Question

What is a net dipole?

Answer 1

We know that if all the dipole moments (bond) are exactly opposite to each other in a molecule or cancel Each Other completely, the net dipole moment of the molecule becomes zero. The bond dipole moment of all the bonds is the same and also all the bonds have a positive vector so they cancel Each Other dipole moment. Hence, the net dipole moment of the molecule is zero or it has no net dipole moment.

Complete answer:
Dipole moment of a chemical bond is an entity which is used to measure or calculate the polarity of that Bond within the molecule. Bond dipole moment comes into existence whenever positive and negative charges’ separation occurs in a molecule. The idea behind the discovery of bond dipole moment is that of an electric dipole moment. Bond dipole moment takes into concern the partial positive and the negative charges. Diatomic molecules possess only one Bond which can be either single or double, so the net dipole moment of the diatomic molecule is equal to the individual Bond dipole.
Dipole moments occur when there is a separation of charge. This occurs due to an atoms' electronegativity - where one atom has the ability to attract electrons towards it (In other words, electrons want to spend another time around it) giving it a negative charge and the other a positive charge.
Net dipole operates on the same idea - but it focuses on the direction and magnitude (vector) of the dipole moment. Carbon dioxide - oxygen being more electronegative than carbon pulls electrons towards it as demonstrated through the arrow. As the molecule is linear, both the bond dipoles cancel each other out (i.e. the vector addition of the dipoles equals zero) and the overall molecule has a zero net dipole moment.

Note:
Remember that the dipole moment of individual bonds of $C\to Cl$ in carbon tetrachloride is not zero. Only the net dipole moment of the molecule is zero. Bond dipole moment is the dipole moment between the single bond of a diatomic molecule, while the total dipole moment in a polyatomic molecule is the vector sum of all the bond dipoles.