 # Alternating Current

## Two types of electric current used in the transmission of electricity - direct current and alternating current. Direct current is the unidirectional flow of electric charges and alternating current, which changes or alternates its magnitude and direction periodically. Current which occurs due to charging or discharging of capacitors is known as transient current. The current drawn through different elements is different, when an alternating e.m.f is applied to them. An alternating e.m.f. applied across a resistor gives an alternating current having same phase as that of the alternating e.m.f.  An alternating current lags behind the e.m.f. by pi by 2 in case alternating e.m.f applied across the inductor. When an alternating e.m.f 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 electric current can be found by either phasor diagram technique or analytical method. The phasor diagram is used for the analysis of an a.c. circuit. 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. The phase relationship between the voltage and current is shown by Phasor diagram. 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. Transformer is a device that is used to change an alternating voltage from one value to another greater or smaller value. It works on the principle of mutual induction between two circuits linked by a common magnetic flux. Transformer which is 100% efficient is known as Ideal transformer. 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 Step-up transformer is the one which increases the alternating voltage 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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