Question

When a silk handkerchief is rubbed with glass the charge acquired by silk is:
A. Positive
B. Negative
C. Neutral
D. None of the above

Answer 1

Hint: Basically there are 4 fundamental forces in nature. Hence whatever the force we talk about, that must be derivative of one of these 4 fundamental forces. Now the friction force is a type of contact force which arises due to interaction between two objects. It comes under the category of electromagnetic fundamental force. This is responsible for charging silk cloth too.

Complete answer:
Usually for wet surface friction will be very less. But if the surface is too wet then friction might be more. Take an example of white marble. If the surface of white marble is too wet and if we place another white marble on it, it will be sometimes difficult to separate them or rub one on another due to friction.
But the dry surfaces have more friction. So if we comb the dry hair with the comb then due to more friction due to intermolecular interactions the electrons will transfer from the comb to the hair. Then comb will attain the positive charge due to the loss of electrons.
When we take this charged comb near small pieces of paper this comb will attract electrons from the paper towards it. Hence after combing, the comb will attract a bit of paper.
Similar case happens in case of silk cloth and glass. Compare silk cloth with hair and the glass with comb.
When we rub silk cloth with glass then glass will lose electrons and attain positive charge while silk cloth will gain electrons and acquire negative charge.

So, the correct answer is “Option B”.

Note:
When we rub the wet silk cloth with the glass then the friction between the silk and the glass will be less due to wetness. The free charges are created due to rubbing of smooth silk cloth with the glass.