Magentic Dipole.docx

Magnetic dipole, generally a tiny magnet of microscopic to subatomic dimensions, equivalent to a flow of electric charge around a loop. Electrons circulating around atomic nuclei, electrons spinning on their axes, and rotating positively charged atomic nuclei all are magnetic dipoles. The sum of these effects may cancel so that a given type of atom may not be a magnetic dipole. If they do not fully cancel, the atom is a permanent magnetic dipole, as are iron atoms. Many millions of iron atoms spontaneously locked into the same alignment to form a ferromagnetic domain also constitute a magnetic dipole. Magnetic compass needles and bar magnets are examples of macroscopic magnetic dipoles. The strength of a magnetic dipole, called the magnetic dipole moment, may be thought of as a measure of a dipole’s ability to turn itself into alignment with a given external magnetic field. In a uniform magnetic field, the magnitude of the dipole moment is proportional to the maximum amount of torque on the dipole, which occurs when the dipole is at right angles to the magnetic field. The magnetic dipole moment, often simply called the magnetic moment, may be defined then as the maximum amount of torque caused by magnetic force on a dipole that arises per unit value of surrounding magnetic field in vacuum. When a magnetic dipole is considered as a current loop, the magnitude of the dipole moment is proportional to the current multiplied by the size of the enclosed area. The direction of the dipole moment, which may be represented mathematically as a vector, is perpendicularly away from the side of the surface enclosed by the counterclockwise path of positive charge flow. Considering the current loop as a tiny magnet, this vector corresponds to the direction from the south to the north pole. When free to rotate, dipoles align themselves so that their moments point predominantly in the direction of the external magnetic field. Nuclear and electron magnetic moments are quantized, which means that they may be oriented in space at only certain discrete angles with respect to the direction of the external field. Magnetic dipole moments have dimensions of current times area or energy divided by magnetic flux density. In the metre–kilogram– second–ampere and SI systems, the specific unit for dipole moment is ampere-square metre. In the centimetre–gram–second electromagnetic system, the unit is the erg (unit of energy) per gauss (unit of magnetic flux density). One thousand ergs per gauss equal one ampere-square metre. A convenient unit for the magnetic dipole moment of electrons is the Bohr magneton (equivalent to 9.27 × 10−24 ampere–square metre). A similar unit for magnetic moments of nuclei, protons, and neutrons is the nuclear magneton (equivalent to 5.051 × 10−27 ampere–square metre).

Interaction of Magnetic Dipoles in External Fields Torque By the F = iL × Bext force law, we know that a current loop (and thus a magnetic dipole) feels a torque when placed in an external magnetic field: τ = μ × Bext The direction of the torque is to line up the dipole moment with the magnetic field: F μθ i Bext Potential Energy Since the magnetic dipole wants to line up with the magnetic field, it must have higher potential energy when it is aligned opposite to the magnetic field direction and lower potential energy when it is aligned with the field. To see this, let us calculate the work done by the magnetic field when aligning the dipole. Let θ be the angle between the magnetic dipole direction and the external field direction.

W =∫F⋅ds =∫Fsinθds=−∫rFsinθdθ (whereds=−rdθ) =−∫r×Fdθ ⇒W =−∫τ dθ Now the potential energy of the dipole is the negative of the work done by the field: U =−W =∫τdθ The zero-point of the potential energy is arbitrary, so let’s take it to be zero when θ=90°. F Then: U=+∫θ τdθ=+∫θ μBsinθ′dθ′ π/2 π/2 The positive sign arises because τ ⋅ dθ = −τ dθ , τ and θ are oppositely aligned. Thus, U=−μBcosθ θ =−μBcosθ π/2 Or simply: The lowest energy configuration is for μ and B parallel. Work (energy) is required to U =−μ⋅B re-align the magnetic dipole in an external B field. Bμ B μ Lowest energy Highest energy The change in energy required to flip a dipole from one alignment to the other is ΔU = 2μB