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When we comb our hair (with a plastic comb) and then bring the comb close to bits of paper, the bits get attracted to the comb. I understand this is because the comb is now electrically charged.

Why then, don't these same bits of paper get attracted to a live wire, or a terminal of a battery? Isn't the battery electrically charged?

Bhanu Nepal
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    The voltage of the battery, and the electric field is much smaller. – S.s. Jun 10 '20 at 01:27
  • Well, I was thinking the same actually. What would be the magnitude of voltage induced in a comb, would it be in kilo volts then? – Bhanu Nepal Jun 10 '20 at 01:29
  • Please see this video. There, you will in fact see the charge on the surface of a conductor actually attract a pith ball. Worth seeing. And it will help answer some of your questions. – jonk Jun 10 '20 at 02:31
  • @jonk thanks, I watched the video. How come such high voltages don’t kill us? A comb should kill me every morning. – Bhanu Nepal Jun 10 '20 at 03:17
  • @BhanuNepal Voltage is Joules per Coulomb. You can have lots of Joules for high voltage (that's bad.) Or you can have only a very small amount of Coulombs (which can be harmless.) In the case of your comb, there's only a very tiny amount of Joules and an even *smaller* amount of Coulombs of charge. So it's not bad. Most of us really don't appreciate the powerful nature of electric charge force. If you equate 1 Coulomb of electric charge to 1 kg of mass, then the electric force is 10 to the 20th power times stronger, for example. But if there's only a little charge, it can't harm you. – jonk Jun 10 '20 at 03:25

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If you charge up a plastic comb or balloon, etc., enough to get a tiny spark, that surface charge can be on the order of a few thousand volts, due to the tiny capacitance of the object. Whereas the chemistry inside a typical battery can only create a voltage of on the order of a single volt.

The difference in electric field potentials, and thus the force they can create, is thus several orders of magnitude.

hotpaw2
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  • So, an overhead transmission line (say 100 KV line), would actually attract bits and pieces of paper (or perhaps any material), right? – Bhanu Nepal Jun 10 '20 at 01:51
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    An overhead DC transmission line would. AC ones attract/repel 50x per second. –  Jun 10 '20 at 11:23
  • @BrianDrummond, indeed. So an overhead AC wouldn't really have any effect? This is interesting. – Bhanu Nepal Jun 10 '20 at 17:36
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As mentioned above, the difference is the magnitude of voltages in play. Where walking across a carpet can generate a voltage of 1000V, and the human body is known to accumulate charges of up to 25,000V to give you an idea of what range of voltage might be transferred to the comb. However, the voltage of a typical battery you are likely to come into contact with is maybe 5V, 12V, 18V or 24V for consumer electronics or cordless tools for instance. For further contextualization, the voltage of a battery in a Tesla vehicle is around 350V give or take.

Here is actually a research paper on measuring the charge in hair, referencing a study that measured the electric field strength ranging between 20kV/m to 120kV/m:

https://cg.cs.uni-bonn.de/project-pages/hairmodeling/documents/HairElectricity.pdf

And here is a source discussing the noticeable effects on electric field strength on noticing changes or impacts on one's hair with ranges of field strength within the range of what was stated in the referenced study: https://books.google.ca/books?id=fhWlDwAAQBAJ&pg=PT212&lpg=PT212&dq=kv/m+hair+charge+voltage&source=bl&ots=7ZoVoa31nZ&sig=ACfU3U1rSCrz8vsI2eCokdgyX2v71yZjNg&hl=en&sa=X&ved=2ahUKEwiY9p2Jl_bpAhWSknIEHdHcA8EQ6AEwAXoECAwQAQ#v=onepage&q=kv%2Fm%20hair%20charge%20voltage&f=false

So in general, the energy that can be discharged between objects is much smaller for a battery than that of a generated electrostatic charge. To give you some more insight, this particular phenomenon you are describing is known as the triboelectric effect, where 2 materials that come into contact with each other exchange electrons (one gives, one takes).

jwkeitho
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