Why Does Current Lag Voltage in an Inductor?

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

As it's nature inductor always tries to be the same state as it is. Whenever a change is aaplied, it will try to vanish the cause of change. AC voltage always dynamic and it is getting changed it's magnitutude. Inductor is a rebel against the changes, so when voltage starts from +ve peak to zero it will tries to increase the current to resist the cause of change (Inductor intended to be at peak voltage condition at that time). As a result when voltage starts from +ve peak to zero current wiil be from zero to +ve peak. After that quarter cycle voltage is at zero point (current is at +ve peak point) and starts to increase to +ve peak. But inductor tries to be in the zero voltage condition as a result current will be reduced to zero level. After this quarter cycle current is at zero point and voltage is at +ve peak point. Apply concept to the other two quarter cycle and plot the graph of voltage and current. Then you can see that current is lagged 90 degrees behind the voltage.

As it's nature inductor always tries to be the same state as it is. Whenever a change is applied, it will try to vanish the cause of change. AC voltage always dynamic and it is getting changed it's magnitutude. Inductor is a rebel against the changes, so when voltage starts from +ve peak to zero it will tries to increase the current to resist the cause of change (Inductor intended to be at peak voltage condition at that time). As a result when voltage starts from +ve peak to zero current wiil be from zero to +ve peak. After that quarter cycle voltage is at zero point (current is at +ve peak point) and starts to increase to +ve peak. But inductor tries to be in the zero voltage condition as a result current will be reduced to zero level. After this quarter cycle current is at zero point and voltage is at +ve peak point. Apply concept to the other two quarter cycle and plot the graph of voltage and current. Then you can see that current is lagged 90 degrees behind the voltage.

I am afraid you will need the theory to understand why.

If you lack the theory then all that I say will mean nothing to you... try wikipedia.

John.

Are you after a way of visualising it...? Maybe imagine the current is subject to inertia due to the nature of the inductance...so it takes a while to get going! Dunno if this helps or even makes sense...the fact that the inductor resists change of current explains the 'inertia'.

Any comments from the 'team' ?

Del

I liked this explanation and the analogy, in fact it is the inertia which prevent progerss of current.

thanks

Current will always lag the voltage. It is just a matter of how much.

Think of it this way:

The voltage is the pressure and the current is the flow. You need pressure difference to cause the flow to happen.

In an inductor, there is little resistance to the flow so the flow is only slightly behind the pressure difference.

For a capacitor there is a large enough lag to look like it is actually before the next cycle so we call this leading. (we rarely use only one cycle. Even pulse networks have a burst of a few cycles)

The current lagging the voltage in an AC circuit by exactly 90deg is the definition of a perfect inductor.

I believe the 90 degrees also relates to the angle of incidence of the magnetic field resisting the current in the coil.

Quite.

Well, the current does not lag the voltage in a real inductor by 90^o unless you cool it to cryogenic temperatures to make the coil a super-conducting coil. Wire has DC resistance, and that makes a real inductor more of a complex impedance with an inductive reactance.

Isn't that the same as reluctance?

God! The 19th Century must have been Whack!!!

The 19th Century must have been Whack!!! I'll bet the 18th century was a hoot too!

Reluctance is the resistance of a magnetic path. Non-saturable core transformers were built once upon a time with variable air gaps, for example, and called "variable reluctance transformers."

Do a search on the term "inductive time constant" to learn more about the "why" of it. You will find sites such as this one.

http://www.allaboutcircuits.com/vol_1/chpt_16/5.html

As current increases in an inductor, it produces a voltage which opposes the applied voltage. In other words, it takes voltage to increase the current in an inductor. If you apply a sine wave to a pure inductor (assume no resistance), the applied voltage will be proportional to the change in current. The rate of change of a sine wave (derivitive) is another sine wave leading by 90 degrees. Therefore, the voltage leads the current by 90 degrees.

Current through an inductor generates magnetic flux. Changing the current requires a change in flux. As the current increases the flux increases but changing flux opposes the current required to change it - kind of like a negative feedback. So, the current can not change instantainiously but changes linearly with a step voltage change. Through the magic physics and calculus we come up with Vl = L * di / dt. As we can see di / dt = Vl * 1 / L and since L is " assumed " constant the dervititive of the applied voltage is directly proportional to the current. The derivative of a sine wave is a cosine wave or the sine wave shifted 90 degrees. All this happens perfectly everytime in the " Ideal World ".

They threw me out of the ideal world last week cuz I had some no ideal veiws!

Does that mean that di/dt = V?

di/dt = V/L

The derivative of the current with respect to time equals the voltage divided by the inductance.

Gotcha. Thanks.

consider the current to be a wave functon.... say i=i sin wt....

now find out the voltage that is.... Ldi/dt.....

this would be L*i cos wt...... which is L*i sin(90-wt).... which is -L*sin(wt - 90).... this the voltage leads current in an inductor.....