THE PHYSICS OF... POWER

[FionaZuppa]

I don't quite follow your question. But let me point out that in your graph, when V = 0, I is also non-zero. So having a point in time where V = 0, I is non-zero shouldn't be surprising. It happens any time V and I are out of phase.

Cheers, Wayne

yep, and unless you can show that X_L somehow becomes zero, at any time I(t) != 0 the power in the inductor !=0, why? because 1/2 LI² tells us so.

 

[winnie]

yep, and unless you can show that X_L somehow becomes zero, at any time I(t) != 0 the power in the inductor !=0, why? because 1/2 LI² tells us so.

1/2 LI² is the _energy_ stored in the inductor, not the power being delivered by the inductor.

At the point in the cycle where V across the inductor is zero, but current is flowing, this just means that the power flowing into or out of the inductor is zero, but there is still energy stored in the inductor. In other words, if you graph the energy stored in the inductor versus time, that graph would be at zero slope when V=0.

-Jon

 

[FionaZuppa]

1/2 LI² is the _energy_ stored in the inductor, not the power being delivered by the inductor.

At the point in the cycle where V across the inductor is zero, but current is flowing, this just means that the power flowing into or out of the inductor is zero, but there is still energy stored in the inductor. In other words, if you graph the energy stored in the inductor versus time, that graph would be at zero slope when V=0.

-Jon

whaaaaaaaaa? the energy in the inductor at any time t is 1/2LI(t)², and it follows the current waveform.

 

[FionaZuppa]

the graphs can show offset voltage waveforms due to the phase shift, but you can only have one amps waveform, so how do you escape I²*[ohms]=Power ??

 

[wwhitney]

whaaaaaaaaa? the energy in the inductor at any time t is 1/2LI(t)², and it follows the current waveform.

Yes. That's energy, E, stored in the inductor at any given time.

Power, P, is the rate at which energy is flowing into or out of the inductor.

So the energy in the inductor can be non-zero, while the power flow into/out of the inductor is instantaneously zero. In fact, that occurs precisely when the energy in the inductor has reached a maximum or minimum.

Cheers, Wayne

 

[wwhitney]

the graphs can show offset voltage waveforms due to the phase shift, but you can only have one amps waveform, so how do you escape I²*[ohms]=Power ??

As already mentioned, that equation is only true for a resistor, not an inductor or a capacitor. See the end of post 796.

Cheers, Wayne

 

[Besoeker]

whaaaaaaaaa? the energy in the inductor at any time t is 1/2LI(t)², and it follows the current waveform.

It's 1/2Li²

 

[Carultch]

yep, and unless you can show that X_L somehow becomes zero, at any time I(t) != 0 the power in the inductor !=0, why? because 1/2 LI² tells us so.

Energy stored in the inductor at any instant, is equal to 1/2*L*I(t)^2, where L is the inductance and I(t) is the instantaneous current.

The power delivered to or from the inductor is the time derivative of this. Because 1/2 and L are constants, this equals 1/2*L* d/dt (I(t)^2), which simplifies to L*I(t)*I'(t), where I'(t) is the time rate of change in current.

When I(t) and I'(t) are both in the same direction, the power is positive. This means that the current is speeding up, therefore the inductor is accumulating energy in its magnetic field. When I(t) and I'(t) are in the opposite direction, the power is negative. Meaning that the current is slowing down, and the inductor's magnetic field is releasing energy to the rest of the circuit.

 

[Besoeker]

Energy stored in the inductor at any instant, is equal to 1/2*L*I(t)^2, where L is the inductance and I(t) is the instantaneous current.

I used lower case i. That indicates an instantaneous value - at least when I was taught as an undergraduate.

 

[junkhound]

I used lower case i. That indicates an instantaneous value - at least when I was taught as an undergraduate.

yeseriee ! yep, Taught us the same way in USA in the 60's also.
Have seen I(t)=CV = Q lots of times too! Whole different meaning. could even throw in Q = s(I/dt) to further add mud to water or ......

Some other variations seen in the last 100 posts, such as V= L*di/dt; say what - oh yeah, at the peak current of a transient voltage is zero, like in 90 deg phase shift eh, <G>

Very interesting thread on the way different folks interpret different phenomena ! Not all of which are correct. :weeping: