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B Static electricity questions

  1. Nov 28, 2016 #1
    This is all related to GCSE content, but it doesn't go into enough depth for it to make sense:

    When a ballon sticks to a wall after becoming negatively charged, why does the surface of the wall become positively charged? I would assume that the repulsion between negative charges forces the electrons to move, but does this mean that atoms can change their structure? Theoretically, the atom still has the same amount of protons and electrons, so its overall charge is neutral - does this mean that the electrostatic attraction has a very short 'range' and only reaches the surface of an atom? And why is it that the wall that the subatomic particles in the wall move instead of those on the balloon? Is it because charged particles 'stronger' than neutral particles?

    Another question I have is regarding lightning. Why do the negatively charged ions sink to the bottom of the cloud - electrons are lighter than protons, so shouldn't it be the other way round? Actually (I just now thought of this)but is it because the ice and water have the same chemical formula and therefore the same amount of electrons and protons, which would mean that the negatively charged ones are heavier because they have the same amount of protons, but more electrons?

    Also, with paint sprayers. I understand why you get an even coat, but what confuses me is the fact that the car becomes charged as well. The car was presumably neutral to begin with, so how exactly do the paint sprayers cause the car's particles to become ions?

    I am also confused by earthing. I understand that allowing the charge to flow elsewhere will stop it from shocking you, but how does the charge just 'disperse'? My revision guide says that negative ions flow down, and positive ones flow up - if it flows up, wouldn't it just still cause a shock? So if the negative ones flow down, does that mean the earth is positively charged? I thought it had a neutral charge though, so does it flow down because it has a more positive charge than the ions?
  2. jcsd
  3. Nov 28, 2016 #2
    Yes, the electrons in the wall's atoms are found a bit farther away from the balloon than they were before, and the nuclei are a bit nearer. This means the atoms get a small positive charge on the side near the balloon, and a small negative charge on the other side. This is called electric displacement, from which the atoms get a dipole moment.
    The range of the electrostatic force isn't important for this. The particles in the balloon move to, but that doesn't make much difference, the electrons are still there. (This is about movements of the order of less than an atom's size. None of the particles move to the other side of the balloon or the wall.)
    Also, there are no neutral particles involved. Both electrons and nuclei are charged.
  4. Nov 28, 2016 #3
    Do you mean a lightning rod or the earthing of an electric plug? In the latter case, no ions flow, just electrons. The earthing is connected to long wires (which run alongside the power wires), which are in turn connected to the ground, which isn't a bad conductor – there is a lot of moisture and electrolytes like sodium and chloride ions. And the ground is large. So no problem with dispersion.

    For a lightning rod, keep in mind that it's all about charge differences, so given that the bottom of a cloud is negatively charged, the earth is positive relatively to the cloud. You can think of negative charges being repelled by the cloud rather than being attracted by the earth; the effect is the same: they flow down.
  5. Nov 28, 2016 #4
    So you are essentially saying that the electrons go to atoms of the elements within the ground? Yes, I accidentally said ions, when I meant electrons - sorry about that. Also, my issue is to do with earthing(of, for example, refuelling pipes) is that if the atom has had electrons scraped off and gained a positive charge, my book says the electrons flow up - but wouldn't this still mean there is a risk of a spark??

    Also, thanks for taking the time to reply ☺
  6. Nov 28, 2016 #5

    This makes sense, but I still have one slight issue, I thought that atoms were fixed in structure in that they had their nucleus(with protons &neutrons) orbited by shells with electrons. So, just to clarify, are you saying that this can change via electric displacement??
  7. Nov 28, 2016 #6


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    Atoms aren't super rigid. You can compress them. You only need to change the shape of many atoms very slightly to have a net effect at macroscopic scales. The charge on the balloon induces a dipole moment on the wall, which attracts the balloon.

    For lightning, look here: http://www.lightningsafety.noaa.gov/science/science_electrication.htm
  8. Nov 29, 2016 #7
    Not when they are going through the earthing, i.e. a good conductor. You get sparks when charges try going through the air, which is a bad conductor. The air gets ionised by the electricity, and ionised air (plasma) is a good conductor, facilitating the spark. But this won't happen if there is earthing, since that's the easier way for the electrons.

    My pleasure :smile:
  9. Nov 29, 2016 #8
    The paint spray carries ions with it, and when it reaches the car, the ions sit there.
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