Question

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

1. Estimate the number of electrons transferred (from which to which?)
2. 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" = "q"/"e"`

= `(3 xx 10^-7)/(1.6 xx 10^-19)`

= 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 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 × 10¹²

= 1.706 × 10⁻¹⁸ kg

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