# Ampere’s law

We know that a straight conductor creates a circular magnetic field. The direction of the magnetic field, $B→,$ at a given point is simply the tangent to that circle. The magnitude is given by Ampere’s law:

$|B→|=μ0I2πr,$

where $B→$ is the magnetic field in teslas (T), ${\mu }_{0}$ is the permeability of free space (it has a constant value of 4π × 10−7 T⋅m/A), and $r$ is the distance from the conductor in metres (m).

For a coiled conductor (a solenoid), we use the equation

$|B→|=μ0NIL,$

where $B→,$ ${\mu }_{0}\text{,}$ and $I$ are the same as before, and where $N$ is the number of loops (unitless; always an integer; counted number, therefore $\infty$ significant digits; if you are solving for it, round up, never down) and $L$ is the length of the solenoid in metres (m).

## Example

Find the magnitude of the magnetic field 3.2 cm from a straight conductor with 0.75 A of current.

All we have to do is plug everything into the equation:

Therefore, the magnitude of the magnetic field is 4.7 × 10−6 T.