dipole moment

Meaning: A dipole is a charge +q and a charge −q held a distance d apart. Its dipole moment is a vector of magnitude qd that points from the negative charge to the positive charge. A polar molecule such as water has a net dipole moment, found by adding its bond dipoles as vectors. A symmetric molecule such as CO₂ has polar bonds but a net dipole moment of zero.

Equation: p = qd, where p is the dipole moment (C·m), q is the magnitude of each charge (C) and d is the separation (m). In a uniform field, τ = pE sinθ, where τ is torque (N·m), E is the field strength (N/C or V/m) and θ is the angle between p and E. The potential energy is U = −pE cosθ, in J.

On the MCAT: A passage places a polar molecule or a model dipole between charged plates. You predict that the net force is zero in a uniform field, so the dipole rotates and does not translate. Torque is greatest at θ = 90° and zero at 0° and 180°. Alignment (θ = 0) is the stable, lowest-energy orientation. You may also be asked how p changes when q or d is doubled.

Common confusion: Students expect a dipole in a field to move toward one plate. In a uniform field the forces on +q and −q are equal and opposite, so they cancel and only a torque remains. Only a nonuniform field gives a net force. Direction also depends on the convention. Physics points p from − to +, while chemistry dipole arrows usually point toward the more electronegative, partially negative end.