Have always wondered this.
Why can I touch a 7VDC 140A system with no problems, but 120V with less than a mA can kill?
In: Physics
You’ve got to remember that Ohm’s Law (V=IR) always holds true, at least out of extreme laboratory conditions like absolute zero or quantum mechanics. Given that resistance is mostly dependent on the material, that means current and voltage are proportional.
So a 7V source, across your body that is anywhere between 10-100kohm, could only push a tiny current you’d likely not even feel. If this 7V can deliver 140A, that means the system has enough energy and low enough internal resistance that it can supply that to a load of low resistance.
If you want 140A through 100kohm, you’re going to need very high voltage. There is no way to make 7V push 140A through your body. Even a car battery, which is 12V, can supply 500A or more and yet its fully safe to touch. There is enough energy stored within the battery to do serious harm, but not enough pressure to make it do that harm. The water jet analogy somebody has already provided is a good comparison, the same amount of water can be harmless or lethal based on the pressure, and voltage is like pressure.
120V is a higher voltage, so it has more ability to force that current through your body. That current might only be milliamps, but those milliamps are enough to stop the heart. In fact, typically 50V-1000V is considered low voltage, whereas below 50 is extra low voltage and generally wont cause harm on its own. So 120V is above the 50V “where the real danger begins” threshold.
If your 120V is coming out of a wall socket, its mains voltage which means it has a lot of energy, so it will keep pushing a current through a load whether that be a lightbulb or a human being, it can’t discriminate between the two.
Most sources won’t have sufficient energy to do anything to you. A van de graff generator for instance, can be charged to 200kV or more, yet all it can provide is a painful shock because the charge dissipates in a nanosecond leaving it depleted of energy. But out of the wall it will keep coming, unless you have a protective device like an RCD or GFCI (depending on what you call it where you live).
If that 120V is coming from a power supply, its generally supplied from circuitry that is electrically isolated from mains voltage and can make adjusts to current and voltage based on what it detects, allowing it to be safe to healthy people, though maybe painful. It may limit the current flowing to it by dropping the voltage, or having a target current so increasing voltage.
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