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How can I negatively charge a piece of lead? (A Skeptic's question)

  1. Jan 1, 2013 #1
    Hi everyone, I don't mean to offend anyone (actually I'm looking for help), it's just that I am having a hard time believing the superposition principle and below is the reason why I cannot believe it and what I want to do because of it. so I was reading chapter 21 on my physics book and something caught my attention. You see, there is an equation that states the net force between two objects with equal or different charges and later it says something about a "principle of superposition". The formula is:
    [itex]F = k\frac{Q_{1}Q_{2}}{r^{2}}[/itex]

    Obviously, according to the equation if there is a third object between the two objects of interest, such an object does not influence the force of repulsion or attraction between the other two objects and that is why they say this whole thing about superposition, but I find such a thing hard to believe. I mean, really? Electric force acts over a distance and disregards everything in between the two objects? I don't believe it one bit. This sounds like magic, not science. Is such a statement about "superposition" experimentally proved? Mathematically talking it is proved. I mean, the equation is clear and ignores anything in between the two charged objects, but that's the equation, what about experimentally?

    Later in the same chapter it says the following:
    I say, hell yes he did. Anyone would. I'm not an "early thinker" and this is 21st century, but this superposition thing is ridiculous; nonsense; preposterous! I've tried, but I cannot possibly understand it, to me the superposition principle is simply crazy. Unlike what the superposition principle says, I believe objects in between do have an effect. I cannot believe in such a statement (superposition) until I see it with my own eyes. That is why I want to make an experiment with charged objects.

    I want to know how to negatively charge a piece of lead to experiment with it and other pieces of lead. What I want is basically saturate it with electrons. I know that rubbing objects with a cloth will add static electricity to them, but a cloth won't give them enough charge to possible see the repulsion of them with my eyes. I need to really really saturate them with one charge to see the effects. Anyone here knows how can I accomplish such a task?

    I chose tiny pieces of lead because it is not attracted by magnets so that way I eliminate any interference of other things around generating magnetic fields (for that matter even the Earth's magnetic field) and see the full effects of the phenomena.
    Last edited: Jan 1, 2013
  2. jcsd
  3. Jan 1, 2013 #2


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    This is where things get complicated. If you have two charged objects and you put a metal plate between them, the electric field between the two objects will move charges in the metal plate, so that metal plate produces its own electric field which will add up with the field produced by the original charged objects... It gets complicated.

    I like the general challenge though. Superposition principle is rather central in physics. It comes up in some far more interesting contexts. And it certainly does work. But showing a simple and clear demonstration of that, that would be neat. I'll try to think of something.

    In the mean time, why doesn't it bother you that the gravitational pull of the planet isn't shielded by anything placed between you and ground?
  4. Jan 1, 2013 #3
    That is not what the superposition principle says. Superposition simply means the force or the electric field at a point is the sum of each individual charge's contribution to the force or electric field.

    To show this experimentally simply measure the electric force between two objects, A and B, then measure the force between another, A and C. Add these forces together and that is the force you would get if A, B and C were all there at once.
  5. Jan 1, 2013 #4
    you could rub fur on a piece of glass to charge it and see the effects with your eyes.
    And see how it reacts to a charged balloon that you rubbed with fur.
    A superposition you could do with magnets is have 2 bar magnets use one bar magnet
    to affect a compass needle and then bring the other bar magnet by the first one but have the poles oppositely aligned and you will see that it wont affect the needle because the field is basically zero and this is just like adding the fields together.
    You can also see the effect of charged objects using scotch tape
    take two strips and stick the sticky side of one to the other one but on the other ones dry side, and then rip them apart, now the two strips of tape are charged and you can see them affect each other and even other neutral objects.
    Another cool thing you can do Is take a coat hanger and hang it from the ceiling on a fine thread and then attach two bags of water at each end make sure they weight the same.
    Now if you have a strong rare earth magnet you can push the water around in a circle.
    Before you start make sure your torsion setup is unwound and steady and balanced.
    I used to have this setup in my house and it would baffle and amaze people when I showed it to them.
    Last edited: Jan 1, 2013
  6. Jan 1, 2013 #5
    Oh... that which you just said hit me in the head. I never thought of it that way but yeah, there's some truth to it that I cannot deny. It's practically the same problem, but with gravity. When I was a kid I was once told gravity keeps us attached to the ground and I simply believed it without questioning it. I never even tried to imagine how it worked. I suppose that because I am grown up now and electricity is something new to me, there is something that bothers me about it and that's why I am questioning these things.

    I attached an image of what I wanted to do, but what you just said about electric fields is indeed a situation that has to be taken into account. I was thinking about putting at one point a negative charged piece of lead against a wall, in the middle a glued piece of positively charged lead and in the other extreme another negatively charged piece of lead but this one would be connected to a spring. Restrain the movement of the spring piece of lead to move exactly in the X axis only and align all the pieces in the same axis. Then measure the distance between them and see how much the spring compresses because of the other negative piece of lead at the other side. Then calculate the force exerted over the piece of lead in the spring according to the distance that the spring moved.
    My bad. I expressed that wrong. Here attached is an image of the problem that I feel uneasy about. Look what I marked in red to see what the book says about superposition. However, I believe that Q2 does block the effect of Q1 acting on Q3. The book says it doesn't, that Q2 in no way blocks the effect of Q1 over Q3. Don't you think there is something weird about it? Like it is in the middle, but like it is not there at all? Like interactions between two of them is private and not public.

    I know magnetic fields are a different subject and its ruled by different equations, but let me illustrate what I believe that must happen too with electric charges by comparing them. Magnetic fields, like electric charges get weaker as distance increases. However, if you put mu-metal between two magnetic fields, regardless of the distance, the field will be weaker and hence their attraction or repulsion will be weaker too because the mu-metal attenuates the field. I believe the same must happen with electric fields, an electric charge in the middle must attenuate the effects of one electric charge at one side over another at the other side and not ignore it. The problem is that the equation doesn't say that and I want to make an experiment to see with my own eyes if superposition is true.

    I like the compass one, but I didn't quite got it. Like how I do it, do I bring the magnets closer to each other with opposite poles facing each other and the compass needle in the middle of them?

    Attached Files:

  7. Jan 1, 2013 #6
    ya just have the magnets as close together as you can. Just stack the magnets together like 2 bricks. on each end a north pole should be next to a south pole like
    [N S]
    [S N]
  8. Jan 4, 2013 #7
    Nice, I did it and it happened what you said, as if nothing was influencing the needle at all! Hehe. But still I want to make mine. I found this and I thought that maybe I could charge the lead pieces of my experiment using capacitors and make a relation of these equations with the Coulomb's equation (you know to have everything in the terms I want, in Newtons) although I'm not sure I understand it:

    [itex]E= \frac{1}{4\pi \epsilon_0}\frac{Q}{R^2}[/itex]

    On the metal sphere stuff for which the above equation is given in the pdf, how can I know the value of epsilon?
    Last edited: Jan 4, 2013
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