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6. What is a dipole moment?
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A dipole moment is a vector quantity that measures the separation of positive and negative electric charge and the strength of that separation.
Key points
- For two point charges +q and −q separated by a displacement vector d (pointing from the negative charge to the positive charge), the dipole moment is
μ = q d
(vector direction: from negative charge toward positive charge).
- SI unit: coulomb·meter (C·m). Common chemistry unit: the Debye (1 D = 3.33564×10^−30 C·m).
- Physical meaning: it quantifies how polar a charge distribution or molecule is. A larger μ means a stronger separation of charge.
- Consequences: a dipole in an external electric field E experiences a torque τ = μ × E and potential energy U = −μ·E. Far from the dipole, the electric potential falls off as 1/r^2 (field ∝ 1/r^3).
- Types: permanent dipole (e.g., H2O, ≈1.85 D) arises from molecular structure; induced dipole arises when an external field distorts an otherwise nonpolar distribution.
Note: chemistry diagrams often draw an arrow pointing toward the negative (electron-rich) end; this arrow symbol is opposite the conventional physics vector definition, so be careful with sign conventions.
Key points
- For two point charges +q and −q separated by a displacement vector d (pointing from the negative charge to the positive charge), the dipole moment is
μ = q d
(vector direction: from negative charge toward positive charge).
- SI unit: coulomb·meter (C·m). Common chemistry unit: the Debye (1 D = 3.33564×10^−30 C·m).
- Physical meaning: it quantifies how polar a charge distribution or molecule is. A larger μ means a stronger separation of charge.
- Consequences: a dipole in an external electric field E experiences a torque τ = μ × E and potential energy U = −μ·E. Far from the dipole, the electric potential falls off as 1/r^2 (field ∝ 1/r^3).
- Types: permanent dipole (e.g., H2O, ≈1.85 D) arises from molecular structure; induced dipole arises when an external field distorts an otherwise nonpolar distribution.
Note: chemistry diagrams often draw an arrow pointing toward the negative (electron-rich) end; this arrow symbol is opposite the conventional physics vector definition, so be careful with sign conventions.
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