sf1 said:

i study RLC circuits in school but i just dont understand why i study

it for...

voltage leads current and current leads voltage 90 degrees, what does

that mean and why is it important to study it?

First, try to really understand what a resistor does. It's pretty easy,

because the relationship between the voltage across a resistor and the

current through that resistor is always fixed -- at all points in time, and

for all types of applied signals. The current through a resistor is ALWAYS

in phase with the voltage across the resistor regardless of the applied

waveshape of the voltage (or current).

Also, understand that resistors do NOT store electrical energy, they only

radiate the energy as heat. Disconnect them from a circuit and you will not

be able to recover any of the energy that has been radiated by them.

Inductors and capacitors ARE electrical energy storage devices. If an

un-energized L or C is somehow enticed to store some electrical energy then

it takes time to change that stored energy from one level to another

(regardless of whether you're increasing or decreasing its energy level).

Here's an easy way to understand a capacitor:

If you start with an un-energized C (i.e., its initial voltage = 0V), and

then connect that C to a constant current source, then the voltage across

the capacitor will increase linearly with time. The voltage is "lagging" the

current because it takes time for the voltage (and energy) to build up.

If you start with an un-energized C, and then connect that C to a current

source that fluctuates with a sinusoidal characteristic, then the voltage

will also have a sinusoidal characteristic, but it will have a phase shift

of 90degrees with respect to the current through that C. This 90degree

relationship is ONLY true for a sinusoidal stimulus. The voltage is said to

"lag" the current by 90degrees (for a sin stimulus).

A similar thing happens with an inductor. If you connect that L to a

constant voltage source then the current will increase linearly with time.

If you apply a sinusoidal voltage then the current will have a phase shift

of 90degrees with respect to the voltage across that L. This 90degree

relationship (yada, yada, yada...). The current is said to "lag" the voltage

by 90degrees (for a sin stimulus).

Think and experiment.

Bob