Định luật Biot-Savard

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Định luật Biot-Savard định nghỉa tương quan giửa Dòng điện và Từ trường . Định luật Biot-Savart Law cho rằng:


, where is the magnetic constant
is the current, measured in amperes
is the differential length vector of the current element
is the unit displacement vector from the current element to the field point and
is the distance from the current element to the field point

Phương trình trên chỉ đúng với dòng điện ổn

Mật độ dòng điện


is the unit vector in the direction of r.
= is the differential unit of volume.
Dòng điện ổn
Điện tích điểm với vận tốc không đổi

In the special case of a charged point particle moving at a constant velocity , then the equation above reduces to a magnetic field approximately of the form:

This formula, however, is wrong. This is because a point charge moving in a straight line does not constitute a steady current nor a constant charge distribution, both of which are essential to magnetostatics. In particular, in this case, there is a changing electric field and an induced magnetic field that must be added to correct the above formula. Even though the equation is not precisely correct, it is a very good approximation (unreasonably good, in fact).

Cực nhỏ

and hence,