Now that's new. First I heard of pain with MRI. Wonder if it's common?

 

Well, my back is pretty mucked up with tingling and numbness in the hands and some minor shoulder pain. Physical therapy primarily consisted of traction 20 minutes. I think it was 2 minutes on, 30 s off duty cycle.

Well, a quick internet search: https://www.spine-health.com/forum/...rgery-and-neck-surgery/severe-pain-during-mri

It wasn't even on the pain scale of 1-10. I had an "ice-pick" headache once in my life and the pain was comparable, but muchlonger lasting. See: https://www.healthline.com/health/headache/ice-pick-headaches#outlook

 

I had a few MRI's done recently. One came with horrific pain. They sent me home to try again. I pre-medicated with stuff I already had and designed the doses to peak at the time of the MRI. I also asked the tech to restrain my arm. It still hurt, but not as much. They were imaging the cervical spine and my arm hurt with very sharp pain. The non-oral med I used was a Lidocaine patch and some other oral Rx meds.

If you close your eyes before going into the chamber, the claustrophobicness goes away. Then it usually just the noise.

OK, here is another instance of discomfort from the magnetic pulses. In my case there was no claustrophobic issue as I was quite interested in the mechanism of the operation, plus I don't do fear. So evidently really intense magnetic fields can affect the iron in our blood, at least for some folks and probably certain conditions. I was still very sore from a violent physical trauma, KISS was suffering from some sort of spinal issue, and so it might not feel the same to everybody. My whole point being that there is a magnetic effect even if it is not noticed.

 

And back to the original question about higher powered wireless charging. There was a recent petition to the FCC to allow a whole lot of much higher powered wireless charging radiation of RFI. That would probably result in the end of a whole lot of other wireless use because of the intense interference. Fortunately it is possible to demonstrate interference with math, and so unless the FCC justices are paid off the permission would not be given. Such interference might not reach cell phone frequencies, but broadcast AM radio would be doomed, as well as ham radio and all other HF and MF communications uses. That is because the equipment would wind up producing harmonics that would be radiated, no matter what lies would be told about it not being the case.

 

Two things that nobody seems to have mentioned.

First, in the Qi charging common with mobile phones etc, there is communication between the transmitter and receiver. The transmitter doesn't start sending 'serious' power until it has established comms with the receiver. If you get in the way, that won't happen, so no power! In any case, the maximum gap between Tx and Rx is normally around 5-10mm - I'm not that thin!

Second, as I understand it, high-power wireless charging relies on resonant charging rather than the 'air-cored transformer' principle. This requires the receiving apparatus to be resonant with the transmitter - which a human body is unlikely to be. I suspect that this is how wireless charging of electric vehicles is likely to be done. Again, there is communication between sender and receiver, so just walking over a charging pad won't produce any result.

Thus I don't believe there is any problem in either case.

 

Two things that nobody seems to have mentioned.

First, in the Qi charging common with mobile phones etc, there is communication between the transmitter and receiver. The transmitter doesn't start sending 'serious' power until it has established comms with the receiver. If you get in the way, that won't happen, so no power! In any case, the maximum gap between Tx and Rx is normally around 5-10mm - I'm not that thin!

Second, as I understand it, high-power wireless charging relies on resonant charging rather than the 'air-cored transformer' principle. This requires the receiving apparatus to be resonant with the transmitter - which a human body is unlikely to be. I suspect that this is how wireless charging of electric vehicles is likely to be done. Again, there is communication between sender and receiver, so just walking over a charging pad won't produce any result.

Thus I don't believe there is any problem in either case.

It's still a 'air-cored transformer' at resonance because the energy transfer is mainly by the magnetic field while the energy capture at the load coil is enhanced by the Q of the combined system. Our bodies are 'safe' in this type of magnetic coupling because they are only weakly ferromagnetic.
Resonance it not required for energy transfer it just makes it more efficient in this case. That food in the Microwave is not resonant but it gets hot because the molecules move and transfer RF energy (mainly from the electric field) into thermal energy.

 

That food in the Microwave is not resonant

Actually it is. The microwave frequency is chosen to excite a specific vibrational mode in water molecules. This is why the containers are not heated except secondarily to the food in them.

Bob

 

Actually it is. The microwave frequency is chosen to excite a specfic vibrational mode in water molcules. This is why the containers are not heated except secondarily to the food in them.

