15.1. Static Electricity http://www.ck12.org
Two Ways to Charge Using Induction
If the charged object is withdrawn from the electroscope, the charges will no longer remain separated. The electrons
will move back into the leaves and the leaves will collapse. No permanent charging of the electroscope will have
occurred. What if we did wish to charge the electroscope?
Method 1: Charging by Induction and Conduction
Allow the coin to touch the metal plate inFigure15.8. Electrons on the plate would then transfer to the coin. The
electroscope would be left with a deficiency of negative charge, and the leaves of the electroscope would remain
positively charged and separated even after the coin was withdrawn.
Method 2: Charging by Induction through Grounding
In order to properly describe the second method, we must first explain what is meant by electrically grounding an
object.
The Earth can act as a huge reservoir for charge. It can readily accept a flow of electrons or it can deliver a flow
of electrons. An object that is grounded is connected to the earth by a conductor. A pathway is established which
allows electrons to flow freely between the object and the Earth. The symbol for electrical grounding is shown in
Figure15.10.
InFigure15.9, Disk A is negatively charged and Disk B is neutral. When Disk A is brought close to Disk B (without
touching it), the negative charges on Disk B move as far away as possible from the negative charges on Disk A. If
a conducting wire is attached to Disk B and to the Earth as shown inFigure15.10, the negative charge on Disk B
will move to the “ground.” Removing the conducting wire between Disk B and the Earth,Figure15.11, while Disk
A remains in place, ensures a permanent loss of negative charge from Disk B. Disk B is now positively charged.
FIGURE 15.9
Disk A (blue) on the left and Disk B
(green) on the right.Permanently charging with induction by grounding:
Check Your Understanding
1a. The leaves of the electroscope are negatively charged. How would the leaves behave if a negatively charged
object is brought close to the top of the electroscope?
Answer: The leaves would separate more, since additional negative charges would be repelled into the leaves,
increasing the repulsion.
1b. The leaves of the electroscope are negatively charged. How would the leaves behave if a positively charged
object is brought close to the electroscope?
Answer: The leaves would move closer together since the negative charges on the leaves would be attracted to the
positively charged object, leaving the leaves less negative.