mathematical expression for voltage waveform between these two "holes" is shown on picture below:
If we represent voltage between these two "holes" as voltage generator like on picture below and if we connect to it some real bulb which is supposed to work with this AC voltage we will see light from bulb because we have completed circuit and alternating current can flow:
Why is it dangerous for human being to touch one of contacts in "holes" in AC wall outlet? I mean, according to previously discussed situations, there should be no closed path for current to flow If I stand on earth and touch one of contacts in AC wall outlet. I should touch both contacts in wall outlet for current to flow through me and that will be very dangerous. Thanks in advance.