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Question 11

# 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?

(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 = = 1.87 × 1012

Therefore, the number of electrons transferred from wool to polythene is 1.87 × 1012.

(b) Yes. There is a transfer of mass occur. This is because an electron has mass,

as we know

me = 9.1 × 10−3 kg

Total mass transferred from wool to polythene,

m = me × n

= 9.1 × 10−31 × 1.85 × 1012

= 1.706 × 10−18 kg

Hence, a negligible amount of mass is transferred from wool to polythene.

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