How Do Electric Dipoles Work? Key Principles Explained for Students
An electric dipole is an arrangement of electric charges. Let’s say if one charge is negative then the other needs to be positive, provided that these two charges are of equal magnitude. Also, there should be a certain distance between these two charges.
Let’s suppose + q and - q are the two charges separated by a distance ‘2a’, now joining the centre of these charges with a line, as shown below:
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The mid-point of this line is the centre of the dipole. The dipole has a certain length and also the moment.
Here, we are going to discuss electric dipole and dipole moment.
What is Electric Dipole and Electric Dipole Moment?
We understood that an electric dipole is an arrangement of equal and opposite charges. Since the two charges are separated by a distance ‘2a’, which is called the dipole length. The distance between the charge and the centre of the dipole is ‘a’.
So, what is an electric dipole moment?
The dipole moment is a vector quantity and is denoted by a symbol \[\overrightarrow{p}\]. Its magnitude is equal to the magnitude of either of the two charges. Since we don’t specify the sign for the dipole moment, we multiply either of the two charges with the dipole length. It is given by:
\[\overrightarrow{p} = q \overrightarrow{d}\] ….(1)
Here, d = 2a, so, we can rewrite the equation as:
\[\overrightarrow{p} = q(2a)\] …..(2)
So, equation (2) is the magnitude of the dipole moment.
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So, what is the SI unit of the dipole moment?
We know that the unit of the charge is ‘C’ and that of distance is ‘m’. So, the unit of the dipole moment becomes:
Can we see the difference in equations (1) & (2)?
Yes, there is a big difference between the two, but how, let’s understand this:
See, in equation (1), we considered the distance ‘d’ as a vector quantity, however, in equation (2), both the quantities viz: charge and distance or dipole length are the scalar quantities.
Now, when the product on the R.H.S of eq (2) are scalar, so, how can dipole moment be a vector quantity? Yes, p can be a vector quantity only when we somehow convert the distance ‘2a’ as the displacement.
So, how can a distance be displaced, as these two quantities are different?
Now, let’s look at the following arrangement:
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When we take ‘2a’ displacement of the charge - q with respect to charge + q, .i.e., from L.H.S to R.H.S or of the charge + q with respect to charge - q (from R.H.S to L.H.S). Now, this ‘2a’ becomes a vector quantity. Hence, equation (2) becomes:
\[\overrightarrow{p} = q(2 \overrightarrow{a})\]…..(3)
Or, \[|\overrightarrow{p}| = q \times (2a)\] …..(4)
So, what do you mean by electric dipole moment?
From eq (3), we define electric dipole moment as the product of charge and the displacement of + q charge with respect to - q charge.
Sometimes confusion arises in considering the direction of electric dipole moment. In Chemistry, we consider the displacement of + q charge with respect to charge - q, i.e., from right-to-left, while in Physics, we take the displacement of - q w.r.t. + q, i.e., from left-to-right.
There are two more units of the dipole moment, possessing a relationship between each. These are:
StatC . cm
Debye (D)
Also, 1 D = 10-18 StatC . cm, which is approximately = 3.33564 x 10-30 C.m.
There’s something ideal included in the concept of dipole moment, let’s discuss it.
What is an Ideal Electric Dipole?
From eq (4): If the charge ‘q’ gets larger, the distance between the two charges becomes smaller and smaller, to keep the product of these two quantities viz: ‘q’ and ‘2a’ as constant, i.e., \[\overrightarrow{p}| = q \times (2a)\] = constant, this is what we call it as an ideal dipole or point dipole.
Therefore, an ideal dipole is the smallest dipole having almost no size.
Significance of Electric Dipoles
We can find the application of electric dipoles in atoms and molecules, but how?
Consider one atom of Hydrogen and the other two of Oxygen in the water molecule. Now, what happens here is, a covalent bond forms between H and two O-atoms, and they come close to each other.
Since there is a difference in the electronegativity of H and O, there is a shift of charges; also, the centres of these atoms don’t coincide.
O is an electronegative element and a shared pair of electrons between H and O towards O. Therefore, the centre of the negative charge shifts towards O, and the positive one to the H-atom; also, a partial positive and the negative charge develops on H and O-atom, respectively. Now, the arrangement of H and O in the water molecule behaves like a dipole.
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FAQs on Electric Dipole: Concepts, Formulas & Uses
1. What is the fundamental concept of an electric dipole?
An electric dipole is a system consisting of two equal and opposite point charges, +q and -q, separated by a very small, finite distance, typically denoted as 2a. The line connecting the two charges is called the dipole axis. This arrangement is fundamental to understanding the behaviour of atoms and molecules in external electric fields.
2. What is the difference between an electric dipole and an electric dipole moment?
The key difference lies in what they describe:
- An electric dipole refers to the physical arrangement itself—the pair of equal and opposite charges separated by a small distance.
- The electric dipole moment (p) is a vector quantity that measures the strength and orientation of the dipole. It quantifies the overall polarity of the system.
3. What is the formula for electric dipole moment, and what are its SI unit and direction?
The formula for electric dipole moment (p) is the product of the magnitude of either charge (q) and the distance of separation between the charges (2a).
The formula is: p = q × 2a.
- The SI unit is the coulomb-meter (C·m).
- Its direction is a vector that points from the negative charge (-q) towards the positive charge (+q) along the dipole axis.
4. What happens when an electric dipole is placed in a uniform electric field?
When placed in a uniform electric field (E), an electric dipole experiences:
- Zero net force: The force on the positive charge (+qE) is equal and opposite to the force on the negative charge (-qE), so the dipole does not undergo translational motion.
- A net torque: The two equal and opposite forces create a turning effect, or torque (τ = p × E), which tends to align the dipole moment with the direction of the electric field.
5. How does the behaviour of an electric dipole change if placed in a non-uniform electric field?
In a non-uniform electric field, the forces on the positive and negative charges are no longer equal in magnitude. Consequently, the dipole experiences both:
- A net torque, which tries to align it with the field.
- A net force, which causes it to accelerate and undergo translational motion towards the region of the stronger electric field.
6. How does the electric field generated by a dipole differ at its axial and equatorial points?
The electric field of a dipole varies significantly with position:
- At an axial point (a point on the dipole axis), the electric field is strong and its direction is parallel to the dipole moment. For a short dipole, its magnitude is E_axial ≈ (2kp)/r³.
- At an equatorial point (a point on the perpendicular bisector), the electric field is weaker and its direction is opposite to the dipole moment. For a short dipole, its magnitude is E_equatorial ≈ (kp)/r³.
7. Why do some molecules like H₂O have a permanent electric dipole moment, while others like CO₂ do not?
This difference is due to molecular geometry.
- In water (H₂O), the molecule has a bent shape. The individual bond dipoles between oxygen and hydrogen do not cancel each other out, resulting in a net, permanent dipole moment. This makes water a polar molecule.
- In carbon dioxide (CO₂), the molecule is linear and symmetrical. The two C=O bond dipoles are equal in magnitude but point in opposite directions, causing them to cancel each other completely. Therefore, CO₂ has a zero net dipole moment and is a non-polar molecule.
8. What is the importance of the electric dipole concept in Physics and Chemistry?
The concept of the electric dipole is crucial for several reasons:
- It explains the behaviour of dielectric materials when placed in an electric field.
- It is fundamental to understanding the nature of polar and non-polar molecules, which determines properties like solubility and boiling points.
- It is used to explain phenomena like the functioning of a microwave oven, where oscillating electric fields interact with the dipole moment of water molecules to generate heat.
