The magnetic field (B) defined above is also known as the magnetic induction field, magnetic induction, or magnetic flux density. However, there is another type of magnetic field, denoted by H, called the magnetic field intensity. H and B have different units and a somewhat different physical significance. H may be thought of as an externally applied "magnetizing force," whereas B represents the actual magnetic field induced within a region of space. It is necessary to distinguish between H and B because the electromagnetic field at a given point in space depends not only on the distribution of electrical currents giving rise to that field (reflected in H) but also on the type of matter occupying the region (reflected in B).
When no matter is present (i.e., in a vacuum), B and H are essentially equivalent, except for a factor µo to adjust units of measurement. Thus, we can write: Bvac= µoH. The factor µo is called the permeability of free space and has the value 4π × 10-7 newtons/ampere² in SI units. Since B is measured in tesla (newtons per ampere-meter), the SI units for H must therefore be amperes per meter.
Whenever matter is present within a given region of space, the induced field (B) is generally not equal to the applied field (H). When H encounters matter, various electromagnetic interactions occur that can be thought of as tending to "concentrate" or "disperse" the magnetic lines of force. This phenomenon results primarily from the action of unpaired orbital and delocalized electrons, which set up circulating currents and secondarily induce an internal magnetization (Mi), also called the bulk magnetic moment, that serves either to augment or to oppose the applied field (H). This magnetization is proportional to the applied field by a dimensionless constant known as susceptibility (χ), expressed by the relationship
When no matter is present (i.e., in a vacuum), B and H are essentially equivalent, except for a factor µo to adjust units of measurement. Thus, we can write: Bvac= µoH. The factor µo is called the permeability of free space and has the value 4π × 10-7 newtons/ampere² in SI units. Since B is measured in tesla (newtons per ampere-meter), the SI units for H must therefore be amperes per meter.
Whenever matter is present within a given region of space, the induced field (B) is generally not equal to the applied field (H). When H encounters matter, various electromagnetic interactions occur that can be thought of as tending to "concentrate" or "disperse" the magnetic lines of force. This phenomenon results primarily from the action of unpaired orbital and delocalized electrons, which set up circulating currents and secondarily induce an internal magnetization (Mi), also called the bulk magnetic moment, that serves either to augment or to oppose the applied field (H). This magnetization is proportional to the applied field by a dimensionless constant known as susceptibility (χ), expressed by the relationship
Mi = χH.
In matter we can therefore write:
B = µo (Mi + H)
= µo (χH + H)
= µo (1+ χ) H
= µo µ H
= µo (χH + H)
= µo (1+ χ) H
= µo µ H
The new term µ is a dimensionless factor known as the magnetic permeability of the material. It is related to magnetic susceptibility by the expression µ = 1 + χ.
When µ >1 (or χ>0) the magnetic field can be thought of as "concentrated" relative to that in a vacuum and the substance is called paramagnetic. When µ <1 (or χ<0) the field can be considered relatively "thinned" or "dispersed" and the substance is called diamagnetic. These concepts will be covered in much more detail in subsequent questions.
When µ >1 (or χ>0) the magnetic field can be thought of as "concentrated" relative to that in a vacuum and the substance is called paramagnetic. When µ <1 (or χ<0) the field can be considered relatively "thinned" or "dispersed" and the substance is called diamagnetic. These concepts will be covered in much more detail in subsequent questions.
Relationship between B and H in matter