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JEE MAIN & ADVANCE 12th PCM Physics -Alternating Current Module-1 Demo Videos

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Hello, students, welcome back to the chapter of alternating current. In the first module we have already discussed, what do we call as a direct current. A current that is constant or variable but does not change its direction in the interval of time is called as a direct current. Whereas an alternating current is one which can be positive for some amount of time and can be negative for some amount of time, is called as an alternating current. Further we have discussed, how can we construct something which is called as an AC generator. In this module we will discuss, how an AC generator works.

Let’s first revise how we have constructed an AC generator. It consists of two magnetic poles, one north the other south, in between which is placed a rotatable coil which is connected to slip rings and which are connected to an external circuit through two carbon brushes.

Let’s discuss how this combination works to provide us an alternating current. If we see in the figure as the coil rotates the magnetic flux through the coil, that is the number of field lines crossing the area in a unit time or at any instant changes and thus producing an EMF which can be read according to the relation Epsilon is equal to minus d pi by dt, where pi is the magnetic flux through the coil. Now, if we recall, the magnetic flux through a coil can be changed in three ways. First, by changing the magnetic field, second by changing the area of the coil that means either reducing the area or extending the area. But in this particular case the AC generator works on the principle that the magnetic flux can also be changed by changing the angle between the magnetic field and the area vector. Let’s discuss this phenomena in detail. Let’s say that t equal to zero the area vector is anti parallel to the magnetic field. Therefore the flux in this particular case is negative. Now, if the time taken to rotate one complete circle that is 360 degrees is T, then what would be the time to rotate by an angle of 90 degree? Yes, it’s T by 4. So, at T by 4, the coil rotates by an angle of 90 degree, therefore we can see that the magnetic field and the area vector are perpendicular to each other. Therefore the magnetic flux through the coil is zero. Now a further rotation of 90 degrees the time elapsed would have been T by 2. But as the coil rotates in the following manner then the magnetic flux and the area vector become parallel. And therefore the flux through the coil becomes positive. Now, if you note from T equal to zero, at T equal to zero the flux was negative. At T equal to T by 4 it becomes zero and at T by 2 it becomes positive. Therefore the flux from T equal to zero to T equal to T by 2 increases. Let’s discuss what happens after the time instant T by 2. On further rotation of 90 degree angle, the coil is placed like as shown in the figure. Again we can see that the area vector is perpendicular to the magnetic field and therefore the flux is zero again. On further rotation of 90 degree angle the time elapsed is T and the area vector is again anti parallel to the magnetic field, thus if we note from T equal to T by 2, the flux was positive. At T equal to 3T by 4 it is zero and T equal to T it becomes negative. So, we can say from T equal to T by 2 to T equal to T it is decreasing. And thus the coil rotates and the flux increases for half of the revolution and decreases for the other half. This change in magnetic flux thus produces an EMF, changing the direction of EMF itself which produces an alternating current. Now, this alternating current is produced through the area vector changing its angle with the magnetic field.

In the next module we will discuss the working and construction of what is called as a DC generator, thank you.

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