trans-pent -2-ene is polar but trans-but -2-ene is non-polar- why ?

Explanation

In trans-but-2-ene molecule, the two `C-CH_3` bond moments oriented in opposite directions cancel each other and so the net dipole moment in trans-but-2-ene molecule is zero. On the other hand , in trans-pent-2-ene molecule, although the `C-CH_3 and C-C_2H_5` bond moments act in opposite directions ,they are not equal in magnitude and so they cannot cancel each other. Hence the molecule possesses a net dipole moment.