Case: Read the passage and answer the following questions.
Historically the credit for the discovery of the fact that amber rubbed with wool or silk cloth attracts light objects goes to Thales of Miletus, Greece, around 600 BC. The name electricity is coined from the Greek word ‘electron’ meaning amber. Many such pairs of materials were known which on rubbing could attract light objects like straw, pith balls and bits of paper. For example, cut out long thin strips of white paper and lightly iron them. Take them near a TV screen or computer monitor. We can see that the strips get attracted to the screen. In fact, they remain stuck to the screen for a while.
A glass rod is rubbed with a silk cloth. During the rubbing, \(2.5\times10^{10}\) electrons move from the rod to the silk every second. The rubbing goes on for 4 s. What is the charge on the glass rod at the end? (Take \(e=1.6\times10^{-19}\ \mathrm{C}\).)
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(+1.6\times10^{-8}\ \mathrm{C}\)
The rod loses electrons, so it is left with a positive charge. The charge on a body is \( q=ne \), where \( n \) is the number of electrons lost or gained.
Number of electrons that leave the rod in 4 s:
\( \begin{aligned} n &= 2.5\times10^{10}\times4 \\ &= 1.0\times10^{11} \end{aligned} \)
Charge on the rod:
\( \begin{aligned} q &= ne \\ &= 1.0\times10^{11}\times1.6\times10^{-19} \\ &= +1.6\times10^{-8}\ \mathrm{C} \end{aligned} \)
Therefore, the glass rod carries \( +1.6\times10^{-8}\ \mathrm{C} \), and the silk carries an equal negative charge.
NCERT (2026-27):
Electric Charge – “When we rub a glass rod with silk, some of the electrons from the rod are transferred to the silk cloth. Thus the rod gets positively charged and the silk gets negatively charged.”
Quantisation of charge – “Experimentally it is established that all free charges are integral multiples of a basic unit of charge denoted by e. Thus charge q on a body is always given by”
\( q=ne \)