Beatrice Lang
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, or its overall polarity. It is a vector quantity with a defined direction from the negative charge to the positive charge. The SI unit for electric dipole moment is the coulomb-metre (C⋅m), and it is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges. The dipole moment measures the strength of an electric dipole and occurs due to the existence of an electric dipole.
| Characteristics | Values |
|---|---|
| Definition | A measure of the separation of positive and negative electrical charges within a system, or a measure of the system's overall polarity. |
| Formula | p = q * d where d is the displacement vector pointing from the negative charge to the positive charge. |
| Direction | From the negative charge to the positive charge (in physics), and the opposite in chemistry. |
| SI Unit | Coulomb-metre (C⋅m) |
| Other Units | Debye (D) |
| Idealization | Two infinite charges infinitesimally separated, but with a finite p. |
| Torque | An object with an electric dipole moment p is subject to a torque τ when placed in an external electric field E. |
| Energy | The scalar dot product and the negative sign show that the potential energy is minimized when the dipole is parallel to the field, maximized when it is antiparallel, and zero when perpendicular. |
| Net Force | In a non-uniform electric field, a dipole may experience a net force that is generally parallel to the dipole moment. |
| Polarization | The study of dipole moments measures the tendency of a dipole to align with an external electric field, resulting in the development of a net dipole moment. |
| Examples | H2O, HCl, CH3COOH are examples of molecules with permanent dipole moments. |
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What You'll Learn
Electric dipole moment definition
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, or a measure of the system's overall polarity. It is a vector quantity with a defined direction from the negative charge to the positive charge. This convention, however, is specific to physics, with chemistry adopting the opposite convention, from positive to negative. The SI unit for electric dipole moment is the coulomb-metre (C⋅m), while another unit of measurement used in atomic physics and chemistry is the debye (D).
An electric dipole is defined as a couple of opposite charges ("q" and "-q") separated by a distance "d". The midpoint "q" and "-q" is called the centre of the dipole. The simplest example of an electric dipole is a pair of electric charges of two opposite signs and equal magnitude separated by distance. The dipole moment is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges or a neutral conductor in a uniform electric field.
The total charge of the dipole is zero, but the electric field of the dipole is not zero as charges "q" and "-q" are separated by some distance, resulting in a non-zero electric field. Molecules like H2O, HCl, and CH3COOH are examples of electric dipoles with permanent dipole moments. These molecules are called polar molecules because the centre of their positive and negative charges do not coincide. On the other hand, molecules where the centre of positive and negative charges coincide are called non-polar molecules, with carbon dioxide and methane being examples of zero dipole moment molecules.
The electric dipole moment can be expressed mathematically using vector algebra. The dipole moment vector "p" points from the negative charge to the positive charge. When an object with an electric dipole moment "p" is placed in an external electric field "E", the torque tends to align the dipole with the field. The potential energy is minimised when the dipole is parallel to the field and maximised when it is antiparallel.
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Electric dipole moment vector
The electric dipole moment is a vector quantity. It has a defined direction, which is from the negative charge to the positive charge. This convention is only followed in physics, with chemistry employing the opposite convention. The line along the direction of an electric dipole is known as the axis of the dipole.
The dipole moment is a measure of the separation of positive and negative electrical charges within a system, or the system's overall polarity. It is defined as a couple of opposite charges, "q" and "-q", separated by a distance "d". The midpoint "q" and "-q" is called the centre of the dipole.
The dipole moment is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges or a neutral conductor in a uniform electric field. In a non-uniform electric field, a dipole may receive a net force as the force on one end of the dipole may no longer balance the force on the other end.
The dipole moment can be expressed in vector form as:
P = qd
Where d is the displacement vector pointing from the negative charge to the positive charge. The dipole direction tends to align itself with an external electric field.
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Electric dipole moment in physics and chemistry
The electric dipole moment is a fundamental concept in physics and chemistry, providing a measure of the separation of positive and negative electrical charges within a system. This separation of charges results in a non-zero electric field, and the dipole moment quantifies the strength of this field. The SI unit for the electric dipole moment is the coulomb-metre (C⋅m), although the debye (D) unit is also used in atomic physics and chemistry.
An electric dipole is formed by two equal and opposite charges separated by a distance. In other words, it consists of a positive charge and a negative charge, with the distance between them defining the dipole. This separation results in a non-zero electric field, and the dipole moment quantifies the strength and direction of this field. The direction of the dipole moment is from the negative charge to the positive charge, although it's important to note that this convention is specific to physics. In chemistry, the direction is typically defined from the positive charge to the negative charge.
