Question

Akshay and Sauranh are the students of class IX. They have recently studied the chapter on structure of atom in the class. Both Akshy and Saurabh were performing some activities in the science laboratory. Akshy took a plastic comb and rubbed it in his dry hair. When he brought this plastic comb (rubbed in dry hair) near tiny pieces of paper, the comb attracted the pieces of paper towards it. Mean while, Saurabh took a glass rod and rubbed it with a piece of silk cloth. When he brought this glass rod (rubbed with silk cloth) near the tiny pieces of paper, the glass rod also attracted the pieces of paper towards it (just like the plastic comb). Akshy and Saurabh had studied an instrument called electroscope in class VIII. So, they decided to make use of a positively charged electroscope having diverged leaves (or opened up leaves) in their activity. Akshy took the plastic comb (rubbed in dry hair) and touched the metal top of positively charged electroscope with it. This made the diverged leaves of electroscope to fold up. Saurabh then tool the glass rod (rubbed with silk cloth) and touched the metal top of another positively charged electroscope with it. This made the diverged leaves of the electroscope to diverge (or open up) even more. Askshy did not understand the various conclusions which could be drawn from all these observations. Saurabh explained him everthing very clearly.
(a) What conclusion can be drawn from the observation that a plastic comb rubbed in dry hair and a glass rod rubbed with silk cloth, both attract tiny pieces of paper ?
(b) What do the above observations tell us about the atoms present on plastic comb and glass rod ?
(c) (i) What conclusion do you get from the observation that when a plastic comb rubbed in dry hair is touched with the metal top of a positively charged electroscope, then its diverged leaves fold up ?
(ii) What conclusion do you get form the observation that when a glass rod rubbed with silk cloth is touched with the metal top of a positvely charged electrosope, then its leaves diverage even more ?
(d) What are the two types of electric charges present in atoms as shown by the above observations ? Name the subatomic particles which carry these charges.
(e) (i) Which electric charges are agained by a plastic comb on rubbing in dry hair ?
(ii) Which electric charge are lost by a glass rod on rubbing with silk cloth ?
(f) What values are displayed by Saurabh in this episode ?

Answer 1

(a) It is a known fact that an electrically charged object can attract an uncharged object. So, both a plastic comb rubbed in dry hair and a glass rod rubbed with silk cloth are electrically charged (having electric charges on them) due to which they attract the uncharged pieces of paper.
(b) These observations tell us that some charged particles are present in the atoms of plastic comb as well as glass rod
(c) (i) This observation shows that the charge on plastic comb is opposite to that of positively charged electroscope. In other words, the plastic comb rubbed in dry hair carries negative electric charge.
(ii) This observation shows that the charge on glass rod is of the same type as that of positively charged electroscope. In other words, the glass rod rubbed with silk cloth carries positive electric charge
(d) The atoms contain negative electric charges as well as positive electric charges. The subatomic particles having negative charge are electrons whereas the subatomic particles having positve charge are protons. (A neutral atom contains an equal number of electrons and protons)
(e) (i) The plastic comb gain negative charges (electrons) from dry hair on rubbing
(ii) The glass rod loses negative charges (electrons) to silk cloth on rubbing
(f) The various values displayed by Saurabh are (i) Awareness that atoms contain two types of electric charges : negative (electrons), and positive (protons), and (ii) Knowledge that only loosely held negative charges (electrons) can be transferred from atoms of one substance to another by friction during rubbing (the strongly held positive charges or protons cannot be transferred by friction during rubbing).