The inductance of a circuit is defined as the flux linkage per unit current. Therefore, in order to find the inductance of a circuit, the determination of flux linkage is of primary importance. We shall discuss two important cases of flux linkages.

### 1.Flux linkage due to a Single Current Carrying Conductor:

Consider a long straight cylindrical conductor of radirus r metres and carrying a current I amperes (r.m.s.) as shown in Fig. 9.4 (i). This current will set up magnetic field. The magnetic lines of force will exist inside the conductor as well as outside the conductor. Both these fluxes will contribute to the inductance of the conductor.

Flux linkages due to internal flux: Refer to Fig. 9.4 (ii) where the X-section of the conducĀ­tor is shown magnified for clarity. The magnetic field intensity at a point x metres from the centre is given by;

Assuming a uniform current density,

If Ī¼ ( = Ī¼0Ī¼rĀ )Ā is the permeability of the conductor, then flux density at the considered point is given by;

Now, flux dĪ¦ through a cylindrical shell of radial thickness dx and axial length 1 m is given by;

Therefore, flux linkages per metre length of the

Total flux linkages from centre upto the conductor surface is

Flux linkages due to external flux: Now let us calculate the flux linkages of conductor due to external flux. The external flux extends from the surface of the conductor to infinity. Referring to Fig. 9.5, the field intensity at a distance x metres (from centre) outside the conductor is given by ;

Now, flux dĪ¦ though a cylindrical shell of thickness dx and axial length 1 metre is

The flux dĪ¦ links all the current in the conductor once and only once.

Total flux linkages of the conductor from surface to infinity,

### 2.Flux linkage in Parallel Current Carrying Conductors:Ā

We shall now determineĀ the flux linkages in a group of parallel current carrying conductors. Fig. 9.6 shows, the conductors A,B,C etc. carrying currents IA,IB,ICĀ etc. Let us consider the flux linkages With one conductor, say conductor A. There will be fluxĀ linkages with conductor A due to its own current as discussed previously. Also there Will beĀ flux linkages with this conductor due to the mutual inductance effects of IB,IC,IDĀ etc. We shall now determine the total flux linkages with conĀ­ductor A.

Flux linkages with conductor A due to its own current

Flux linkages with conductor A due to current IB

Flux linkages with conductor A due to current IC

Total flux linkages with conductor A

Similarly, flux linkage with other conductors can be determined. The above relation provides the basis for evaluating inductance of any circuit.