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I asked a similar question here about this, but I still don't quite understand.

Say I grab onto two wires from a standard 120V 20A wall socket. I would fry. I would have 20A at 120 forced on my body.

But, when speaking in terms of circuits, as in my linked question, if I have a power supply outputting 100mA maximum at 5v, for some reason my device only gets exactly what it needs? Why is this?

Most things flow from an area of high concentration to low concentration, taking the path of least resistence. If you connect two wires to a power supply, why don't the maximum number of electrons want to go through your circuit?

If current behaves this way...why do you have to regulate voltage?

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    \$\begingroup\$ "I would have 20A at 120 forced on my body." no, you wouldnt. But you would still be fried... \$\endgroup\$ – PlasmaHH May 4 '15 at 12:37
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    \$\begingroup\$ Current behaves this way because you regulate the voltage. So you have a constant voltage and resistance and the current is defined by Ohm's law. \$\endgroup\$ – Bence Kaulics May 4 '15 at 12:47
  • \$\begingroup\$ You can draw more than 20A from the socket. But it would be a fire hazard. 20A is not a technical limit, it is more of an advice. \$\endgroup\$ – ilkhd May 4 '15 at 17:37
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The "120 V 20 A" rating of a wall outlet means it will supply 120 V and up to 20 A.

What is implicit in this spec, and most power supply specs, is that the power supply is considered a voltage source. That means it will try to keep its output voltage constant. It only has this single degree of freedom. The load then decides how much current to draw at that voltage.

For example, let's say your overall resistance (mostly due to relatively dry skin where the current enters and leaves your body) is 10 kΩ. You grab the two wires from the 120 V outlet and (120 V)/(10 kΩ) = 12 mA will flow thru you. That's way way less than 20 A, but still enough to kill you.

There are power supplies that regulate the current. These are unusual, and will be clearly labeled as such. A constant current supply might be labeled 1 A 50 V. That means it will put out 1 A of current, but can only go up to 50 V. If it would take more than 50 V to get 1 A of current thru the load, the output will sit at 50 V and the current will be below 1 A.

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Current does not behave this way "I would have 20A at 120 forced on my body."

You would have V = I * R(Resistance of your body). Rearranging terms I = V / R. Just for fun lest say the resistance of your body is 50k Ohms.

I = 120 / 50k = 2.4 mA. Not 20 Amps.

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"120V 20A" means "This power socket supplies 120V, and you can draw up to 20A from it". The actual current drawn by any circuit connected depends on its resistance/impedance.

For a human, from hand to hand, that resistance varies from a few kohms to a few hundred kohms, depending on whether you just got out of the bath.

But that ohm range of a human body is still enough to kill you, because only a few tens of mA of current is needed to put your heart into fibrilation.

And yeah, if the voltage is high enough & you haven't dried off from the shower before engaging in the experience, you'll also start to cook, because there's amps going through you :)

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Your reasoning is incorrect. There are 2 factors that determine how much current flows in a circuit: the voltage of the source and the resistance of the circuit. If you grasp onto a 120 VAC source, the current will be determined by how much resistance your body has. This is a function of the dampness of your hands and the characteristics of your body. Typically, your body resistance is low enough that enough current will flow to electrcute you, athough it is not going to be many amperes but closer to many milliamperes. In an electrical circuit, when you connect the power supply to the circuit, the current is also limited by the equivalent resistance of the circuit. Ohms law, in general, determines the current: I = V/R (I is current, V is voltage, and R is resistance). Either your body or the circuit determines the value of R. Hence, given V, the current is determined and is not infinite.

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Most things flow from an area of high concentration to low concentration, taking the path of least resistence.

Yes - at a particular rate. Just because you have damp in your basement does not mean your house will instantly flood. Punching a small hole in the bottom of a full bucket does not cause it to empty instantly.

Voltage does not correspond to number of electrons, rather it corresponds to the force that can be applied to electrons that are free in a conductor. Metals have plenty of electrons that are attached to the crystal structure as a whole rather than a particular atom, so they conduct well.

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  • \$\begingroup\$ Thanks... I was thinking that this was the perfect place for a water flow analogy! \$\endgroup\$ – R Drast May 5 '15 at 11:32

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