Dipole

In physics, a dipole is a special pattern that happens with electric and magnetic forces. The word comes from Ancient Greek, where δίς means "twice" and πόλος means "axis."

Dipoles can be electric or magnetic. An electric dipole happens when tiny bits of positive and negative electric charge are pulled apart, like in atoms or molecules. A magnetic dipole is like a tiny magnet, found in atoms, molecules, and electrons.

Both electric and magnetic dipoles have something called a dipole moment, which tells us how strong they are. This dipole moment acts like an arrow pointing in a certain direction. For electric dipoles, this arrow points from the negative charge to the positive charge. For magnetic dipoles, we can think of them as tiny loops of electric current.

These dipoles affect their surroundings. An electric dipole creates an electric field around it and feels a pull or twist when placed in another electric field. The same idea works for magnetic dipoles, but with magnetic fields instead. The math describing magnetic dipoles looks almost exactly like the math for electric dipoles.

For more details, see the articles on electric dipole moment and magnetic dipole.

Classification

Electric dipole

Main article: Electric dipole

Some objects, like atoms or molecules, have both positive and negative charges but no overall charge. These can be thought of as having two opposite charges very close together, called an electric dipole. The strength of this dipole is measured by something called the electric dipole moment. This moment tells us how strong the dipole is and points from the negative charge to the positive one.

Magnetic dipole

Main article: Magnetic dipole

A magnetic dipole describes a tiny magnet, such as one found in an atom or an electron. All magnets can be thought of as magnetic dipoles when we look at them from far away. The strength of a magnetic dipole is given by its magnetic dipole moment. There are two ways to think about magnetic dipoles. One simple way is to imagine the magnet as having two opposite poles, but this isn't completely accurate because magnetic poles don't really exist on their own. A better way is to think of the magnet as a tiny loop of electric current, which is a more correct description.

Physical vs. ideal dipole

A physical dipole has two equal but opposite charges very close together. When we move far away from these charges, the dipole's strength is what matters most. An ideal dipole is what we get when the two charges are moved infinitely close together while keeping their strength the same.

Dominant term in multipole expansion

Main article: Multipole expansion

When we look at charges or currents from far away, we can think of them as a series of simpler patterns. The first pattern is the monopole, which is just the total charge. For magnets, this pattern doesn't exist in nature. The next pattern is the dipole, which is usually the most important one when looking from far away. The strength of a dipole decreases more quickly with distance than other patterns. Even though magnetic monopoles don't exist, magnetic dipoles do, and they behave similarly to electric dipoles.

Potential of static dipoles

In electromagnetism, we can make things easier by first figuring out two important values, called the scalar potential Φ and the vector potential A.

The electrostatic potential at a point r because of an electric dipole at the start point is given by a special formula. This formula shows how the electric dipole moment p relates to the distance r. This idea is a big part of a larger way to figure out electric potentials.

Similarly, the vector potential A_dip at a point r because of a magnetic dipole moment m at the start point also has its own formula. This helps us understand magnetic fields from small magnets, like those from tiny parts of atoms. This idea is also a key part of a bigger way to figure out magnetic potentials.

Field of a static dipoles

The electric field and magnetic field around a dipole can be described using special math formulas. These formulas help us understand how electric and magnetic forces act at different distances from the dipole.

These ideas are important for studying very small particles like atoms and molecules, where such dipoles naturally occur. The formulas work perfectly for ideal, point-like dipoles and give good approximations for real dipoles when we look from far away.

Torques and forces on static dipoles

Electric fields point away from positive charges and toward negative charges. When an electric or magnetic dipole is placed in a steady electric or magnetic field, it experiences forces on both sides that create a turning effect called torque.

This torque tries to turn the dipole to match the direction of the field. For an electric dipole, this matching creates a certain amount of energy, and the same idea applies to magnetic dipoles.

Molecular electric dipoles

See also: Chemical polarity and Dipole moments of molecules

Electric dipole moments help explain how substances behave when placed in electric fields. These dipoles line up with the field, whether it stays the same or changes over time. This idea is used in a technique called dielectric spectroscopy.

Many everyday molecules, like water, and even important biomolecules such as proteins, have dipole moments. This happens because the positive and negative charges in their atoms are not spread out evenly. A molecule’s dipole is an electric dipole, which creates its own electric field, different from a magnetic dipole that makes a magnetic field. The scientist Peter J. W. Debye studied these dipoles a lot, and we measure them using a unit named after him, called the debye.

For molecules, there are three kinds of dipoles:

Permanent dipoles happen when atoms in a molecule pull electrons differently — one atom becomes more negative and the other more positive. Such a molecule is called a polar molecule. See Intermolecular force § Dipole–dipole interactions.

Instantaneous dipoles occur by chance when electrons are more in one place than another in a molecule, making a temporary dipole. These are smaller but still important. See instantaneous dipole.

Induced dipoles happen when a molecule with a permanent dipole pushes another molecule’s electrons, creating a dipole in that molecule. When this happens, the molecule is polarized. See induced-dipole attraction.

Quantum-mechanical dipole operator

Imagine a group of tiny particles, like the parts of a molecule, each having a small electric charge. We can describe how these charges are arranged using something called the dipole operator. This helps us understand how the charges are spread out in space.

This idea works best when the total charge of all the particles adds up to zero, meaning the group is balanced. If the group isn’t balanced, we adjust the calculation to focus on the center of the group.

Atomic dipoles

An atom in its simplest state cannot have a permanent dipole. This is because the atom’s symmetry means that any positive and negative charges balance out perfectly.

For atoms with more complex energy levels, a dipole moment can sometimes be created if certain energy states behave differently under symmetry operations. This is a special case and does not happen often. One example is in excited hydrogen atoms where specific states have opposite symmetry properties.

Degenerate energy level Stark effect Parity Laplace–Runge–Lenz vector

Dipole radiation

See also: Dipole antenna

Dipoles can also be thought of when they are moving back and forth over time. This is a step beyond static dipoles and leads to what we call spherical wave radiation.

When we look at a dipole that moves back and forth in a regular way, we can describe it using something called angular frequency. The strength of this moving dipole creates waves of electricity and magnetism that spread out in all directions. These waves are not the same in every direction; they are strongest where the dipole points and weaker where it points straight.

The amount of energy these waves carry away depends on how fast the dipole moves and how strong it is. This helps explain why the sky looks blue, since blue light moves in ways that match these kinds of waves well.

A dipole that spins in a circle can be thought of as two dipoles moving side by side.