question_answer

For Brownian motion of particles of suspensions in liquids, what should be the typical size of suspended particles? Why should not the size of the particles be too small (say of atomic dimensions \[\mathbf{1}{{\mathbf{0}}^{\mathbf{-10}}}\mathbf{m}\] ) or too large (say of the order of \[\mathbf{1m}\])?      

Answer:

                The typical size of the suspended particles should be \[{{10}^{-6}}m\]. If the size of the suspended particles is too small \[(\approx {{10}^{-10}}m),\] the net momentum imparted to the suspended particle due to bombardment of neigh- bouring fluid particles would be zero and hence no Brownian motion is possible. If the size of the suspended particle is too large \[\text{(}\approx \text{1m),}\] the particle will not move due to its large inertia, even when an unbalanced force is acting on it.