Electric Field: Newsletter Challenge (08/28/07)

Sounds like Tom is walking up on a dipole, but John?? There's the puzzle. Sounds like this'll be good.

Tom

John is broadside to the dipole.

Tom is off of the end.

So far as I know, there are no zeros in a dipole field (regardless of whether magnetic or electrostatic). If you walk around a dipole maintaining constant distance, the change is in the direction of the field, rather than the magnitude.

Yehbut, yehbut, yehbut!

What if he only has an electrometer that only knows amplitude.

Ive used these, and theyre only an exposed capacitor plate

connected to the gate of a JFET which drives a meter movement.

It doesnt care about the direction, only the intensity of the local field.

If I were broadside to the dipole,

the pos and neg field would cancel in a device such as this.

Al D.

Not claiming to be a sparky of any kind, but if dipole field drops off proportionally as the inverse square, and a quadrupole field drops off as the inverse ^4, would the field that drops off as the inverse cube not be a tripole?

And.... yes StL, I'll beat you to the pun, if it only drops off a little bit it would have to be a tadpole.

I think you're using a fishingpole!

ROFL

I've probably been causing confusion here, for which I abjectly apologise - imagine me as a slave in the presence of the great Ramesses (and I really loathe the taste of Egyptian dust).

. The field of a monopole (effectively a single point charge, or even a gravitational object) falls off as the inverse square.
. A pair of equal-and-opposite charges that are close together create opposing fields that tend to cancel each other. The residual field falls as the inverse cube of distance from the centre of the dipole. There are no regions of identically zero field in the pattern of a dipole (although the central orthogonal plane is a region of zero potential)
. To improve the cancellation using three poles, you still need the sum of the charges to add to zero. So one of the charges will be equal-and-opposite to the sum of the other two. To obtain an inverse fourth law, the third pole would need to be made so that you the arrangement is exactly equivalent to two equal-and-opposite dipoles, separated so that the charges of one sign coincide in position. Because it's best modelled as a pair of dipoles, it has generally been called a quadrupole. The physical origins of many quadrupoles as four "unit" charges may also have driven the terminology.

I hope this has not simply added to any confusion.

Fyz

For every question you answer, i find a new one to ask. That answer summed it up pretty good. When I grow up I want to be just like you. I am familiar with quadrupoles only as they apply to mass spectrometry, but the electrical mechanism is somewhat muddy to me. I only asked about tripoles from googling it: ttp://jnaudin.free.fr/html/tripole.htm . and i know that it is a magnetic phenomenom rather than electrical field.

Even were it possible, I couldn't recommend that you grow up like me. Fortunately, there is no chance of it, as I've spent at least the last 60 years successfully avoiding maturity. Even so, it's not possible to avoid growing old...

Regarding your question: I believe 'tripole' to be a word specifically invented by the originator of the site. What Naudin has is usually called a quadrupole; so far as the field outside the near-field region is concerned, all that matters is the orientation of the quadrupoles. However, you will see Naudin is interested mainly in the near-field region, where the specific locations of the poles is significant. In that regard, a finite-diameter solenoid is not strictly a dipole anyway - but that is probably carping.
Ignoring that and treating each single solenoid as a dipole, the way Naudin has arranged the solenoid places two identical physical poles in the same position. In other words, he has placed the poles as x, -2x, x, with x (not standard notation) representing the magnitude of the poles.

BTW, SFAIK, your spectroscopic quadrupole is generally spin-based, with opposite spins being self-aligning - analogous to a pair of identical bar magnets with the north pole of each magnet abutting the south pole of the other. (I'll try to remember to check that with my brother whenever I next see him)

Fyz

I'm assuming that both are using their Gaussmeter/magnetometers correctly?? i.e. orientation of the field in relation to orientation of the detector. It cant be that simple.

Gaussmeter/magnetometer measures magnetic field, which is set up by a current or permanent magnet. The question was posed as an electric field so we should assume that they are equipped with electric field meters

True for the magnetometer - If it wasn't oriented perpendicular to a magnetic field it still registers a needle deflection. The Hall effect generator (gaussmeter) on the other hand measures the current and magnetic field the same way doesn't it? I was under the understanding that the inductive properties of the current field are what is being measured (see US patent #5416407). When you order a probe for a hall effect generator gaussmeter you have to specify the direction you want the detection circuit placed, so I'm guessing that if it is parallel rather than perpendicular to the field, no measurement would be read.

A voltage sets up the electric field, and a current sets up the magnetic field. These are orthogonal to each other. A Hall Effect sensor (you referred to Hall Effect Generator???) responds to the magnetic field which is set up by a current. The direction you referred to would give the indication of current polarity. In practice, what you think is perpendicular or parallel, may not be in agreement with what Mother Earth indicates, because there could be some stray field or error in the practical setup that could skew the observation.

