Electric Charges and Fields

Electric Dipole

Electric charge is present around us and there are many different examples to prove this phenomenon. Have you ever tried rubbing a comb-over a towel and brought it close to your hair? You will see that some of your hair tend to get attracted to the comb. This is basically due to the generation of Electric Charge. In this section, we will try to decode the behavior of opposite charges when kept at a distance. This is the concept of the Electric Dipole which is a vital portion of electrostatics.

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Introduction to Electric Dipole

Electric Dipole

An electric dipole is tagged as a pair of objects which possess equal & opposite charges, parted by a significantly small distance. Let us take two charges having equal magnitude ‘Q’, which are separated by the distance ‘D’.

Here we assume the first charge to be negative, while the second charge stays positive. You can call this particular combination as an electric dipole. Hence, we can state that an electric dipole is formed due to the grouping of equal & opposite charges when separated by an assured distance.

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What is the Dipole Moment?

It is basically the exact measure of the strength associated with an electric dipole. Based on scientific and mathematical conclusions, the dipole moment magnitude is the product of either of the charges and the separation distance (d) between them. Do remember that, the dipole moment is a vector measure whose direction runs from negative to a positive charge.

The formula for electric dipole moment for a pair of equal & opposite charges is p = qd, the magnitude of the charges multiplied by the distance between the two.

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Dipole Placed in Electric Field

Although the two forces acting on the dipole ends cancel each other as free vectors, they do act as different points.  Hence, it does develop a torque on the dipole. Further, there is a rotating effect due to this torque which is experienced by the dipole.

The torque (t) magnitude considering the dipole center is the sum of the two forces times their respective distance arms, that is:

|t| = 2q |E|a sin q

=   |p||E| sin q

t = p × E    (expressed in newton-meter)

Therefore, in the presence of the uniform electric field, a dipole tends to align itself parallel to the concerned field. For this to happen, there are other conditions too, that is, orientation stays at some non-zero angle denoted as ‘q’. Further, potential energy needs to be stored in the dipole at a preferred orientation, which starts from q = 0 to a nonzero q.

Solved Examples for You

Question: What is the dipole moment for a dipole having equal charges -2C and 2C separated with a distance of 2cm.

Solution: The calculated dipole moment for this condition is, p = q x d. Thus, p = 2 x 0.02 = 0.04 C-m.

Question: What is electric potential for a dipole? Electric potential due to a Dipole (V)

Solution: Let us assume there are two charges, –q, fixed at point A, and +q fixed at point B. These two are separated by a distance d, thus creating a dipole. Now, suppose the midpoint between AB is O. Therefore, the electric potential as a result of the dipole placed at any point P, when OP = r, is calculated as:

V = (1/4πε) x pcosΘ / r2

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