5.7 Electric Dipoles

Electric dipoles are fundamental to understanding charge distribution in molecules and materials. They arise from the separation of positive and negative charges, creating a dipole moment that interacts with electric fields.

Permanent dipoles exist in asymmetric molecules like water, while induced dipoles form in neutral atoms exposed to external fields. These concepts are crucial for grasping how materials respond to electric fields and interact with each other.

Electric Dipoles

Permanent electric dipoles in molecules

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Top images from around the web for Permanent electric dipoles in molecules

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  Molecular dipole - Knowino View original

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  The electric dipole moment (p) in the water molecule | TikZ example View original

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  Intermolecular Interactions View original

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  Molecular dipole - Knowino View original

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  The electric dipole moment (p) in the water molecule | TikZ example View original

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• System with two equal and opposite charges separated by a fixed distance charges not free to move independently
• Dipole moment ($\vec{p}$) vector quantity characterizes strength and orientation of dipole $\vec{p} = q\vec{d}$, $q$ magnitude of each charge, $\vec{d}$ displacement vector from negative to positive charge
• Many molecules have permanent electric dipoles due to asymmetric structure water (H2O) and ammonia (NH3)
  • In water, oxygen atom attracts electrons more strongly than hydrogen atoms creates charge separation
  • Bent geometry of water molecule results in net dipole moment
• Molecular geometry plays a crucial role in determining the presence and strength of permanent dipoles

Formation of induced electric dipoles

• Induced electric dipole occurs when neutral atom or molecule subjected to external electric field causes redistribution of electron cloud creates charge separation
• Positive charges (nuclei) slightly displaced in direction of electric field, negative charges (electrons) displaced in opposite direction
• Magnitude of induced dipole moment depends on strength of external electric field and polarizability of atom or molecule polarizability measures how easily electron cloud can be distorted by external field
• Induced dipoles are temporary exist only in presence of external electric field
• Polarization occurs when many molecules in a material align their dipoles in response to an external electric field

Calculation of electric dipole moment

• Electric dipole moment ($\vec{p}$) calculated using formula $\vec{p} = q\vec{d}$
  • $q$ magnitude of each charge in dipole
  • $\vec{d}$ displacement vector from negative to positive charge
• SI unit for electric dipole moment coulomb-meter (C·m)
• Larger dipole moment indicates stronger charge separation more significant effect on surrounding electric field
• Potential energy ($U$) of electric dipole in external electric field ($\vec{E}$) given by $U = -\vec{p} \cdot \vec{E}$ dipole tends to align itself with electric field to minimize potential energy
• Torque ($\vec{\tau}$) experienced by electric dipole in external electric field given by $\vec{\tau} = \vec{p} \times \vec{E}$ causes dipole to rotate and align with field

Behavior of dipoles in electric fields

• In uniform electric field, electric dipole experiences only torque no net force
  • Torque causes dipole to rotate and align with field
  • Once aligned, dipole remains in stable equilibrium
• In non-uniform electric field, electric dipole experiences both torque and net force
  • Net force directed towards region of higher electric field strength
  • Phenomenon known as dielectrophoretic effect
• Force on electric dipole in non-uniform electric field given by $\vec{F} = (\vec{p} \cdot \nabla)\vec{E}$
  • $\nabla$ gradient operator represents rate of change of electric field in space
• Behavior of electric dipoles in non-uniform fields basis for various applications dielectrophoresis in microfluidics and particle manipulation

Dipoles in materials and electrostatics

• Dielectric constant measures a material's ability to store electrical energy in an electric field, related to its molecular dipole properties
• Electrostatics studies the behavior of stationary electric charges and fields, including the effects of electric dipoles
• Dipole-dipole interactions occur between molecules with permanent or induced dipoles, influencing material properties and behavior