very important questions of chapter-4

Millikan's oil drop experiment

Point charge, Conductors, Insulators And Semiconductors

Point Charge : It is a point where all the charge of body is concentrated. 

Conductors : Some substances which allow electricity to pass through them easily  are called conductors. They have electric charges (electrons) that are comparatively free to move inside the material. 

e.g. Metals, human and animal bodies and earth are conductors.

Insulators :  Some substances which don't allow electricity to pass through them easily  are called insulators.

e.g. Most of the non-metals like glass, porcelain, plastic, nylon, wood offer high resistance to the passage of electricity through them.

Semiconductors : There is a third category called semiconductors, which offer resistance to the movement of charges which is intermediate between the conductors and insulators.

e.g. gailuam arsenide , silicon and germanium etc.

Note :

Earthing or Grounding : When we bring a charged body in contact with the earth, all the excess charge on the body disappears by causing a momentary current to pass to the ground through the connecting conductor (such as our body). This process of sharing the charges with the earth is called grounding or earthing. Earthing provides a safety measure for electrical circuits and appliances.