Feeling Current Through the Human Body

why we cannot feel current in potentiometer experiment rather wire is in our hands??
- yash (age 22)
chandigarh

I'm not sure what your experiment was like, but if you didn't feel any current, it's probably because the current was too small. That's a good thing for you--currents large enough to feel can be dangerous if they flow through your body, especially if they take a path that includes your heart or brain.

This Wikipedia page has some information on the minimum current a human can perceive: http://en.wikipedia.org/wiki/Electric_shock

Assuming your experiment used a DC source, the minimum current needed for you to feel something is about 5 milliamperes (mA). To calculate the voltage needed to produce a current of 5 mA, we need the resistance of the human body. This Q&A has a detailed discussion of the resistance of the body, and how it varies depending on the points of contact and the path the current takes: The Human Body's Resistance. Using a multimeter, I measured the resistance through my body when I held the two leads in opposite hands to be about 1 million ohms. (You can try this yourself--measuring your own resistance in this way is safe because the multimeter doesn't use a high current.)

Ohm's law describes the linear relationship between voltage (V), resistance (R), and current (I): V = I*R. Plugging in 1 million ohms for resistance, Ohm's law says that it would take 5,000 volts to generate a 5-mA current through my body. I assume your experiment didn't use anywhere near that much voltage! This is an overestimate of the required voltage, however, because the resistance of the body starts to decrease in a nonlinear way at voltages higher than a couple hundred volts. This is due to damage to the skin and other factors that you can read about in the other Q&A linked above.

The resistance of the body and the relative danger of different current strengths is quite variable--it depends on the exact path through the body, whether the current is DC or AC, and on other factors like whether your skin is wet. Being able to feel the current would definitely be a warning sign that you're in or near the danger zone. I wouldn't deliberately pass any current through your body unless the experiment was specifically designed to do it safely.

You might also wonder why you can feel a painful electric shock from static electricity, and yet that shock isn't (usually) dangerous. The voltage of a static electric discharge can be large: 20,000 to 50,000 V for a typical, everyday shock (more detail here: http://en.wikipedia.org/wiki/Static_electricity). However, this voltage (technically a voltage difference) is present in the air between your skin and the shock source, not inside your body. The current only flows a very short distance inside your body because it isn't part of a complete circuit. The painful sensation you feel is caused both by the short-lived current of charge from the shock source, and by opposite charges traveling through your body to neutralize the build-up of charge at the shock site; these charges stimulate your nerves and cause the feeling of pain.

Rebecca Holmes

(published on 08/05/2014)