B IOT -S AVART L AW · Part 2

The magnetic field is produced by a vector source I d l . * The sense of dl×r is also given by the Right Hand Screw rule : Look at the plane containing vectors dl and r . Imagine moving from the first vector towards second vector. If the movement is anticlockwise, the resultant is towards you. If it is clockwise, the resultant is away from you. FIGURE 4.9 Illustration of the Biot-Savart law. The current element I d l produces a field d B at a distance r . The ⊗ sign indicates that the field is perpendicular to the plane of this page and directed into it. 144 E XAMPLE 4.5 (iii) The electrostatic field is along the displacement vector joining the source and the field point. The magnetic field is perpendicular to the plane containing the displacement vector r and the current element I d l . (iv) There is an angle dependence in the Biot-Savart law which is not present in the electrostatic case. In Fig. 4.9, the magnetic field at any point in the direction of d l (the dashed line) is zero. Along this line, θ = 0, sin θ = 0 and from Eq. [4.11(a)], |d B | = 0. There is an interesting relation between ε 0 , the permittivity of free space; µ 0 , the permeability of free space; and c , the speed of light in vacuum: