Alternating Current

There are two types of electric current used in the transmission of electricity - direct current, which is unidirectional flow of electric charge and alternating current, which changes or alternates its magnitude and direction periodically. Another type of current which occurs due to charging or discharging of capacitors is known as transient current. When alternating e.m.f. is applied to different elements, the current drawn through them is different. An alternating e.m.f. applied across a resistor gives an alternating current having same phase as that of the alternating e.m.f. When applied across an inductor, it gives rise to an alternating current which lags behind the e.m.f. by pi by 2. However, when applied across a capacitor, it gives an alternating current that leads the voltage by pi by 2. When an alternating voltage is applied to LCR circuit, the current can be found by either phasor diagram technique or analytical method. The analysis of an a.c. circuit is facilitated by the use of phasor diagram. A phasor is a vector, which rotates about the origin with an angular speed. In the phasor diagram, peak value of a.c. and alternating e.m.f. are represented by arrows called phasors. Phasor diagram is drawn to show the phase relationship between the voltage and current. It is an easy way to represent the relationship of voltage and current. When a charged capacitor is connected to an inductor, the charge on the capacitor and the current in the circuit exhibit the phenomena of electrical oscillations. These oscillations are known as LC oscillations. The process of oscillating charge and current will repeat itself till the system reverts to its original state. So, the energy in the system oscillates between the capacitor and the inductor. Mainly, there are two main types of oscillations (i) Damped oscillations (ii) Undamped Oscillations. A device used to change an alternating voltage from one value to another greater or smaller value is called transformer. It works on the principle of mutual induction between two circuits linked by a common magnetic flux. Ideal transformer is the one which is 100% efficient. It has no power loss and the power input in it is equal to power output. Practically, it is not possible to build an ideal transformer, which is efficient. But a well-designed transformer may have an efficiency of more than. A transformer, which increases the alternating voltage, is known as step-up transformer while that which decreases the alternating voltage is known as step-down transformer. There are energy losses in transformers due to magnetic flux leakage, eddy currents and hysteresis loss.

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