Source: High school physics (Chinese)
Problem Sets:
Problem
Three identical, contacting metal blocks are placed on a plastic table. Two objects with a large positive charge are placed at the two ends of this row of metal blocks, close but not touching. Then, an uncharged insulating rod is used to separate the three metal blocks. After that, the positively charged objects are removed.
Left block: negative charge. Middle block: positive charge. Right block: negative charge.
The two external positive charges induce a charge separation in the row of three conducting blocks. Free electrons are attracted towards the ends, causing the two outer blocks to accumulate a net negative charge. Consequently, the central block is left with a net positive charge. When the blocks are separated while the inducing charges are still present, this charge distribution is trapped. Removing the inducing charges afterward does not change the net charge on each isolated block.