The essence of the Langevin formula is that when an electric field is applied to molecules with permanent dipole moments there is a competition between the torque exerted by the electric field and the torques exerted on the molecules as they collide with each other. If the gas is very hot thermal motion wins and the dipole moments are randomly oriented all the time. If the gas is very cold then the electric field wins and the dipoles are nearly perfectly aligned with the applied field with only minor thermal fluctuations. The animation below shows an in-between situation where both effects are important. You will see the molecules relax from a state of random orientation to the thermal steady-state where the two effects are in constant competition. You can tell that the simulation is starting by watching for the dipole arrows to dissolve into the initial random state.

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