Biot-Savart law is suitable for calculating the magnetic field generated by a steady current. This current is the charge that keeps flowing through the wire. The magnitude of the current does not change with time, and the charge will not accumulate or disappear at any position. The international system of units is adopted and expressed by the following equation:

Where I is the source current, L is the integral path, dl is the tiny line element of the source current, the unit vector of the current source pointing to the field point to be solved, μ0 is the magnetic permeability of vacuum, and the value is

The direction of dB is perpendicular to Idl and the determined plane. When the right hand bends and the four fingers turn to R from the direction less than the angle, the direction pointed by the straight thumb is the direction of dB, that is, the directions of the three vectors of dB, dl and R conform to the right hand rule.

The following are specific solutions to this problem:

(1) The contribution of horizontal current from left to right to the magnetic field at point O is zero. The magnetic fields of the above two semicircle currents are offset at the O point, so it is only necessary to calculate the magnetic field from the vertical current to the O point. Considering that the magnetic field generated by an infinitely long straight charged wire is B=μ0I/(2πr), and the length of the wire here is unidirectional infinity, the magnetic field at point O is μ0I/(4πR).

(2) Similarly, the contribution of the straight wire on the right to the magnetic field at point O is zero, and the contribution of the semicircle current: for Biosavart's law, by integrating the semicircle current, B 1=μ0I/(4R) is obtained, and the contribution of the current in the unidirectional long straight wire is μ0I/(4πR) calculated in (1), and both directions are perpendicular.