Basic Understanding of Current

We did a hire for a first year physics instructor. One interview question we asked was:

“A student comes to you very concerned that they are expected to use current on the order of amps in the lab. They found on the internet that current much less, on order of 100 milliamps, can seriously hurt, even kill a person.”
What do you tell this student.

Surprisingly, not a single interviewee gave a suitable answer with many even seemingly lost to correctly understanding the situation. Even when prodded to the right answer, some interviewees where at a loss.

The question arises as to why? Interviewees where mostly Physics PhDs with reasonable understanding of Physics. Some even experimentalists. And yet, confounded with a very basic question of current.

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2 Responses to Basic Understanding of Current

  1. Mike Coombes says:

    Hi Bernd,

    Congrats on new blog. I hope it is a success.

    I have to confess that I read the question, loved it but did not know the answer and I thought to myself, is there really a simple answer?

    Isn’t the physiology of electrocution rather complex? Most physics textbooks point out that low currents can be more dangerous that large currents because they override the nervous system and can stop the heart (or restart it like in the TV shows). Now starting and stopping the heart requires the current pass through your heart and the defibrillator paddles are placed to ensure that happens. If you just grab a set of live wires with your hands, how much current takes a path through your heart? What resistance would that path have?

    Then again in the TV shows, you require 300 to 500 J to restart a heart, so with the typical voltages allowed in labs, say 12 Volts, and a current of 0.1 Amp you only have 1.2 J/s. It’s not clear how long the defib shock lasts but if TV is a guide about a second, so the power delivered in the lab setting is probably way too low to be dangerous. This seems consistent with the fact I do recall my undergrad professors being extremely leery of voltages over 100 V in the lab. The electric chair operates at higher voltage, 10, 000 V I think, but the poor sod in the the chair can burst into flame.

    So with all that meandering, I guess my answer to the student is that even if 0.1 A gets through to your heart, the voltage is kept low enough so there isn’t a danger.

    Is this anything like the answer you guys were looking for? Am I even in the right ballpark?

    So if you ask me the reason the interviewees have a hard time answering is that they have been asked an icky biology question rather than a proper, god-fearing physics problem with blocks and particles and the like. It is not a domain most non-biophysicists apply their physics to, so one lacks the ordinary mental supports.

    Mike Coombes

    • Hi Bernd and Mike,
      I would tell the student not to worry, as while the current in his/her circuit in the lab may indeed be a lethal 100 mA using a voltage source of 1.5 or 9 or 12 volts, the current through his/her body with this source would be much, much less due to the high resistance of his/her skin. The key point is that the current adjusts to the load and does not exist independent of it.
      -Stan-

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