Flux-gate magnetometer could also be used to measure magnetic flux.

If you want to measure DC electric field, you need to resort to something like an electric field mill, whereas for AC electric fields, you can use a capacitive sensor.

Is it possible that John has unnaturally insulated himself such as to interfer with the meter's operation? Any meter needs to have some point of reference to work properly.

Had an interesting thing happen out at a power plant once. A guard, with his heavy rubber boots and shotgun slung over his shoulder with the barrel pointed up, was checking out the switchyard on a Pea-Soup night. When he went to grab the lock to check it, he found himself on his back, looking up at a worker frantically trying to revive him. Strange things can happen in electric fields.

how about this-we had a worker go to do some cleanup inside an electrostatic dust collection unit. He locked it out as per policy, but forgot to dissipate the residual charge on the cathode. He was sweeping out the floor to prepare the unit for some electricians to work on it and noticed the hair on the back of his neck standing on end. When he bent over to sweep the dust into a dustpan, his rear came within critical proximity to the charged cathode. The subsequent lightning bolt that lit up his hind quarters (as he described it - the finger of God) left a 8 in. second degree burn on his gluteal area.

The subsequent lightning bolt that lit up his hind quarters (as he described it - the finger of God)

I knew that all MD, including Proctologists, think they are "God", but I didn't know that the converse was true!

ROFLMAO hoping I never feel the "finger of God"!

Since an electric field is present and being generated over the area where Tom is, I would say that Tom is standing between two metal plates that have a potential difference, a really large capacitor. The chamber is also a conductor and part of the circuit and is grounded to the plate in the center, between Tom and John. Therefore, there is no potential difference in the area between the plate and the chamber, where John is and therefore, no electric field.

To make it even more dramatic, and to explain the increasing field strength, let us say that the chamber is divided into two halves which are electrically isolated. Tom's half is connected to one lead of a voltage source and the other lead is connected to a metal object, preferably a sphere, in the center of the chamber. Therefore, an electric field would be detected anywhere on Tom's half. John's half is grounded to that sphere and so no voltage potential exists anywhere where John is, and so no electric field. Since the field is generated between a small plate (the sphere) and a large plate (Tom's half of the chamber), the field becomes concentrated and stronger the close Tom comes to the small sphere and follows the inverse proportion law for a three-dimensional field.

The person posing this problem has not been clear about the circumstances of the system. Like what are the two men using to measure an electric field? If the device is a hand held unit with two probes protruding out the end, then the potental difference measured would be the result of what is picked up in the space between the probes. If the device has a reference lead connected to something and one arial probe then the potental diffence can be much larger depending where each man choose to connect his reference lead. I think this definitely requires clearification.

My guesses:

1. John doesn't have a meter.

2. If John has a meter, it is defective.

3. If John's meter is not defective, he doesn't know how to use it.

4. Tom is using an analog meter where the needle simply reacts to the electric field. John is using a digital meter that requires a battery to power the display, and the battery is dead.

5. John is shielded from the electric field.

6. If the field is caused by a direct current, and the current enters the chamber at the center and flows towards one side, Tom is in the half of the chamber with the current.

7. John is measuring the magnetic field and Tom is measuring the electric field

8. Half of the chamber is carpeted. Tom is on the carpet, and the air is dry that day, He's shuffling his feet on the carpet so when they meet at the edge of the carpet and shake hands, John gets zapped and Tom retains his position as the better prankster.

9. The chamber is the brig on a starship. Tom is checking the strength of the force field holding the prisoner. John doesn't have a meter because he is the prisoner.

10. Tom is insane and the electrical field is a delusion. John has a large net instead of a meter.

11. Tom is obsessed with electric fields, so he measures them everywhere he goes, no matter how weak they are. John doesn't care unless he is close to a power line or a high-power transmitting antenna.

12. I need to review electromagnetic theory to come up with a better explanation!

3Doug,

Your "guesses" range from ridiculous, through banal and trivial to "spot on!"

1,2,3,4,7, all fall in the same category and make the assumption that this is somehow a trick question and the writer is not sincere in seeking a technical answer to his challenge. Given the background of the question and this short format which is allowed we must believe that the question is sincere and that both men have working detectors that would read the field strength correctly if the electric field was present at their location.

Your number 5 comes closer to a real answer, but your number 6 is quite correct in my opinion, matching the answer I gave earlier, though not in quite as much detail, not explaining why "current*... flows [only] towards one side". *By which I assume you mean the electric field forms only between the center and one side. Neither do you explain why his readings increase in inverse proportion to the cube of the remaining distance.

