I was watching someone use a pith ball electroscope. The guy charged an ebonite rod by rubbing it with fur ( got it ). As the ebonite rod was brought closer to the pith ball, the pith ball was attracted to the rod ( got it ). After touching the rod the ball got repelled which clearly indicates transfer of charge to the ball (HOW!).

I feel that ebonite is an insulator and the charges in it cannot move freely. So if I touch it with a conductor the charges must not get transferred but the experiment states otherwise.

Can somebody pls explain what I am doing wrong here ? Thx

  • $\begingroup$ The additional charges on the rod's surface can move a little. $\endgroup$ – Jasper May 16 '18 at 13:07

Charges can be added or removed to or from the surface of an insulator. This is what the experiment shows. Insulators only inhibit the flow of charges in its volume or along its surface. Such an addition or removal of charge to or from the surface of an insulator is, e.g., used in electrostatic generators like the van de Graaff generator.

  • $\begingroup$ Can you pls elaborate $\endgroup$ – user194517 May 16 '18 at 13:36
  • $\begingroup$ @user194517 - The removal/transfer of charge from the surface of an insulator can take place through any conductor, even through ionized air when you observe sparks in connection with a rubbed insulator. $\endgroup$ – freecharly May 16 '18 at 14:17

Although a small charge could potentially get transferred between the ebonite rod and a pith ball on contact, most charge transfer happens through the air ionized by a strong electrical field.

If you watch carefully this video or similar videos, you'll notice that the pith ball bounces off before it actually touches the rod. It happens very quickly because, once started, the air tends to ionize very quickly - you can think of it as a mini-spark.


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