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Dipoles are electromagnetic phenomena with two opposing forces. Electric dipoles have positive and negative charges, while magnetic dipoles have north and south poles. Molecular dipoles occur when a molecule has charge separation, creating a partial positive and negative charge. Magnetic dipoles are found in bar magnets, compasses, and the spin of electrons.
The term dipole is used in physics and chemistry to describe an electromagnetic phenomenon in which two poles, or opposing forces, exist in a given region. A dipole can arise in many different situations, but most dipoles can be classified as either electric or magnetic. Electric dipoles are often encountered in chemistry in the form of molecular dipole moments – the separation of positive and negative electrical charge across a molecule. Magnetic dipoles can be observed in ordinary magnets and compasses, as well as in the microscale activity of electrons and other particles.
Electric charge consists of two opposing, or polar, forces: positive and negative charge. These two forces attract each other when brought together, but repel other charges of the same type, i.e. negative repels negative. The type of charge a substance has is determined by the distribution of its subatomic particles. Electrons carry a negative charge, while protons, found in atomic nuclei, carry a positive charge.
A molecular dipole occurs when a molecule has charge separation throughout its structure. For example, a water molecule has a structural arrangement that attracts electrons to one side of the molecule, leaving the other side with fewer electrons. Thus a partial positive charge is formed on one end of the molecule, while a partial negative charge is formed on the other, which makes the molecule polar. In other words, the water molecule has a molecular dipole.
The charge separation of the water molecule does not change or disappear over time, so it is said to be a permanent dipole. A temporary alignment called an instantaneous dipole can also form in some molecules when electrons temporarily migrate to one part of the structure. Induced dipoles occur when electrons in a molecule are attracted to or repelled by a polar molecule.
Magnetic dipoles form the second major category of naturally occurring dipoles. Like electric dipoles, they consist of two opposite poles that attract each other. Bar magnets and compass needles are both examples of this type of system.
The north and south poles of a magnet create a dipole by aligning themselves with a magnetic field. Physicists think of a magnetic dipole as a loop of electric current that rotates around an axis as it aligns itself. For example, a compass needle rotates to point north, aligning itself with the earth’s field. The spin of an electron around the nucleus of an atom is also considered a magnetic dipole.
