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