The definition of power is rate of energy conversion (E/t), or rate of work done(W/t). wiki: Power (Physics)
P = VI is for Electrical Power, that ain't wrong.
But there's also Mechanical power: P = ( F * d ) / t, where F is force and d is displacement. Force can be simply cause by steam or wind pushing a turbine, and displacement can simply be a bucket filled with water being lifted from a well.
Says who current and voltage must be involved in power?
There's also human power, horse power, solar power, wind power, etc... With so many types of energy in this world, how can ALL energy conversion i.e. power involved current and voltage?


So you neither rebut nor accept my statement this time.
Look if you read back my first 2 posts in this topic, I've already said efficiency is the main problem with wireless power transfer. The only reason we went around a big bush is because you disagree electrical current is not necessary for power transfer.
I'm not questioning your electrical knowledge, but you said my statement is wrong so the onus is on you to provide proof to back yourself up. We've already provide 2 examples where current is not involved in at least one of the power transfer path.
In fact, this whole side-tracked discussions appears to stem from your insistence that power must involved electricity.


That's exactly how passive RF sensors (access card like touch'n'go cards). Certain radio frequencies transmit relatively better over the air e.g. 27Mhz, 455Mhz, 900Mhz. These passive access card draws power from RF carriers waves generated by the transceiver. Range is very short but it's another example of no wires power transfer.

Assuming a 100% efficiency conversion,(which is not possible btw if you didn't realise it),

P= VI

You need to have approximately 4.5V to charge a Li-On battery.
So the amount of current you can charge is

0.001 = 4.5I
I = 0.2mA

In real world I expect say 40% efficiency that would mean around 0.1mA

Assuming a 1000mAh battery, it would take approximately 10000 hours or 416 days to charge a battery.
However since a battery itself has it's own self discharge and your self discharge is higher than your charging current.
1mW will never be enough to charge a battery.

But transferring information and transferring useful power are slightly different things right? Information can be digital signals (transferred using EM waves) and only require very small amount of energy. I'm only a mechanical engineer but I'm guessing the receiving end of the Touch n Go thing has some sort of amplifier to boots input signal from the card. Low frequencies should suffice for this application.

But for powering devices, wouldn't we need a higher frequency for greater current?

Hi,

Thanks 4 the definition.

Now we are talking about "electricity" rite? So, i'm talking in the context of electrical power. If u take wat i said about power involving current out of the context....

Tun Abdullah Badawi "transferred power" to DS Najib - NO CURRENT INVOLVED ("wireless" too )

OK, U WIN


Solar "Power" or Solar "Energy"?

Can we decide a term to be used amicably here? Well, I have said in my reply to Senior Member befitozi:




OK, let's look at SOLAR ENERGY:



So, we can see the nuclear fusion gives out energy.

This energy is transferred to earth, and the "light" that we receive during daytime is from photons.

(The cycle starts with the thermal collision of two protons (1H + 1H) to form a deuteron (2H), with the simultaneous creation of a positron (e+) and a neutrino (v). The positron very quickly encounters a free electron (e-) in the sun and both particles annihilate, their mass energy appearing as two gamma-ray photons) Nuclear Fusion

It is this energy that we "harvest" at the photovoltaic cells:

This energy causes the photovoltaic to emit electrons and this electron causes current flow, which in a complete circuit connected to load, gives us power.

So, i dun call it solar power when it's not yet realized as "power"... while it's still in the transfer process as "energy".

Another story:

Money gives us "buying power" & i must earn money 2buy something.

U can say u dun need to involve "money transaction" to buy something (by barter trade )

Money, is like current, it flows from 1 party to another, giving the person having it "buying power"....

So, I plant durian in my orchard (sun).

Passers-by asked: 'What r u doing?"
Me: I'm planting "buying power"
Passers-by: ???

So, during durian season (nuclear fusion), i collected my durian & transport to pasar malam to sell.
Passers-by: Wat r u doing?
Me: I'm transferring my "buying power" to pasar malam
Passers-by: ???

So i sold my durian (photon) & i got money (current) & now i have "buying power"... I can buy 10 nasi lemak for my family

OK, sorry. Went to far off the track LOL



May i ask something...

Touch n' Go cards, u need to touch it rite? The "reader" reads the info in the microchips via RF.

Those SmartTAG, (active RFID) has battery & thus it transmit the signal back to the reader via RF.

Err.. RF... = power transfer?

Nevermind.

Guys, interesting discussion here. No hard feelings OK?

