Question

NaCI when placed in a uniform electric field 20VM^-1 , both Na⁺ and Cl^- ions experience a force (average distance between Na⁺ and Cl^- is 1.3A°).

1. In what way the NaCI will orient in the electric field. 

2. Arrive an expression for torque experienced by the dipole.

3. Calculate the maximum torque experience by the NaCl molecule in the electric field.

Answer 1

1. ‘Na’ is orienting towards -ve side and cl ions towards +ve side.

2.

 

Consider an electric dipole of dipole moment P = 2aq kept in a uniform external electric field, inclined at an angle θ to the field direction.

torque = any one force × perpendicular distance 

\(\tau\) = qE × 2 a sin θ

Since P = 2aq

\(\tau\) = p E sin θ

Vectorialy

\(\overrightarrow \tau=\overrightarrow P\times \overrightarrow E \)

3.  \(\tau\) = q2a.E

\(\tau\) = 1.6 × 10^-19 × 1.3 × 10^-10 × 20 Nm

= 41.6 × 10^-29 Nm.