## Magnetic Field Strength and Magnetic Flux Density

### 1. Magnetic Field Strength

A magnetic field is a force field generated by moving electrical charges. An electric current running through a loop of wire generates a magnetic field. The strength of the field depends on the current and the area of the wire loop.
In the UK and the USA, the strength of the magnetic field is normally expressed as Magnetic Field Strength (SI unit: amperes per metre, A/m), or Magnetic Flux Density (B).
Consider the interior of a long wire coil in air.
Suppose that the linear current density in this coil is 1 ampere per metre along the coil axis.
Then the magnetic field strength in the interior of the coil is 1 A/m.

Figure 1. Diagram to illustrate magnetic field strength

For a given coil, the magnetic field strength is directly proportional to the linear current density - if the linear current density doubles, the magnetic field density doubles. If the linear current density diminishes by a factor of 10, then the magnetic field strength also diminishes by a factor of 10.

### 2. Magnetic Flux Density

The Magnetic Flux is the rate of flow of magnetic energy across or through a (real or imaginery) surface.
The Magnetic Flux Density is a measure of the amount of magnetic flux in a unit area perpendicular to the direction of magnetic flow, or the amount of magnetism induced in a substance placed in the magnetic field.

The SI unit of magnetic flux density is the Tesla, (T).

One Tesla, (1T), is equivalent to one weber per square metre (1 Wb m2).
weber = the magnetic flux which, linking a circuit of one turn, produces in it an electromotive force (emf) of 1 volt as it is uniformly reduced to zero in one second.

In the USA, cgs units (Gauss, G) are often used, where 1T = 10,000 Gauss, or 1 µT = 10 mG.

The intensity of the Magnetic Flux Density, (B), is affected by the intensity of the Magnetic Field, (H), the quantities of the substance and the intervening media between the source of the magnetic field and the substance.

The relationship between magnetic field strength and magnetic flux density is:
B = H × µ

where µ is the magnetic permeability of the substance.
When dealing with exposure of non-ferromagnetic material such as animals or cells, Magnetic Flux Density and Magnetic Field Strength can be assumed to be equal.