Physics (NTSE/Olympiad)  

2. Magnetic Effect of Current


AC Generator
Construction of an AC Cenerator:
It consists of a rectangular coil of insulated copper wire. This coil is placed in a magnetic field between the poles of a strong permanent horseshoe magnet. This magnet is known as the field magnet. In actual practice, a large number of turns of the insulated copper wire are suitably wound on an iron core called armature.

The two free ends of the coil are connected to the two slip rings R1 and R2. Current generated in the coil is taken out through the two carbon brushes B1 and B2 pressed lightly against the slip rings.
Working of an AC Generator : Let the coil ABCD be initially in the horizontal position, and is rotated in the anticlockwise direction. When the coil is rotated anticlockwise, the arm AB moves downwards and the arm CD moves upwards. The coil during this movement cuts the magnetic lines of force, and produces induced current in the coil.
According to the Fleming’s right–hand rule, during this downward motion of the arm AB, the induced current flows from B to A in the arm AB, and from D to C in the arm CD. The current so produced is taken out through the two slip rings, and the carbon brushes.
After half the rotation (after rotating through 180º), the arms of the coil interchange their position; the arm AB becomes the right arm and the arm CD becomes the left arm. Then, the arm CD starts moving downwards and the arm AB upwards. During this half–rotation, the induced current flows from C to D in the arm CD, and from A to B in the arm AB. The two slip rings also rotate with the coil. As a result, their polarities (+ and – poles) keep changing at every half–rotation.
The current which changes its polarity after regular intervals of time is called alternating current (AC). So, this electric generator produces alternating current (AC).
Frequency of the Alternating Current: Alternating current (AC) so produced has a definite frequency. This frequency is equal to half the number of times the polarity changes in one second. In our country, the frequency of the alternating current supplied by the power generation units is 50 cycles per second (or Hz). This means, the alternating current (AC) produced in our country changes polarity 100times in one second.

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