Answer
For cis-isomer (right, top) the chlorine atoms are on the same side of the double bond and so the bond polarities reinforce and results in a large dipole moment. For trans-isomer (left, bottom) the chlorine atoms are on opposite ends and hence cancel out the bond polarities resulting in zero dipole moment.
Work Step by Step
1. Two carbon atoms are attached with a double bond and have both three bond pairs (sp2 hybridization).
2. Carbon is more electronegative than hydrogen, so the dipole moment in C-H bond will be towards carbon. (The arrow points towards the most electronegative atom giving it a partial negative charge and away from the less electronegative atom giving it a partial positive charge).
3. On the other hand, chlorine is more electronegative than carbon, so dipole moment in Cl-C bond is towards chlorine.
4. For cis-isomer (right) the Chlorine atoms are on the same side of the double bond and so the bond polarities reinforce and results in a large dipole moment.
5. For trans-isomer (left) the chlorine atoms are on opposite ends and hence cancels out the bond polarities resulting in zero dipole moment.
