Question

A polythene piece rubbed with wool is found to have a negative charge of \(3 × 10^{−7} C\). 
(a) Estimate the number of electrons transferred (from which to which?) 
(b) Is there a transfer of mass from wool to polythene?

Answer 1

(a) When polythene is rubbed against wool, a number of electrons get transferred from wool to polythene. Hence, wool becomes positively charged and polythene becomes negatively charged. 
Amount of charge on the polythene piece, \(q \) =\( −3 × 10 ^{−7} C\)
Amount of charge on an electron, \(e\) = \(−1.6 × 10^{−19}\,C\) Number of electrons transferred from wool to polythene =\( n\)
n can be calculated using the relation, \(q = ne\)
\(n = \frac{q }{ e} =\frac{ -3 × 10^{-7} } {-1.6 × 10 ^{-19 }}= 1.87 × 10^{ 12}\)
Therefore, the number of electrons transferred from wool to polythene is \(1.87 × 10^{ 12}\).

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(b) Yes.
There is a transfer of mass taking place. This is because an electron has mass, \(m_e = 9.1 × 10^{−3} kg\)
Total mass transferred to polythene from wool, \(m = m_e × n\)
\(= 9.1 × 10^{−31} × 1.85 × 10^{12}\)
\(= 1.706 × 10^{−18 }kg\)
Hence, a negligible amount of mass is transferred from wool to polythene.