The dipole moment is particularly useful when studying neutral systems, such as pairs of opposite charges or neutral conductors in a uniform electric field. In these systems, the dipole moment helps analyse the overall polarity and behaviour of the charges. For example, in a neutral molecule, if the centre of mass of positive charges coincides with that of negative charges, the molecule behaves as a non-polar molecule. Conversely, if the centres do not coincide, the molecule is considered polar and exhibits permanent dipole moments.
The presence of an external electric field influences the behaviour of dipoles. When a dipole is placed in an external electric field, it experiences a torque that tends to align the dipole with the field. The energy of the dipole-field system is minimised when the dipole is parallel to the field and maximised when it is antiparallel. This alignment results from the interaction of the electric flux lines produced by the dipole charges and those of the external field.
The electric dipole moment is a vector quantity, possessing both magnitude and direction. Vector algebra is used to express the dipole moment mathematically, capturing the quantity's directional nature. The dipole moment vector is denoted as "p" and points from the negative charge to the positive charge, consistent with the convention in physics.
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Electric dipole moment and polar molecules
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system. It is a vector quantity, with a defined direction from the negative charge to the positive charge. This convention, however, is specific to physics, with chemistry adopting the opposite direction. The SI unit for electric dipole moment is the coulomb-metre (C⋅m), while the debye (D) is another unit of measurement used in atomic physics and chemistry.
An electric dipole is defined as a pair of equal and opposite charges separated by a distance. The simplest example of an electric dipole is a pair of electric charges of two opposite signs and equal magnitude separated by a distance. The dipole moment is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges.
The polarity of a molecule is a property of the bonds between the atoms that make up the molecule. It refers to the separation of electric charge, leading to the presence of an electric dipole in the molecule. Molecules with permanent dipole moments are called polar molecules, where the centres of positive and negative charges do not coincide and are separated. Examples of polar molecules include HF, HCl, and water.
In contrast, non-polar molecules have zero dipole moments, with the centre of positive and negative charges coinciding at the same point. This results in a zero distance between the two charges. Examples of non-polar molecules include chlorine, carbon dioxide, and methane. The polarity of a molecule depends on the polarity of a given chemical bond and their mutual arrangement. The dipoles from each of the two bonds can combine to make the overall molecule polar.
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Electric dipole moment calculations
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, or a measure of the system's overall polarity. It is a vector quantity with a defined direction from the negative charge to the positive charge. The SI unit for the electric dipole moment is the coulomb-metre (C⋅m), while the debye (D) is another unit of measurement used in atomic physics and chemistry.
An electric dipole consists of two charges of equal magnitude but with opposite signs, separated by a finite distance. The simplest example of an electric dipole is a pair of electric charges of two opposite signs and equal magnitude separated by a distance. The dipole moment is particularly useful in the context of an overall neutral system of charges, such as a pair of opposite charges or a neutral conductor in a electric field.
Mathematically, the dipole moment of two point charges is expressed as:
> \displaystyle \mathbf{p} = q\mathbf{d}
Where p is the dipole moment, q is the magnitude of one of the charges, and d is the displacement vector pointing from the negative charge to the positive charge. The dipole moment vector p also points from the negative charge to the positive charge.
For a continuous distribution of charge confined to a volume V, the corresponding expression for the dipole moment is:
> \displaystyle \mathbf{p} (\mathbf{r}) = \int_V \rho (\mathbf{r'})(\mathbf{r'}-\mathbf{r})d^3\mathbf{r}'
Where r locates the point of observation and d3r′ denotes an elementary volume in V.
For an array of point charges, the charge density becomes a sum of Dirac delta functions:
> \displaystyle \rho (\mathbf{r}) = \sum_{i=1}^N q_i \delta (\mathbf{r - r_i})
Where each ri is a vector from some reference point to the charge qi.
Let's consider an example to calculate the electric dipole moment. Suppose we have a dipole with charges of −2 C and +2 C, separated by a distance of 0.5 cm. We can calculate the dipole moment using the formula p = qd:
> \\begin{align}\
> p &= qd \
> &= (2\:\text{C})(0.5\:\text{cm}) \
> &= 1\:\text{C}\cdot\text{cm}\
> \\end{align}
So, the dipole moment for this example is 1 C⋅cm, and the direction is from −2 C to +2 C.
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Frequently asked questions
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, or a measure of the system's overall polarity. It is a vector quantity with a defined direction from the negative charge to the positive charge.
The SI unit for the electric dipole moment is the coulomb-metre (C⋅m). Another unit of measurement used in atomic physics and chemistry is the debye (D).
An electric dipole is the separation of two equal and opposite charges by a small distance. An electric dipole moment, on the other hand, measures the strength of an electric dipole and the system's electric polarity.
