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According to my Physics book:

1.) Charging by conduction is as follows: imagine having an electroscope and you place a charged metal rod (negative in this case = excessive electrons) to a neutral knob of an electroscope and the electrons from the charged rod will transfer to the entire length of the electroscope.

2.) Separation of charges: placing a positively charged object near the end of a neutral metal rod will move all the electrons at the end of the rod near the positively charged object (because charges work on a distance and unlike charges attract) and make the other end positively charged (I'm assuming is where there's a higher concentration of positively charged atoms due to repulsion between them and the positive object?).

Questions: 1a.) What causes the neutral metal electroscope to accept the excess electrons of the negatively charged rod that touched it? I can understand the concept if the object that touched the electroscope was positively charged and the electroscope (since it is a metal and therefore has free-floating electrons to easily lose) readily loses electrons. But the other way around? How?

2.a) I understand the concept of separation of electricity in the above example (2.) but wonder why the electroscope mentioned in (1.) doesn't experience the same phenomenon? When the negative rod touches the electroscope why aren't the charges in the electroscope separated, and instead received and distributed the given excess electrons throughout the length of the electroscope?

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2 Answers 2

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My answers are:

(1a) If the metallic rod is negatively or positively charged (excess or deficiency of electrons) and you touch the plate of the electroscope, part of the negative (positive) charge will be transferred to the electroscope because upon touching the rod+electroscope form one conductor with the same potential and the electrons distribute on its surface. Note: if the plate of the electroscope is replaced by a conductive beaker (Faraday's ice pail) and you insert the charged rod into the beaker, the whole charge will be transferred. NB: Negative charge is due to an excess electrons, positive charge is a deficiency of electrons. In both cases the charge moves due to the movement of the mobile electrons only.

(2a) When you approach a neutral electroscope's plate with a negatively charged rod, a separation of charges occurs on the electroscope before electrical contact by electrostatic induction, i.e. positive charges of the electroscope move towards the approaching negatively charged rod. This, of course, happens because the negatively charged electrons move away. When you touch, these positive charges will be neutralized by some of the negative charges of the rod and in the end the negative charge of the rod distributes partly over the electroscope and partly over the rod.

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1a. You are correct that charge in solids is conducted or transferred only by electrons. Atoms do not move. If you accept that electrons can flow from the neutral electroscope onto a +ve charged object, why do you think the opposite is not possible? ie Excess electrons flowing from the rod to the electroscope.

2a. This question is not clear. The excess electrons in 1 are distributed between the rod and electroscope when they come into contact, with the highest concentration at the sharpest points or corners. (Your assumption in 2 that the distribution of +ve charged atoms changes is not correct : the atoms do not move, they are fixed in place, only the electrons move.)

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  • $\begingroup$ oh okay, I mean to say that the mere LACK of electrons at the end of the conducting rod in (2) results in the end of the rod lacking in electrons to be positive. I need to keep reminding myself that atoms don't significantly move in solids! I also forgot to observe the fact that the objects in (1) are TOUCHING and in (2) are merely NEAR each other. I was confused because if the rod in (2) is experiencing a separation of charges why isn't the electroscope in (1) experiencing the same phenomenon? $\endgroup$
    – user138850
    Dec 11, 2016 at 0:47
  • $\begingroup$ Also it was also hard for me to accept the ability of neutral objects to gain excess electrons because I've only heard about how some atoms are always so ready to lose electrons (e^-). $\endgroup$
    – user138850
    Dec 11, 2016 at 0:50
  • $\begingroup$ Also separation of charges only happen in distances right (w/o the factors of insulators or anything else like that) ? $\endgroup$
    – user138850
    Dec 11, 2016 at 0:56
  • $\begingroup$ The rod in 2 is not experiencing a separation of charges. Neither is the electroscope. Conduction electrons in a metal are free to move. They are already separated from atoms, and belong to the whole metal. So when a +ve object is placed nearby the free electrons move towards it; when a -ve object is placed nearby they move away. The difference with 1 is that when a +ve rod makes contact with the electroscope the nearby electrons in the electroscope jump across to the rod and neutralise some of the +ve charge on the rod, so that the electrons are spread uniformly between both objects. $\endgroup$ Dec 11, 2016 at 0:57
  • $\begingroup$ The correct term for the "separation of charges" which you describe is "polarisation". $\endgroup$ Dec 11, 2016 at 0:59

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