#8 starts out interestingly enough, but degenerates into a scientifically meaningless, though quite humorous, joke.

#9 is just annoying. "Earth-to-Doug, please adjust your banality clarifier. Your last transmission was somewhat garbled."

#10, well that's just crazy talk! The size of the net does not matter! Sometimes a meter is just a meter. Wake up and smell the ozone!

#11 is a personality study and not a scientific analysis. This answer is just as weak as the field strength that John is measuring (or not measuring in this case).

#12 if you must but with your shotgun approach, you did find at least one solution that works. After all, the invention of the light bulb was simply trial and error, was it not? Someone said that baseball is the only field of endeavor where you can succeed (get a hit) only three times out of ten and still be considered to be doing well. I think engineering, especially research and development, has that beat by a mile (or a kilometer for you folks who must use metric!)

This guy should be a writer. He has got WAY TOO much imagination.

Hilarious!

I have marked your test paper.

A plus.

You are now a fully qualified senior manager.

OK, this is probably stupid, but... Tom turned his detection device on before John did, so Tom has become part of the electric circuit. John, on the other hand, doesn't detect anything because he is not part of the circuit that Tom is currently (no pun intended) a part of.

(A) John is wearing metallic Boots. Maybe a metallic suit? An anti-shark attack suit with the sensor attached to his wrist beneath the suit.

(B) John is the origin of the electric field. (see the 80s show: THE SPECIALISTS)

John is a goat and couldn't give a rats ass about fields. He just wants more goat pellets.

Chain mail

The easiest explanation is that the radiation source is not omnidirectional, and John is walking on a line where there is no field, while Tom observes an inverse cube law. One should look at the radation diagram of the source and check it out. In it simplest form this diagram can look as two spheres connected t the center of the chamber. In the plane that ideally separated these two spheres the radiation always measures zero.

Depending on the wavelenth and the speed o propagation, the phenomenon of interference should be taken into account. Constructive and destructive interference can cause radiation to be anything from zero to twice as much as expected depending on the position.

P. Fernandes

pfern@triast.pt

You mean, something like this?

Nothing in the puzzle says anything about Tom and John having the same measuring devices. Is it possible that Tom is detecting AC while John's detector is set for DC (or the other way around)?

Radiation + Inverse Cube ???

"The easiest explanation is that the radiation source is not omnidirectional, and John is walking on a line where there is no field"

Who said there was any radiation? Do not confuse an electric field with electromagnetic radiation. Although electromagnetic radiation sources (radio waves in particular) will have a changing electric field (as well as a magnetic field) there can easily be an electric field without radiation, just as the magnetic field of the earth or a magnet exists without any electromagnetic radiation. A good example is a capacitor. An electric field exists between the plates. The field may be changing, if alternating current is charging the plates, or it may be static, if the plates have become fully charged from direct current.

In an electric field there is normally no such thing as a line where there is no field. Unlike electromagnetic radiation, which can be easily blocked (except for higher energy radiation like x-rays and gamma rays, electric field lines, like magnetic flux lines, will tend to flow around objects, unless the blockage is considerable. This is very much like concrete blocks in a river. Toss in a few, or even many and the water flows around them and through the gaps, but fill in the gaps and create a dam and you can effectively block the water from flowing (until it tops the dam anyway, but there the analogy breaks down).

At the centre of the room is a highly directional aerial. A Yage-Array with multitundinous directors and reflectors, properly tuned to a single frequency. This reply send in by Benno

Wouldn't that give an inverse square law if radiative, and a cubic decay in both directions if not?

Tom is measuring his field in a radial direction with respect to the chamber. John is measuring the field at a right angle to the direction Tom is measuring.

Inverse cube - so the electric field (stated in question) must be a non-radiating electric dipole - therefore, static or low frequency relative to the size of the chamber (<< 1MHz).

If the chamber is long and low (and/or narrow), it could be that the floor-and-ceiling (and/or the walls) at John's end are conductive but those at Tom's end are not*. That would impose an exponential decay at John's end. The rate of decay would be at least exp(-2.pi.d/a), where a is the smaller of the height and the width of the chamber.
*For example, the floor might be metal reinforced and the ceiling wiring installed at John's end, but not at Tom's. If the ceiling is about 3-metres high, the field in John's half and 10-metres from the centre would already be less than 1/1000 of the field at the very end of Tom's half of the chamber.

But the above field is still not zero. The explanations that I have found that could find and that give zero measurements are all rather fatuous - I'll give a couple, starting with the less fatuous:
.That one of the ends is inside a Faraday cage, and part of the screen runs down the middle of the room*. The field generator in that case could be a monopole on Tom's side near the centre of the room. (It shouldn't matter whether John is inside or outside the cage - so long as the generator is on the opposite side; however, if the cage is on Tom's side, it needs to be very high and wide, however, to avoid the exponential decay described above ). The problem with this is that the Faraday cage must split the chamber into two parts.
.That one of the two is equipped with an AC field meter, and the other with a DC field meter, and Tom's meter matches the source.

