# Formula Long solenoid Magnetic field    Coil length    Electric current    Number of turns    Relative permeability ## Magnetic field

Unit
Magnetic flux density inside the long coil. Inside the coil $$\class{violet}{B}$$ is homogeneous (constant) and exactly this constant value $$\class{violet}{B}$$ inside the coil is referred to by the formula.

Note that the magnetic flux density of a coil is not homogeneous outside.

## Coil length

Unit
Length from one end to the other end of the coil. The formula is precise only if the coil length is significantly larger than the radius $$r$$ of the coil.

## Electric current

Unit
Electric current is the amount of charge $$Q$$ that flows through the coil per unit time $$t$$. If you double the current, the magnetic field also doubles.

## Number of turns

Unit
Number of turns of the coil ("number of spirals"). The more turns a coil has, the larger the magnetic field generated by the coil.

## Relative permeability

Unit
It is possible to significantly amplify the magnetic field $$\class{violet}{B}$$ inside the coil by pushing a certain material into the coil interior. This material is characterized by the relative permeability.

If there is vacuum (or air) inside the coil, then $$\mu_{\text r} ~=~ 1$$. If you push an iron core into the coil, then the relative permeability can be 300 up to 10000. Thus the magnetic field would be amplified by the factor 300 to 10000.

## Magnetic field constant

Unit
Magnetic field constant is a natural constant and occurs whenever magnetic fields are involved. It has the value:$\mu_0 ~=~ 4\pi \cdot 10^{-7} \, \frac{ \text{N} }{ \text{A}^2 }$