You should probably mention that, although power might be negative for certain periods of time in AC circuits, the pointing vector always points to the source (because the E and H fields flip when power is negative) and therefore energy always flows to the source even when current flows away from it. Love your videos btw!
What you're saying is contradictory. The Poynting vector depicts the direction and rate of transfer of energy (ie. power). By definition, when the power is negative, the Poynting vector points in the opposite direction (compared to when the power is positive).
That video you referenced is nice, but don't get too carried away about it. He claims that it's wrong to think that electrons are pumped around a circuit in DC, and back-and-forth in AC. But that's exactly what's happening!!! He even states that they do in fact move (he says "drift") along the wire in the direction given by the battery (at the 6:42 min mark of his video)! Also, he hypes up the idea that energy is "flowing around the wire", not "in" the wire. But this is totally obvious to anyone who uses a mobile phone, wireless radio, or uses a TV with an aerial. Electromagnetic energy is transmitted ("flows") in the electromagnetic wave radiated by electrical antennas, in all of those examples. That's how a mobile phone sends and receives its signals! I'm sure that if he'd pointed out those examples in his video, then everyone would just say: "oh yeah! that's right - electrical energy does flow through the air". But obviously it wouldn't have got as many "clicks/views/likes".
... to be more clear, you're getting confused by AC signals (where half the time the voltage and current are positive, and half the time they are both negative), and thinking that this means that half the time the Power is positive and half the time it's negative. Not true! Negative times negative equals positive. So the power is always positive (in basic resistor circuits). ... It's not always the case though, when there are inductors and capacitors in the circuit - but in these cases the power (and Poynting vector) does actually flow (point) in the negative direction for periods of time! See this for more details: "What is Reactive Power in Electric Circuits?" ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-HDABrT-YTJU.html
*Summary* - *0:00** - Topic:* Importance of Instantaneous Power Flow - *0:04** - Context:* Discussion on average power, RMS power, and instantaneous power in electrical circuits. - *0:11** - Acknowledgment:* Special call-out to Thand Nyan and Yor Nazal, first members of the RU-vid membership program. - *0:28** - Circuit Example:* Power supply or socket in a house, with current flowing into a load (e.g., fridge, washing machine) using AC power. - *0:51** - Waveforms:* Voltage and current waveforms shown at 1 Hertz for simplicity (typically 60 Hz or 50 Hz in houses). - *1:17** - Equations:* Phase difference between voltage and current demonstrated, with equations showing power as the multiplication of voltage and current. - *1:45** - Mathematical Transformation:* Conversion of the product of two cosines into a sum of two cos terms. - *2:06** - Instantaneous Power Analysis:* Power waveform generated by multiplying voltage and current waveforms, highlighting the average and instantaneous power. - *2:50** - Negative Power Periods:* Instances of negative power corresponding to opposite polarities of voltage and current. - *3:13** - Circuit Implications:* Power flowing back from the load to the source during negative periods, necessitating circuit design considerations. - *3:49**, **4:27** - Extreme Cases:* - Alpha = 0: Voltage and current in phase, power always positive, typical of purely resistive loads. - Alpha = 90°: Average power zero, significant current flow possible, indicative of purely reactive power. - *5:10** - Design Considerations:* Importance of accounting for high current in circuit design to prevent wire overheating and failure. - *5:43** - Further Topics:* Introduction to reactive power and its subtleties, with a promise of future videos. - *6:09** - Phase Offset and Power Direction:* Simple formula to calculate the proportion of time power flows in the opposite direction, based on the phase angle. - *6:44** - Conclusion:* Invitation to like the video, subscribe to the channel, and visit the website for more categorized video listings.
RMS power is something useless (referenced in 2:35). Only involved with audio systems (fake). Only Mean or Average Power has significance in electric circuits..
It's neither higher nor lower (nor equal). Voltage is in volts, current is in amperes. These are different physical quantities. How they will be pictured on a graph depends on what scale of units is chosen for the vertical axis (for each quantity).
Is this the reason why in the power grids the frequency is monitored so closely? If the frequency goes down then also the phase shift between current and voltage changes? And then we dont have the expected power flow?
Yes, since the impedance of capacitors and inductors both depend on frequency, all electric circuits must be designed to match the operating frequency. If the frequency of the source changes, then the performance of the circuit will change.
Thanks, when P is negative, what does that mean for the load? I would say the energy gets negative to E=P*t - how does the load handle the negative energy? Thank you.
It means the energy is sent from the load to the source at the given moment. A phase shift means the load has some reactance, which means the energy is alternately stored and discharged in magnetic/electric fields of the device.
The load doesn't "handle negative energy". That's not what's happening. "Negative energy" is simply energy transfer in the opposite direction. For example, for some time instances a capacitor in the load will be accumulating charge (storing electrical potential energy), and at other instances it will be discharging (releasing electrical energy "back into the circuit", and potentially back into the source).
Thank you for clearly explaining this! I opened up a dead led light bulb last week and found no rectifier, just a 10.5 ohm resistor. I noticed the same in a cheap, plug in, 5v usb supply. I found something on the internet explaining that a resistor caused positive power but not why or what could be without the resistor. I love it when the universe supplies me an answer I didnt know how to find. I'm eager to see the rest of your videos on this topic! Thanks again!
That's great to hear. I'm glad this topic is of interest. I've got a few more videos in mind for this series, but if you have particular suggestions for topics, then please let me know.
By the way, the LED you mentioned is a diode (light emitting diode), so it does the "rectification" itself. It needs the resistor to limit the current flow, since otherwise it acts as a short circuit on the positive voltage part of the cycle.
I'm not sure what you mean, but generally it is desirable for the load to be purely resistive (with no reactance). It means the current is minimized and there's no extra loses in transmission. Of course some devices cannot work without reactance, so we try to make the energy fluctuations local, for example by adding a capacitor to an electic motor.
It all depends on what you are trying to achieve. Sometimes you can't avoid it - for example transmission lines have a natural inductance, so you just have to live with it, or try to design your terminating circuitry to counteract the line's inductance. In other cases you might want to add capacitance to a circuit - for example to smooth out potential voltage spikes. Keep an eye out on my channel for some upcoming videos on these topics.
