Electric Dipole and Electric Potential

Electric Dipole Fundamentals and Dipole Moment

An electric dipole is formed by two charges of equal magnitude but opposite sign, separated by a distance. This system is characterized by a vector quantity known as the dipole moment, symbolized by \(\vec{p}\). The dipole moment points from the negative charge to the positive charge, and its magnitude is the product of one of the charges and the separation distance (\(2a\)), thus \(\left|\vec{p}\right|=q(2a)\). The correct SI unit for the dipole moment is the coulomb-meter (\(\mathrm{C\cdot m}\)). Molecules such as water (H2O), alcohols, and hydrogen chloride (HCl) have dipole moments because of the uneven distribution of their electrical charges.

Electric Potential Defined

Electric potential is a scalar quantity that represents the work done per unit charge to move a test charge from a reference point to a specific point within an electric field. The potential at a point due to a point charge \(+q\) is given by \(V=\frac{1}{4\pi\varepsilon_0}\frac{q}{r}\), where \(V\) is the electric potential, \(q\) is the charge, \(r\) is the distance from the charge to the point, and \(\varepsilon_0\) is the permittivity of free space. Electric charges naturally move from regions of higher potential to lower potential, and movement ceases when there is no potential difference.

Electric Potential of a Dipole at a Point

The electric potential due to a dipole at a point in space is the net work needed to bring a unit positive charge from infinity to that point against the electric field of the dipole. The potential is determined by the dipole's electric field, which emanates from the dipole and influences nearby charges. To find the potential at a point P due to a dipole, one must consider the contributions from both charges of the dipole, using the superposition principle. The potential at point P is then \(V=\frac{1}{4\pi\epsilon_0}\left(\frac{q}{r_2}-\frac{q}{r_1}\right)\), where \(r_1\) and \(r_2\) are the distances from point P to the positive and negative charges, respectively.

Derivation of Electric Potential from a Dipole

To derive the electric potential from a dipole, we consider a point P at a distance \(r\) from the midpoint of the dipole and at an angle \(\theta\) with the dipole axis. Assuming the dipole length (\(2a\)) is much smaller than \(r\), we can use the law of cosines to approximate \(r_1\) and \(r_2\). Applying a Taylor series expansion and neglecting higher-order terms, the potential at P simplifies to \(V=\frac{p}{4\pi\epsilon_0r^2}\cos{\theta}\), where \(p\) is the magnitude of the dipole moment. This expression is useful for calculating the potential at any point in the vicinity of the dipole.

Electric Potential on Axial and Equatorial Lines

The electric potential due to a dipole varies with the position of the point of observation. Along the axial line, which extends through the charges of the dipole, the potential is given by \(V_{\mathrm{axial}}=\frac{p}{4\pi\epsilon_0r^2}\cos{\theta}\), reaching a maximum when \(\theta=0\). The potential decreases with the square of the distance from the dipole and is proportional to the dipole moment. On the equatorial line, perpendicular to the dipole axis, the potential is zero due to the symmetry of the system and the cancellation of the potentials from the two charges.

Summary of Electric Potential and Dipoles

In conclusion, the electric dipole is defined by its dipole moment, which significantly affects the electric potential in its surrounding space. The potential due to a dipole provides insight into the work required to move a unit positive charge within the dipole's electric field. The derived potential formula, \(V=\frac{p}{4\pi\epsilon_0r^2}\cos{\theta}\), is essential for understanding electric field interactions in different scenarios. The potential is maximal along the axial line and null along the equatorial line, highlighting the directional dependence of a dipole's influence on the electric potential in its environment.