1
$\begingroup$

We often hear that the human body has a weak electrical charge. Is it a positive charge or negative charge? Or maybe different parts of me have different electrical charges? Or maybe there's a complete electrical circuit inside of me?

Now suppose I scuff my feet on the carpet and touch metal objects to make sparks. Does any of that affect my electrical charge? Which direction do the sparks travel?

Or maybe a buildup of static electricity is a completely different phenomenon from inherent electrical charge. Even so, why doesn't the buildup of static electricity interfere with my internal electrical flow? It certainly does with old TVs and electronic equipment!

What if two people are scuffing their feet on the carpet and touching each other? You would think that both people, being electrically identical, would have the same charge. Why then would there be a spark at all? And which direction would the sparks travel?

$\endgroup$
  • 6
    $\begingroup$ What sign charge you have eaten just before asking? $\endgroup$ – Vladimir Kalitvianski Jan 14 at 15:43
  • $\begingroup$ There are two classic experiments you want to know about: Rubbing fur on an rubber rod, and rubbing silk on a glass rod. The former results in a negative charge on the rubber, the latter in a positive charge on the glass. When you scuff your rubber shoes on a rug it is like the former case. $\endgroup$ – Paul Young Jan 14 at 16:04
  • $\begingroup$ Just a note: be careful when trying to figure out "which way does the spark travel?" This can be tricky for a couple of reasons. First, remember that the movement of charges past a point is defined as current, and the sign of current is consistent with electrons having negative units of charge. If you deduce that electrons are net moving from A to B, then the current is positive in the B to A direction. Second, some sparks "move" in surprising ways. Watch some super-slow-motion videos of lightning strikes to see what I mean. $\endgroup$ – Eric Lippert Jan 15 at 21:57
5
$\begingroup$

We often hear that the human body has a weak electrical charge. Is it a positive charge or negative charge? Or maybe different parts of me have different electrical charges?

The Earth has a slight negative charge, so by default you have a slight negative charge, if you are standing directly on bare ground without shoes. Since you're a conductor, you can't have positive charge in some areas and negative in others, it should even out.

Now suppose I scuff my feet on the carpet and touch metal objects to make sparks. Does any of that affect my electrical charge? Which direction do the sparks travel?

Yes, you build up charges as you rub your feet on the carpet. The direction of charge flow depends on the relative positions of the materials in the triboelectric series. For example, human skin is very high on the series, so if you do this barefoot, you're almost guaranteed to get a positive charge no matter what the carpet's made of. But nylon is higher on the series than cotton, so if you wear cotton socks on a nylon carpet, you'll get a negative charge.

Why doesn't the buildup of static electricity interfere with my internal electrical flow?

Since you are a conductor, in equilibrium the excess charges will gather on your skin and create no electric field inside your body. Of course, an actual electric field in your body would have significant effects.

What if two people are scuffing their feet on the carpet and touching each other? You would think that both people, being electrically identical, would have the same charge.

If they really were identical, there wouldn't be a spark. It's just that it's quite likely they'll have different shoes, walk on the carpet different amounts, and so on.

$\endgroup$
  • $\begingroup$ Excellent answer, thank you @knzhou. One follow-up question: You are saying that people generally have a slight negative charge, but the Triboelectric Series that you linked shows most human parts (hair, skin, hands) as being positively charged. $\endgroup$ – SlowMagic Jan 14 at 19:24
  • 1
    $\begingroup$ @SlowMagic Most conducting things in direct contact with the Earth's will have a tiny negative charge, because the Earth is negatively charged. But this becomes irrelevant if you put on shoes and walk on a carpet, because these are insulators; at that point what matters more is the triboelectric effect. $\endgroup$ – knzhou Jan 14 at 20:55
  • $\begingroup$ Barefoot people are not negatively charged when they are indoors, which is most of the time. As buildings are slightly conductive, the negative charge is located at the roof. $\endgroup$ – jkien Jan 15 at 20:32
0
$\begingroup$

In my experience, people who get a shock from touching doorknobs are positively charged. The charge is positive because the deflection of an electroscope touched by them diminishes when a negatively charged pvc rod is approaching. Typically the person got the positive charge from sitting in a chair with a synthetic seat. The leaves of an electroscope rise when the person is standing up from the chair, because the work of removing the positive body from the negative seat creates a high voltage, as in an electrophorus.

In my experience, walking around on rubber soles, and even rubbing your shoes in the carpet, in itself does not cause a deflection of the electroscope. Of course, if a person was already charged by rising from a chair before walking around on insulation shoes, that charge will cause a deflection of the electroscope, and finally a spark to the doorknob.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.