Bob

That's not true. It's a dielectric heating effect on molecules that can be polarized (dipole rotation).
The important part is that it has nothing to do with a relationship between 2.4 GHz and water, and nothing to do with resonance.

Most of the original microwave ovens were at 900MHz The current microwave frequency was chosen because it's in the ISM band polluted with other devices using the same band.

http://wtamu.edu/~cbaird/sq/2014/10/15/why-are-the-microwaves-in-a-microwave-oven-tuned-to-water/

The book Electromagnetics Explained, by Ron Schmitt states:

There is a popular myth that explains microwave ovens as operating at a special resonance of water molecules. In reality, this myth is just that, a myth. Referring to the Figure 15.2, you can see that there is no resonance of water at this frequency. The first resonant peak occurs above 1THz, and the highest loss occurs well into the infrared. There is no special significance of 2.45 GHz, except that it is allocated by the FCC as being allowable for microwave oven usage.

That's a very good book.

 

Okay. Must be a common myth I fell prey to since he took the effort to bust it.

Bob

 

Okay. Must be a common myth I fell prey to since he took the effort to bust it.

Bob

It's very common because it seems to be correct at first glance as the water dipole follows the electric field in some fashion.

But when we look at the physics of the water and similar two atom molecules we see the diatomic molecule resonant frequencies are much higher.

Hydrogen chloride 8.66x10^13 Hz
Carbon monoxide 6.42x10^13 Hz
Nitrous oxide 5.63x10^13 Hz
Near infra red part of the spectrum

Short wave diathermy machines heat internal tissue by dielectric absorption at 27MHz so it must be something else. It turns out that simple dipole rotation molecule spin is actually much easier to understand.
http://www.schoolphysics.co.uk/age1...ve properties/text/Microwave_ovens/index.html

I think systems like Witricity might be ok for low power applications. The power loss from remote charging a phone might be only a few bucks a year extra. The problem is see is the voodoo hype of how this is new and different in actual physics from the wireless energy transmission methods Tesla tried and failed at more than a hundred years ago. In 1894 Nikola Tesla used resonant inductive coupling in experiments with wireless energy transfer. Everyone's favorite toy the Tesla coil/transformer is a doubly-resonant air-cored transformer in the stacked hi-voltage configuration instead of the series coupled Witricity mode.

https://spectrum.ieee.org/transportation/mass-transit/a-critical-look-at-wireless-power

 

https://venturebeat.com/2019/03/29/apple-finally-launches-airpower-wireless-charging-mat/

One and a half years after announcing a wireless charging mat for iPhones, Apple Watches, and AirPods called AirPower, Apple has unexpectedly canceled the accessory.
...
Behind-the-scenes scuttlebutt hinted that the release was delayed due to various charging issues, including Apple Watch-specific challenges and potentially long-term reliability of the entire charging system. Twenty-two separate charging coils were intended to be used in the system to facilitate “anywhere” placement of devices, increasing the potential for overheating.

Tough engineering problems related to the laws of physics. My guess is that it ran too hot because of poor transfer efficiency with three randomly placed devices.

 

I have a question about wireless charging. I know that wireless charging works by sending power through em waves, which cant be converted by the human body enough to feel any kind of shock, but what about if high power wireless charging were to become a thing? Would standing between the pathway of the charger and reciever of a high power wireless circuit send a lethal portion the electricity through you if the power were high enough? Or would the amount of power from the circuit being converted by the human body never become dangerous enough no matter how much power was being transferred wirelessly to the receiving end?

This is why they ask you about metals in your body with an MRI machine.... !)

 

Mostly the concern about metals with the MRI is the mechanical forces and the heating. The problems intrinsic to wireless charging start with it being less efficient, and then continue with it being very likely to interfere with other electronics, since producing a harmonic-free power signal costs a lot more than producing one with assorted harmonics. The possible health issues about the powered magnetic fields have been rejected because there is money to be made. That EM radiation is harmless, just like smoking was harmless for so many years. No hazard is allowed to stand in the way of profit, at least not until the evidence is overwhelming.
The strange thing about ignoring the lesser efficiency is that there has just concluded an all-out war on less efficient wall-wart power supplies because of the energy they were wasting. And now the target is wireless charging of electric vehicles, at the kilowatt level. CLEARLY money talks and is listened to.
I realize that this RANT is sort of off-topic but it really does relate to wireless charging.