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Some theoretical questions that got me confused during physics lecture today.

  1. Bringing a conducting balloon to a negatively charged rod close will allow the conducting balloon to have positive charges closer to the rod. When the rod touches the conducting balloon, why is it that the rod(dielectric) transfers its -ve charges to the conductor instead of the other way around since it's harder for charges to move inside a dielectric?

  2. How exactly does charge by friction work? How I recognize it is that the stronger insulator will absorb electrons from the weaker insulator leaving one positively charged and one negatively charged?

  3. When you charge a balloon and stick it to a wall, after some time the balloon slowly loses its charged state. But the balloon is a dielectric(rubber) so how does it lose the charge? I'm assuming friction from rubbing against air slowly picks away at the charged electrons? Some clearer explanation would be great!

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    $\begingroup$ Please read up on the triboelectric effect $\endgroup$
    – QuIcKmAtHs
    Jan 10, 2018 at 5:45
  • $\begingroup$ @XcoderX I've already read that. That also only answers 1 of my questions. $\endgroup$
    – Justin Li
    Jan 10, 2018 at 5:54

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  1. Electron current always flows from negative to positive potential.

  2. The most easy method to distinguish between different combinations and cases is to use the Tribolelectric Series Table.

"A material towards the bottom of the series, when touched to a material near the top of the series, will acquire a more negative charge.". See also this question here fro an example application of the Table.

  1. No dielectric is perfect. Eventually it looses its excessive static charge due dielectric leakage currents from one material to the other in contact but also as pointed out by others also via the surrounding air. Notice, if the static charge voltage goes too high e.g. hundreds of KVolts/m or more you can have dielectric breakdown and excessive charge can be released in the form of spark current.
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Adhesion between different materials can result in a transfer of electrons between the two materials.
This is the reason for bodies gaining net charges after they have been in contact with one another - charging by friction.
Different substances have different affinities for electrons but to predict which substance will gain/lose electrons is very difficult and it is the empirical results which are usually quoted as a triboelectric table.

Your balloon although a good insulator in the bulk might have surface moisture on its surface and as water is a relatively good conductor in electrostatics experiments some of the charge might leak away by conduction through the water.

Although the air is a good insulator there are charged particles (ions and electrons) in the air which may originate from dust, air blown over a wet surface, hot bodies/flames, natural radioactivity and cosmic rays. These charges in the air will neutralise the charges which are on the balloon.
Feynman has a chapter in his book entitled Electricity in the Atmosphere which will give you some more information.

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  • $\begingroup$ I see. That most of the things up, thanks! But I'm still confused on why a conducting balloon is able to 'steal' charges from a -ve charged glass rod? $\endgroup$
    – Justin Li
    Jan 10, 2018 at 15:21
  • $\begingroup$ It will do it by contact with the glass rod which will not be a very efficient process. Transfer might be improved by rolling the glass rod over the balloon? $\endgroup$
    – Farcher
    Jan 10, 2018 at 16:23
  • $\begingroup$ Hmm I don't think that was 100% what I meant. What I meant was, why is it that the conductor is able to take charges from the insulator? The insulator disallows movements within it while a conductor lets electrons move freely. So isn't it easier for insulator to take parts of the -ve charges instead of conductor taking charges from the insulator? $\endgroup$
    – Justin Li
    Jan 10, 2018 at 16:42

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