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
Think of volts like pressure in a water pipe. High flow (140a), but low pressure (7v) wont kill you like a slow flood, but low flow (1a) high pressure (110v) will surely go straight through you like a needle.. The high pressure will semd electricity through your body trying to find a path from the pressure, but the low voltage doesnt have enough pressure to go through you.
As a very crude analogy:
Think of a wide river flowing slowly. This represents a low voltage (low pressure differential) and high current (a lot of water flowing per second). You can pretty easily swim in it.
Now consider a water cutting jet. This is a super high pressure jet (high voltage) but the total rate of water flow is small (low current). But if you put your hand in it, it will cut your hand off.
Or another analogy: Take a 5 lb hammer and rest it on your hand. Now take a pin and poke it into your hand with 5 lb of weight (same as the hammer). The first has the weight spread out (low pressure; ie low voltage) over your hand and there is no injury. The other has very high pressure at the point of the pin (ie high voltage) and will puncture your skin.
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You have pretty high resistance.
The 7V system might have the “capacity” to provide 140A, but it can’t run that much through you. You’re probably 1M ohms, so only .007 ma will run through you.
To get 140A, you need a pretty fat wire that has only 0.05 ohms of resistance.
Because the amount of current that passes through you when you are being shocked is directly proportional to the amount of voltage you are being shocked with. You can grab the posts of a 12v car battery with wet hands and nothing will happen even though it’s capable of delivering 900 amps, if you do that with a wall outlet at 120v you would be dead even though the circuit breaker trips as 20 amps; the higher voltage will force more amperage through your body making it more lethal.
Your body is not just pure resistance either. Your body has some capacitance. Capacitors block DC but pass AC. I mean it’s more complicated than that but ya
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.
because your skin is a decent resistor. a low voltage source can’t push a current through your skin.
One thing to keep in mind, high current low voltage sources can be very dangerous as well, but for different reasons.
They can pose a high arcing hazard, and accidental shorts can be more dangerous because of the high current availability. These sources expect to see high current, so if a fault occurs that causes high current it can be more difficult to detect and clear it because the source can deliver it all day long.
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