Electric Dipoles: Torque, Polarization, and Dipole Fields

All flashcards

Electric dipole

A pair of equal-magnitude, opposite-sign charges separated by a small distance.

Electric dipole moment

$\vec{p}=q\vec{d}$, where $\vec{d}$ points from the negative charge to the positive charge.

Dipole torque in a uniform electric field

$\vec{\tau}=\vec{p}\times\vec{E}$, with magnitude $\tau=pE\sin\theta$.

Induced dipole

A dipole moment produced when an external field shifts the positive and negative charge distributions of a neutral object.

Point-dipole field

At distances large compared with the charge separation, its strength decreases in proportion to $1/r^3$.

Quiz questions

1. How does an external electric field create an induced dipole in a neutral, nonpolar atom?Answer: It shifts the positive and negative charge distributions relative to each other.
2. If the distance from an ideal point dipole is doubled, how does the field magnitude change?Answer: It becomes one-eighth as large.
3. At the same distance from an ideal point dipole, how does the axial field magnitude compare with the field on the perpendicular bisector?Answer: The axial field is twice as large.
4. Which orientation is an unstable equilibrium for a dipole in a uniform electric field?Answer: The dipole moment points antiparallel to the field.
5. What can happen to a dipole's net force when the external electric field is nonuniform?Answer: The forces on its charges can differ, producing a net force.