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When we are negatively charged, and we touch a doorknob for example, why does the shock happen (i.e. the flow of charge)?

I understand that the electrons want to flow to positive charges, and I know that as I approach the doorknob, it gets polarized and the positive charge is closer to my hand.

But what I don't understand is what happens next: where do the electrons go? Why would they want to flow to an object which has net charge zero?

After I touch the doorknob, and the electrons flow (why? No net charge) would the doorknob now become charged and I would become neutral again?

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Humans are fairly good conductors, as well as metal doorknobs. So even though the doorknob has a net neutral charge, the electrons in the doorknob will be repeled by and be able to move away from the excess negative charges on my hand. Therefore there is a net negative charge on my hand and a net positive charge on the part of the doorknob by my hand (the neutral doorknob becomes polarized). The flow of charge of the shock is the attraction of these charges as well as the tendency of the excess negative charges on me to get as far away from each other as possible, which is easily done by "jumping" to the door knob.

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Electrons don't want to flow towards positive charge, they want to flow towards a region less negatively charged than their current location.

As you approach the door, you are negatively charged (I think? Triboelectric effects can be hard to predict, because they depend intensely on the chemistry of the materials involved. If I'm wrong, just use a hole model instead.). The region of the doorknob near you develops a partial positive charge, yes, but that actually doesn't matter. The surplus electrons in your body will be repelled by each other (plus all your "native" electrons) more than they are repelled by the doorknob.

That excess repulsion causes them to flow out onto the doorknob. This process continues, until the surplus electrons in your body are repulsed by your body and the doorknob equally, which is when you and the doorknob are equally negatively charged. So the electrons go half onto the doorknob and half onto you.

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  • $\begingroup$ I think both things matter. For example, let's reduce the problem to something simpler: an electron between another electron and a proton. The middle electron is repelled from the other electron and attracted to the proton. In the OP's example the polarized knob does matter as well as the excess charge on yourself. $\endgroup$ Jan 29 '19 at 5:09
  • $\begingroup$ @Aaron Stevens:Hmm. You have a point, in that a sufficiently negatively charged knob would repel the surplus electrons more. What I meant was, for any amount of local charge density on the knob not exceeding your own surplus local charge density, the precise value of the induced charge density doesn't matter. And (possibly excluding very special materials), I think the Second Law of Thermodynamics forces any induced local charge distribution to be no greater than its inducer. $\endgroup$ Jan 29 '19 at 5:14
  • $\begingroup$ Yeah I'm not saying the induced positive charge on the knob near your hand is any larger than the negative charge on your hand near the knob. I'm just saying I don't think it's right to say that the net positive charge on the knob near your hand doesn't have an effect on the negative charges in your hand. $\endgroup$ Jan 29 '19 at 5:17
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But what I don't understand is what happens next: where do the electrons go?

It is a circuit with two capacitors, Cb and Ck, the self capacitances of the body and the knob. The body (the upper plate of Cb) initially received a usually positive charge Q from a chair. When the switch is closed, the charge is redistributed over Cb and Ck.

enter image description here

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Why would they want to flow to an object which has net charge zero?

You are overlooking two things:

  • Electrons don't like to stick together, this implies that to have a flow of electrons from your hand to the knob it doesn't need to have positive net charge, it's just sufficient that the knob presents less electron density. (Of course this is a qualitative explanation, to describe things a little more formally we should say that unbalanced electrons in your hand form an electric field such as to push electrons away from each other, no external field needed)

  • An object with net charge zero can still produce an electric field if it becomes polarized! Remember that it's the electric field that tells the electrons how to move. Electrons don't have, of course, an intrinsic "will" to pursue charge balance.

Would the doorknob now become charged and I would become neutral again?

Surely the doorknob, if not connected to ground, would become charged, after all the electrons must go somewhere! But you will likely not become completely neutral: that's because after a certain amount of charge flow has occurred the doorknob would become so charged that it would prevent additional electron flow. This is quite intuitive if you think about it: if the doorknob would have become completely charged, leaving you neutral, then we would intuitively expect the doorknob to shock you!

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The electrons flow from the negatively charged hand to the electrically neutral doorknob for two reasons.

  1. The atoms in the doorknob are polarised, as was mentioned, so on average the positive charge is closer to the hand than the negative charge.

  2. The electrons are repelled by other electrons in the hand.

If the doorknob isn't earthed, enough electrons will leave the hand until both objects are slightly negatively charged and the electrons are repelled equally from both objects.

If the doorknob is earthed all the excess charge will leave the hand and also the doorknob, being repelled from both until both are neutral.

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