A metal rod held in hand and rubbed with wool will not show any sign of being charged. However, if a metal rod with a wooden or plastic handle is rubbed without touching its metal part, it shows signs of charging: Explain Why?

Hint: When two objects are rubbed they give off electrons, when a metal rod is rubbed with wool, the electrons escape from the wool and tend to stick to the metal rod because metal has a higher affinity for electrons as compared to wool.

Complete Step by step solution:
It is a well-known observation that when two objects are rubbed together, the loose electrons present around the atoms of one object come off and stick to the object that has more affinity for the electrons. This causes an imbalance in the net amount of protons and electrons in both objects, this causes one of the objects to become positively charged (the one that loses electrons) and the other one becomes negatively charged (the one that gains electrons).
When wool is rubbed with a metal rod, the electrons come off and stick to the metal rod because it has more affinity for the electron, and thus here wool acquires a positive charge while the metal rod acquires a negative charge.
We know that the human body is a good conductor, therefore when the metal rod is directly touched after acquiring the electrons, the electrons move through the hand to the human body and then to the ground, because the ground has more affinity for electrons. Therefore, the metal rod cannot hold the charge and does not show any sign of being charged.
Whereas when a wooden or plastic handle is attached to the metal rod, then electrons do not touch the human body because wood and plastic are bad conductors of electricity. This allows the acquired electrons to remain on the metal rod, and it shows the sign of charging.

Note:
When non-conductor materials lose electrons, they do not neutralize easily because the flow of electrons in them is very tough, whereas the crystal structure of metals and other conductors is such that electrons can flow very easily through them.