Err.. for simplicity sake, I'd assume the 1mW is net received power after transmission and conversion losses. And 1mW is also an arbitrary number for the sake of argument.
My argument that 1mW is sufficient to charge a battery is generally based on the design of solar powered night light.
Assuming the night light LED uses 3mA at 1.2V and needs to run for 8hours per night. That means simple total power drain is just below 29mW. Hence 3mW of continuous charging power for 12 hours per day during daytime will be sufficient for it's night time operation. It's a simple calculations but suggest plausibility. You can see why I think 1mW can charge a battery, just depends on what is the load and application.


First higher frequency doesn't means greater current. Simple example, our 3G network uses 2100Mhz, our 240v AC runs at 50Hz. I'm not an expert in this area though, so that's just the simple idea.

Next consider this. Let's say you are transmitting data using some sort of RF signal from one transmitter to another receiver. Now what's to stop me from say making another receiver that also reads the same RF signal, but only converts the electrical signal I received to powers say an LED?
Both electrical signal and electrical power are made of voltage and current. Whether it's a signal or power is our interpretation and application. Of course in order to create the signal, we modulate data into the signal waveform (commonly called carrier). And usually signal lines doesn't carries much power due to well, it's not designed to transfer power after all.

Yah I can see why you think 1mW can charge a battery because you don't look at the bigger picture and consider things like battery self discharge or switching regulator efficiency . Furthermore you're already assuming you can collect 3mW from a wifi signal. And 3mA at 1.2V, it's probably just a dot of light at night

Are you from EE background?

---

Added on April 4, 2010, 10:56 pmSimple, in a normal reciever there is an amplifier that amplifies the recieved signal. The recieve signal alone is probly in the uA range and is unusable without an amplifier before being fed into a mixer. And the amplifier itself uses power itself what more charging a battery.


This post has been edited by Drian: Apr 4 2010, 10:56 PM

No I mentioned 3mW can be use to charge a battery for very low power applications. I did not say it must comes from wifi. The question for me was merely, can 3mW or 1mW charge a battery?
I interpret 3mW or 1mW as available net power. You interpret is as before conversion losses. So it's a simple misunderstanding. That in no way means I do not understand regulator efficiency and battery self discharging.

And it's only a small pool of light yeah but I'm only pointing out a product available on the market, not arguing about the pros and cons of it. Also the example is meant to point out such low amount of power is only suitable for very low power application whereby stressing the key issue of wireless power transfer --> transmission losses, which in my mind has always been the bigger picture.

I'm a firmware programmer, which in itself is a field that covers programming and a little on electronics. I'm not strong in analog electronics though so I'm curious about the amplifier you mentioned. Since the purpose here is merely to collect power and not the signal data, is there really a need for amplifier and mixer? I'm assuming the right antenna will convert the carrier waveform into similar electrical waveform. Since it's an oscillating waveform, then theoretically we can use the voltage doubler or boost circuitry concept to boost up to the right amount of voltage and current we want right?
And if amplifier requires power to function and is required in order to extract energy from RF signals, it would be a chicken and egg story and doesn't explains why passive RF tags work.

Erm. Didn't bother to read anything, but wireless power is indeed possible. Ultra long range, maybe not for now. But utilizing induction, it's quite possible.

And about current flowing. Maybe someone didn't go to school, but last i figured, when you have a wireless inductive field, the field itself induces a voltage and with a voltage you get a flowing current through the system. That's why you call it induction. The stators and the coils of a generator are inductively coupled, if there is no current flowing, how does we get power?

My background is Electrical Engineering, majoring in Power, current dissertation in power transfer and transformer stat measurement.

Topic: Transfers power wirelessly, Wifi, 3G. can electricity did that too? (Science)

The definition of power is RATE of Energy Transfer.

Power dun "exist" while energy is converted from 1 state to another. It is a measure of rate: how fast work is done.

If I push a box, I'm transferring energy to the box, overcoming friction and cause it to move (kinetic energy).

How fast i perform the work (rate), is a measure of the amount of power i have.

If i can push a box for 1 m in 1 sec and u do it in 2 sec, i'm more "powerful" than u. (example only )

I do not transfer POWER to the box.

Secondly, when u pay for electricity, u r paying for ENERGY = kWh transferred to your house. Not for "POWER" transferred to your house.

Since ur avatar looks like wikipedia , here u go: Electrical Energy

kthanxbai

Passive RF Tags works because it requires very little current and they are limited only to approsiamtely 30-50mW. That makes the effective distance approximately 2-3 feet from a 1 watt transmitter with a 0.03% efficiency.If you want to charge a battery 2-3 feet away at a 0.03% efficiency , it's pointless since it has to be so near the transmitter. If you're using in a wifi environment where things are like 20-30 feet away and from a lower power transmitter. You can forget about charging any battery from it. The signal strength decreases with the inverse square of the distance, so your power requirement increases exponentially as the distance grows.



