Charging by Electrostatic Induction


When an object gets charged by induction, a charge is created by the influence of a charged object but not by contact with a charged object.  The word induction means to influence without contact.  In the example shown below, the electroscope gets charged without any contact with the charged object brought in from the top.  Induction Animation (7k)

  1. The  charged object is brought near the neutral electroscope.   This causes many electrons to move toward the top to be near the positive charge.
  2. While the charged object is still near (but not touching) a connection to ground (via your finger)  is created to the electroscope.  In this case, some electrons from your finger are drawn into the electroscope by the charged object.   The electroscope now has an excess of electrons, (but often the leaves go flat because the majority of them are at the top, the bottom is neutral. - not shown in this animation)
  3. The attachment to ground is removed (you take away your finger).   This strands the electrons on the electroscope so it now has  negative charge.  
  4. Pull the charged object away, the leaves move apart.  It is charged!

There are several advantages to charging something by induction.

The table below compares charging by Conduction to charging by Induction.

Charging by Conduction

Charging by Induction

Charging an electroscope by conduction (7k)

Charging an electroscope by induction.(7k)

Charged object touches the electroscope. Charged object does not touch the electroscope.
Electroscope ends up similarly charged to the object used to charge it. Electroscope ends up oppositely charged to the object used to charge it.
The first charge is strong but gets weaker each time the electroscope is recharged.  (This is due to the original object giving up some charge every time it is touched.) The first charge is strong and stays strong each time the electroscope is recharged.  (This is due to the original object not losing any charge in the process.)

1999 Science Joy Wagon