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Physics Question on Electric Charges and Coulomb's Law

A polythene piece rubbed with wool is found to have a negative charge of 3×10−7C3 × 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

(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, qq =−3×10−7C −3 × 10 ^{−7} C
Amount of charge on an electron, ee = −1.6×10−19 C−1.6 × 10^{−19}\,C Number of electrons transferred from wool to polythene =n n
n can be calculated using the relation, q=neq = ne
n=qe=−3×10−7−1.6×10−19=1.87×1012n = \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 1.87×10121.87 × 10^{ 12}.

(b) Yes.
There is a transfer of mass taking place. This is because an electron has mass, me=9.1×10−3kgm_e = 9.1 × 10^{−3} kg
Total mass transferred to polythene from wool, m=me×nm = m_e × n
=9.1×10−31×1.85×1012= 9.1 × 10^{−31} × 1.85 × 10^{12}
=1.706×10−18kg= 1.706 × 10^{−18 }kg
Hence, a negligible amount of mass is transferred from wool to polythene.