Wow you're expecting voltage doubler to give you double the voltage and current for free?


This post has been edited by Drian: Apr 6 2010, 10:04 AM

So passive tags work and there's really no need for amplifier and mixer in order to just extract power from RF signals, glad we cleared that one.
And yes a 1W RF transmitter probably aren't practical for wireless power transfer except for very low power application and of very short distance.
So can we move along now?

I believe lightning strikes the best natural & ancient example of electricity travels wirelessly.

Lasers, microwaves, infrared etc can do that too if the output power is strong enough.

google witricity... n frm wut i see frm dis thread, all just squabbles

Say what? So, mechanical agitation can amplify electrical current?


There's no current flowing through the air, as many have pointed out. This is an inductively coupled circuit, but EMF flows in the steel core.

Higher dimensional space rarely means a superspace, in most physics calculations it's a subspace i.e. a compactified dimension. If we can reach into these compactified dimensions and meddle with stuff anytime we like, I don't think our universe's physical laws will be the way they are because then, our physical constants would have been very, very different to begin with.

Using higher dimensional space to do stuff is a dumb idea and besides, the maths doesn't permit it. If it has no direct contact with our physical world, it also means we can't touch it and transfer information through it.

We don't need a high frequency for a greater current, we just need a suitable frequency based on the transmission medium. Transmitting power wirelessly would require both devices to be in resonance at the same frequency, so that a resonant cavity can be set up. This resonant cavity's frequency must be optimized based on our requirements, whether we're transferring a little or a lot of electricity, and whether we're transmitting a short distance or a long distance. It's kind of like the carrier wave that's most suitable for the medium or the transmission parameters.

For instance, to transmit over long distances, we'll have to use longer wavelengths. Nikola Tesla suggested the use of the Schumann cavity for resonant-coupled wireless power transmission.

Lightning is plasma. It has a conductive medium to flow through. Yeah, that's wireless if you rigidly define "wireless" as being the absence of metallic conductors. If such were the definition, then transmission through conductive plastics would be considered "wireless" as well.

Lasers, microwaves and others can't do it as easily for a few reasons. It depends on how much energy that ambient air can absorb in those wavelengths. Microwaves can pass through air rather easily, so it won't impart much energy into ambient air. Infrared radiation can heat ambient air, but it'll take a hell lot of energy for IR to get to the breakdown point of ambient air. Lasers get scattered and attenuated somewhat by ambient air, but it still doesn't dump a lot of energy into ambient air because air don't typically absorb visible light spectra anyway.

How about if we condition the air so lasers, infrared & microwave can travel their designated path without much loss of energy?

Also isn't UV considered "wireless power"? They can power up calculators & those headshaking cardisplays.

Well, true that we don't know much about it yet. As energies required to just probe those postulated dimensions are well beyond whats available on earth.


That's simply photo electric effect. It is certainly possible but with immense limitations as said by jswong. But research into photonics is pretty large nowadays so it is the most likely method.

But, that being said, a simple obstacle in the path would beat the system so not practical for large scale transmissions.

Whoa........prety far out right ? So many dimensions and places to go.....

Real power is awesome stuff. Just try sticking your fingers into the wall socket if you wanna find out.

Like most of you guys have said before efficiency is actually the key. Welll actually safety too. But at the end of the day, EM waves that can cause unknown effects on humans, light based transmission that can fry flying birds andcables that as long as nobody messes with them will be fine, I'll take cables anytime.

"Dimensions" is always misunderstood. A dimension is not a parallel universe, like how the science fiction cartoon and movies always speak of it as. A dimension is simply a direction through which space and time are measured. Length, width, height, these are dimensions. We don't say that we can travel into the width dimension and see a separate existence, do we?

No. What I am saying is about control. As a scientist they can work with as many dimensions as they want. But in reality, the task of really implementing it at this moment is too great. Power consumption is too great at this point for anything wireless. Regardless of dimensions, the energy must travel through matter. All beings are also made of matter and may also be present at that very dimension.

But again, my greatest doubt on this would be efficiency. Even currently to convert from AC to DC or vice versa the highest I have ever heard of at the level which we call power is no more than 95%. I doubt for the next 20 years we will have enough technology for energy to be transferred 100kms as efficiently as cables.

Just a quick reply to this, I noted that ppl have quoted from Wiki with links to Nikolai Tesla. And as I remember from Astro, he had many stumbles, not necessarily because his idea of wireless energy was not efficient enough, or not the best solution, but simply because he was unlucky, and didn't live long enough to fulfill his dreams.
(yes, at present it isn't good, but who knows how it would have developed if Nikolai had his way)

If it wasn't because of that guy, we'd all probably still be using Edison's DC for our computers.