Starting from Ampere’s circuital law, find the B field, total flux,and inductance of coaxial cable between the conductor and the shielding.
Question
Starting from Ampere’s circuital law, find the B field, total flux,and inductance of coaxial cable between the conductor and the shielding.
Solution
Sure, let's start with Ampere's circuital law. This law states that the integral of the magnetic field (B) around a closed loop is equal to the product of the current enclosed by the loop and the permeability of the medium. Mathematically, it is represented as:
∮ B . dl = μ0 * I
where: B is the magnetic field, dl is the length element along the closed path, μ0 is the permeability of free space, I is the current enclosed by the path.
Now, let's consider a coaxial cable with inner conductor radius 'a' and outer conductor (shielding) radius 'b'. The current 'I' flows in the positive z direction in the inner conductor and in the negative z direction in the outer conductor.
- Magnetic Field (B): The magnetic field at a point located at a distance 'r' from the axis inside the cable (a < r < b) due to the current in the inner conductor can be found using Ampere's law:
B = (μ0 * I) / (2 * π * r)
- Total Flux (Φ): The total magnetic flux through the surface of the coaxial cable can be found by integrating the magnetic field over the cross-sectional area of the cable:
Φ = ∫ B . dA = ∫ ((μ0 * I) / (2 * π * r)) * dA = (μ0 * I / 2π) * ln(b/a)
where ln is the natural logarithm.
- Inductance (L): The inductance of the cable can be found using the definition of inductance, which is the ratio of the total magnetic flux to the current:
L = Φ / I = (μ0 / 2π) * ln(b/a)
This is the inductance per unit length for a coaxial cable.
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