What are the similarities between electric dipole and current loop ?
Electric field intensity at a point on the axial line of a small electric dipole of electric dipole moment $p$ is given by,
$E = \frac{1}{4 \pi \varepsilon_0} \frac{2p}{x^3}$
where, $x$ is the distance of the point from the centre of the electric dipole.
Magnetic field intensity at a point on the axis of a loop carrying current is given by
$B = \frac{\mu_0}{4 \pi} \frac{2 \pi I R^2}{(R^2 + x^2)^{3/2}}$
where, $R$ is the radius of the loop and $x$ is the distance of the point from the centre of the loop.
But $\pi R^2 = A$ (Area of loop)
$B = \frac{\mu_0}{4 \pi} \frac{2 I A}{(R^2 + x^2)^{3/2}} \quad \dots (1)$
If $x >> R$, we have
$B = \frac{\mu_0}{4 \pi} \frac{2 I A}{x^3}$
Electric field intensity at a point on the equatorial line of small electric dipole is given by
$E = \frac{1}{4 \pi \varepsilon_0} \frac{p}{x^3} \quad \dots (\text{iii})$
Magnetic field intensity at a point on the equatorial line of small magnetic dipole is given by
$B = \frac{\mu_0}{4 \pi} \frac{m}{x^3} \quad \dots (\text{iv})$
But $IA = m$ (magnetic dipole moment of a current loop)
$B = \frac{\mu_0}{4 \pi} \frac{IA}{x^3}$
The comparison between eqn. (i) and eqn. (ii) shows that a current carrying loop behaves like a magnetic dipole at a large distance from the loop. This fact led Ampere to conclude that all magnetism is due to circulating currents.
The electric dipole consists of two elementary units i.e., electric charges known as electric monopoles. However, magnetic monopole (i.e., isolated North pole or isolated South pole) does not exist.