CBSE Class 12-science Answered
Let us consider a particularly simple transformer in which the primary and secondary coils are solenoids sharing the same air-filled core. Suppose that is the length of the core, and is its cross-sectional area. Let be the total number of turns in the primary coil, and let be the total number of turns in the secondary coil. Suppose that an alternating voltage
is fed into the primary circuit from some external AC power source. Here, is the peak voltage in the primary circuit, and is the alternation frequency (in radians per second). The current driven around the primary circuit is writtenwhere is the peak current. This current generates a changing magnetic flux, in the core of the solenoid, which links the secondary coil, and, thereby, inductively generates the alternating emf
in the secondary circuit, where is the peak voltage. Suppose that this emf drives an alternating current
around the secondary circuit, where is the peak current.
The circuit equation for the primary circuit is written
assuming that there is negligible resistance in this circuit. The first term in the above equation is the externally generated emf. The second term is the back-emf due to the self inductance of the primary coil. The final term is the emf due to the mutual inductance of the primary and secondary coils. In the absence of any significant resistance in the primary circuit, these three emfs must add up to zero. Equations 281, 282, 284 and285 can be combined to givesince
The alternating emf generated in the secondary circuit consists of the emf generated by the self inductance of the secondary coil, plus the emf generated by the mutual inductance of the primary and secondary coils. Thus,
Now, the instantaneous power output of the external AC power source which drives the primary circuit is
(290) |
(291) |
(292) |
Equations 286,289 and 293 yield
(294) |
(295) |
Equations 293 and 296 can be combined to give
(297) |
the self inductances of the primary and secondary coils are given by and , respectively. It follows that
(298) |