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?

#### Solution

**(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 × 10^{12}

Therefore, the number of electrons transferred from wool to polythene is 1.87 × 10^{12}.

**(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.