Electrical Engineering ⇒ Topic : Applications of Amperes Work Law
Applications of Ampere's Work Law
Ampere's work law can be used to find magnetising force (H) in simple conductor arrangements. We shall discuss two cases by way of illustration.
1. Magnetising force around a long straight conductor. Consider the case of a long straight conductor 1-pole carrying a current of I amperes as shown in Fig.(a). The conductor will set up magnetic lines of force which encircle it. Consider a circular path of radius r metres. By symmetry, the field intensity H on all the points of this circular path will be the same. If a unit N-pole is moved once around this circular path, then work done is = 2πrH. By work law, this must be equal to the product of current and number of turns enclosed by this circular path.
Note that magnetic lines of force encircle the conductor like concentric circles. The direction of magnetic lines of force can be determined by right-hand rule.
If there had been N turns enclosed by the path, then,
The following points may be noted carefully
(I)If we choose a complete path for which r is smaller, H on that circle will be large. However, 2πrH will be still equal to NI.
(ii) Inspection of above expression reveals that H can also be expressed in ampere-turns per metre (AT/m).
(Ill) It is reminded that the quantity NI (i.e. product of the number of turns in a winding and the current flowing through it) is called magnetomotive force \ (m.m.f.).
m.m.f. = NI ampere-turns(2) Magnetising force due to long solenoid. Consider a long solenoid of length 1 and wound uniformly with N turns [See Fig. (A)]. The length of the solenoid is much
greater than the breadth, say 10 times greater. The following assumptions are permissible :
(1)The field strength external to the solenoid is effectively zero
(2)The field strength inside the solenoid is uniform.
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