TRANSFORMER
The transformer is a practical application of mutual induction. Transformers are used to increase or decrease AC
voltages. Usage of transformers is common because they change voltages with relatively little loss of energy. In fact, many of the devices in our homes, such as game systems, printers, and stereos use transformers for their working.
Working of a transformer
A transformer has two coils, electrically insulated from each other, but wound around the same iron core. One coil is called the primary coil. The other coil is called the secondary coil. Number of turns on the primary and the secondary coils are represented by N, and N_{s} respectively.
When the primary coil is connected to a source of AC voltage the changing current creates a changing magnetic field which is carried through the core to the secondary coil. In the secondary coil, the changing field induces an alternating e.m.f.
The e.m.f. induced in the secondary coil, called the secondar voltage V_{5} is proportional to the primary voltage V_{0} Th secondary voltage also depends on the ratio of the number c turns on the secondary coil to the number of turns on th primary coil, as shown by the following expression:
V_{s}/V_{s} = N_{s}/N_{s}
If the secondary voltage is larger than the primary voltage, th transformer is called a step-up transformer . If th secondary voltage is smaller than the primary voltage, th transformer is called a step-down transformer . In an ideal transformer, the electric power delivered to t secondary circuit is equal to the power supplied to t primary circuit. An ideal transformer dissipates no pow itself, and for such a transformer, we can write:
P_{D} = P_{1}
V_{D}*I_{0} = V_{n}*I_{n}
Second important long
D.C. MOTOR
We can see from Fig. 15.9 that the simple coil placed in a magnet cannot rotate more than 90°. The forces push the PQ side of the coil up and the RS side of the loop down until the loop reaches the vertical position. In this situation, plane of the loop is perpendicular to the magnetic field and the net force on the coil is zero. So the loop will not continue to turn because the forces are still up and down and hence balanced.
How can we make this coil to rotate continuously? It can be done by reversing the direction of the current just as the coil reaches its vertical position. This reversal of current will allow the coil to rotate continuously. To reverse direction of current, the connection to coil is made through an arrangement of brushes and a ring that is split into two halves, called a split ring commutator (Fig. 15.9). Brushes, which are usually pieces of graphite, make contact with the commutator and allow current to flow into the loop. As the loop rotates, so does the commutator. The split ring is arranged so that each half of the commutator changes brushes just as the coil reaches the vertical position. Changing brushes reverse the current in the loop.
As a result, the direction of the force on each side of the coil is reversed and it continues to rotate. This process repeats at each half-turn, causing coil to rotate in the magnetic field continuously. The result is an electric motor which is a device that converts electric energy into rotational kinetic energy.
In a practical electric motor, the coil, called the armature, is made of many loops mounted on a shaft or axle. The magnetic field is produced either by permanent magnets or by an electromagnet, called a field coil. The torque on the armature, and, as a result, the speed of the motor, is controlled by varying the current through the motor.
The total force acting on the armature can be increased by
Increasing the number of turns of the coil
Increasing the current in the coil
Increasing the strength of the magnetic field
Increasing the area of the coil
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