electric dipole
Summary
An electric dipole is a pair of equal and opposite charges, +q and −q, separated by a small distance d. Its dipole moment is p = qd, pointing from − to +. In a uniform field it has zero net force but feels a torque that turns it toward alignment with the field.
Detail
Meaning: An electric dipole is a pair of equal and opposite charges, +q and −q, separated by a small distance d. Its dipole moment p points from the negative charge to the positive charge. In a uniform external field the two forces cancel, so the net force is zero, but they act at different points and produce a torque that turns the dipole toward alignment with the field.
Equation: p = qd and τ = pE sinθ: p is dipole moment (C·m), q is each charge's magnitude (C), d is separation (m), τ is torque (N·m), E is field strength (N/C), θ is the angle between p and E.
On the MCAT: Expect a dipole in a uniform field: net force is zero, torque is greatest at 90° and zero at 0° and 180°, and alignment (0°) is the stable, lowest-energy orientation. The potential is zero along the perpendicular bisector of the two charges. In a nonuniform field the net force is not zero.
Common confusion: Physics draws p from − to +, but the arrow on a chemical bond dipole points toward the more electronegative atom, the opposite direction. Also, zero net charge does not mean no effect: a dipole still creates a field and feels torque.
Sources
- University Physics
- Physics
- Fundamentals of Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.