Phase Angle

Dear Sir,


Can you please guide me why current always lags in inductive circuit and why lead in a capacitive circuit????

If our ind circuit is short on start then why it becomes open after full charge?? if we look at the formula XL=2piFL how can i justify ind short on start and open after charging

Q= 1/2 (L x i2)

YOU HAVE IT REVERSED - IT IS AN OPEN ON START AND SHORT AT FULL CHARGE

For DC it is always considered a short - except for transitions

Read any circuit analysis book to understand the reason behind charging and discharging an inductor

http://en.wikipedia.org/wiki/Inductance

http://en.wikipedia.org/wiki/Capacitance

http://en.wikipedia.org/wiki/Phase_angle

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This is very simplistic but basically a capacitor stores charge, sort of like a tire stores air. Electrons repel each other so it takes more potential (voltage) to force them together, like putting air in your tire, the more air the greater the pressure. As the electrons enter the capacitor the voltage rises (like the internal pressure rising in a tire) and the flow decreases until the electrons are packed as tightly as they can be for a given voltage across the terminals and flow stops (again like when the tire pressure reaches the pump pressure). The initial flow rate is determined by the capacitance and the resistance in the capacitor. (Similar to the size of a tire and the charging hose diameter.).

An inductor creates a magnetic field when current flows through it. It takes energy to create a magnetic field (there's no free lunch) and this resists changes in the flow of electricity. The magnetic field creates a "back pressure" as the magnetic field increases. The inductor has a resistance so it acts as a resistor once the magnetic field is established and the voltage across the inductor is unchanged.

When the potential changes the magnetic field changes and the energy stored in the magnetic field changes. If you abruptly disconnect an inductor and interrupt the flow in an inductor the field collapses and creates a voltage potential with the reverse polarity. This is dissipated by the internal resistance or external circuitry but can result in a big reverse voltage spike as it dissipates, sometimes called inductive kick back. This can burn the contacts of relays driving coils or cause unwanted spikes in circuits.

An analog in nature may simplify your understanding. You understand springs and I am sure you understand trying to push something heavy like a car.

A cap is like a spring, you have to compress it, so when you apply a force it moves quickly (depends of course). The force is the voltage analog, the compression is the stored energy so that the voltage is ahead of the current, Remember energy is V*I*t

With a heavy object, you apply a force to move it but initially it does not move. The force is the voltage which is there 1st then the movement which is the current. So that again the energy comes from the force*distance, so that in this case the current is behind (lagging) the force-voltage.

A swing in the playground is a good analogy.

Current lags in inductive circuit because it offers high opposition to the flow current due to expanding magnetic field around the inductor, current is minimum and voltage is full. So current lags and voltage leads.

In case of capacitor; initially current is maximum through capacitor as there is no opposition for current flow in capacitor due to electrostatic field, but voltage is opposed by electrostatic field. So voltage lags and current leads.

Inductor do not short on start it opens and becomes short on reaching maximum current so formula is rightly tells what I explained.

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