## What isÂ Electric Field of a Dipole?

A dipole is a separation of opposite electrical charges and it is quantified by an electric dipole moment. The **electric dipole moment** associated with two equal charges of opposite polarity separated by a distance â€˜*d *â€™ is defined as the vector quantity having a magnitude equal to the product of the charge and the distance between the charges and having a direction from the negative to the positive charge along the line between the charges.

It is a useful concept in dielectrics and other applications in solid and liquid materials. These applications involve the energy of a dipole and the** electric field of a dipole**.

## How to CalculateÂ Electric Field of a Dipole?

Consider an electric dipole with charges \(+q\) and \(â€“q\) separated by a distance of \(d\). We shall designate components due to \(+q\) and \(â€“q\) using subscripts \(+\) and \(â€“\) respectively.

We shall for the sake of simplicity only calculate the fields along symmetry axes, i.e. a point **P**Â along the perpendicular bisector of the dipole and a point **Q**Â along the axis of the dipole.

### Along perpendicular bisector (Point **P**)

**The electric fields due to the positive and negative charges (Coulombâ€™s law):**

*Similarly,*

\(E_-\) = \(\frac{1}{4Ï€Îµ_0}\frac{q}{r_-^2}\) = \(\frac{1}{4Ï€Îµ_0} \frac{q}{r^2~+~(\frac{d}{2})^2}\)

The vertical components of electric field cancel out as \(P\) is equidistant from both charges.

**â‡’** \(E\) = \(E_+~cos~Î¸~+~E_-~cos~Î¸\)

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{q}{r^2~+~(\frac{d}{2})^2}~cos~Î¸~+~ \frac{1}{4Ï€Îµ_0} \frac{q}{r^2~+~(\frac{d}{2})^2}~cos~Î¸\)

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{2q}{r^2~+~(\frac{d}{2})^2}~cos~Î¸\)

*Now,*

**â‡’** \(cos~Î¸\) = \(\frac{\frac{d}{2}}{r_+}\) = \(\frac{\frac{d}{2}}{r_-}\) = \(\frac{\frac{d}{2}}{âˆš{r^2~+~(\frac{d}{2})^2}}\)

*Substituting this value we get,*

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{2q}{r^2~+~(\frac{d}{2})^2}\frac{\frac{d}{2}}{âˆš{r^2~+~(\frac{d}{2})^2}}\) = \(\frac{1}{4Ï€Îµ_0}~ \frac{qd}{(r^2~+~(\frac{d}{2})^2)^{\frac{3}{2}}}\)

**Dipole momentÂ **\(p\) = \(q~Ã—~d\)

When \(r\gt\gt d\), we can neglect the \(\frac{d}{2}\) term. Thus we have,

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{p}{r^2}^{\frac{3}{2}}\)

**â‡’ \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{p}{r^3}\)**

The dipole moment direction is defined as pointing towards the positive charge. Thus, the direction of electric field is opposite to the dipole moment:

\(\overrightarrow{E}\) = \(-\frac{1}{4Ï€Îµ_0} \frac{\overrightarrow{p}}{r^3}\) |

### Along axis of dipole (Point **Q**)

**The electric fields due to the positive and negative charges are:**

**Since the electric fields are along the same line but opposing directions,**

\(E\) = \(E_+~-~E_-\)

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{q}{(r~-~\frac{d}{2})^2}~-~\frac{1}{4Ï€Îµ_0}\frac{q}{(r~+~\frac{d}{2})^2}\)

**â‡’** \(E\) = \(\frac{q}{4Ï€Îµ_0} \left[\frac{1}{(r~-~\frac{d}{2})^2}~-~ \frac{1}{(r~+~\frac{d}{2})^2}\right]\)

**â‡’** \(E\) = \(\frac{q}{4Ï€Îµ_0} \left[\frac{(r~+~\frac{d}{2})^2~-~(r~-~\frac{d}{2})^2}{(r^2~-~(\frac{d}{2})^2)^2}\right]\)

**â‡’** \(E\) = \(\frac{q}{4Ï€Îµ_0} \left[\frac{4r \frac{d}{2}}{(r^2~-~(\frac{d}{2})^2)^2}\right]\)

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \left[\frac{2rqd}{(r^2~-~(\frac{d}{2})^2)^2}\right]\)

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0}~\left[\frac{2rp}{(r^2~-~(\frac{d}{2})^2)^2}\right]\)

*Factoring* \(r^4\) *from denominator and numerator:*

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{1}{r^4} \left[\frac{2pr}{(1~-~(\frac{d}{2r})^2)^2}\right]\)

Now if \(r\gt\gt d\), we can neglect the \((\frac{d}{2r})^2\) term becomes very much smaller than 1. Thus, we can neglect this term. The equation becomes:

**â‡’** \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{1}{r^4} \left[\frac{2pr}{1^2}\right]\)

**â‡’ \(E\) = \(\frac{1}{4Ï€Îµ_0} \frac{2p}{r^3}\)**

*Since in this case the electric field is along the dipole moment,* (\(E_+ \gt E_-\))

\(\overrightarrow{E}\) = \(\frac{1}{4Ï€Îµ_0} \frac{2\overrightarrow{p}}{r^3}\) |

Notice that in both cases the electric field tapers quickly as the inverse of the cube of the distance. Compared to a point charge which only decreases as the inverse of the square of the distance, the dipoles field decreases much faster because it contains both a positive and negative charge. If they were brought to the same point their electric fields would cancel out completely but since they have a small distance separating them, they have a feeble electric field.

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