*To be identically zero, the grid size of the cage would need to be zero, so there is a continuous conductive screen across the middle of the room. The decay would be fast with a 2-metre grid, for example, but that would also apply to the floor and false ceiling both being conductive.

Fyz

The chamber has a wool floor and John is wearing rubber shoes, giving him and his instrument a static charge. Tom's shoes are glass made of glass rods. Repeat the experiment in a room with a silk floor to reverse the results.

I think you've swapped the protagonists. And how do you explain the inverse cube variation with distance from the centre of the room that Tom sees?

The current being produced by the electric field will take the shortest route to ground and can only be measured by one meter at a time.

Could it be that there is a sphere shaped conductor in the center of the room emitting the field in 3 dimensions (inverse cube), and that Tom is close enough to the center to measure that field. Since John is 150 meters away from Tom, let us assume he is 150 meters farther from the center, that the field reduces to an immeasurable amount by the time it reaches John? If this were the case then they could both move towards the center of the room, but John would have to move much farther to actually measure the emitted field.

Amusing interpretation - and not excluded by the wording of the question; I hope it was not a trick question of that sort.

Fyz

At last we agree, Fyz. I second the notion. I certainly hope it is NOT a trick question, otherwise 3Doug had lots of valid and very different answers above! That is the trouble of these short "challenges", not enough words available to exclude the stupid and idiotic, although often entertaining, and leads to the possibility of trick questions.

I guess I just don't understand why they are so artificially short, especially after having submitted several myself, only to have them rejected as too long, requiring edited or rewrite, even if they only had a couple more sentences beyond the limit.

I would like to see that CR4 either set some ground rules for these challenges, or allow the writers more space to qualify them better.

As I recall (referring to post 1) The field strength around a dipole is inverse square. Since this is inverse cube, there must be a curved, probably parabolic, reflector facing toward Tom. The chamber must be transparent to the field, so none gets reflected back to John.

How do you reflect a field? Once again, people are confusing and electric field with elctromagnetic radiation. NOT the same thing!

I agree completely here. An electric field is the product of two differentially charged electrodes or surfaces!

Tom is an electrifying person with a magnetic personality obviously John does not possess these characteristics.

QED

Gee, that's a shocker! Maybe he has a Bi-polar personality.

Comedians!! Are you planning to compete with Robin Williams?

They are just trying for a higher unemployment check.

Lets take a closer look at the question..

Tom and John are standing in a large chamber;

1. It doesn't say where they were standing in the chamber?

they are about 150 meters apart.

2. It only gives a distance of separation, there is no information as to their direction or relative position to each other, or there position in the chamber.

Tom is measuring a weak electric field,

3. The instrumentation is not specified. This could be with a "field mill" or a "Leyden jar" with an antenna or what ever. The field mill requires a reference, and measures a gradient in volts per meter, The Leyden Jar measures a static charge and does not necessarily require a reference.

but John is not detecting any field at all.

4. So John may not be in the field.

Tom notices that if he walks towards the center of the chamber the electric field increases rather quickly, inversely proportional to cube of the distance.

5. Note, this says Cube of the distance. Would Gods law for inverse squares apply? Does this infer a direction relating to the source location? This also indicates that the field source in near the center of the chamber and is not uniform in the entire chamber.

When John walks toward the same point, he still measures no electric field.

6. Could it be that the field is being generated by a single charged surface over head near Toms position and the chamber floor? John could be standing 150 meters away and not even near the field.

Why is Tom measuring an electric field but John isn't?

Tom is in (or ajacent to) the vertical field gradient ahead (or above) him, but John is still 150 meters away and has not entered the field.

I suspect that this is a trick question and data has been intentionally omitted.

At no point does it say that John and Tom are standing next to each other in the same field and have the same readings.

I hope I have said this right, I'm having trouble keeping my Tom's and John's straight.

Location. Location. Location

Snakers

Since we have the inverse cube thing going on, we are probably talking about a dipole. John could be walking along an equipotential path, and would not detect a change in the magnitude of the electric force field, but he would still feel/measure a finite magnitude. However, the problem seems to scream out an anti-symmetric situation. John could be between two dipoles, walking along an equipotential path at zero field strength (opposing dipole fields cancel out), while Tommy-boy is approaching from the far end.

If you are on the line of a zero between two dipoles, their individual fields must have opposite sign. If you move further from one and closer to the other, the absolute magnitude of the first will reduce and the second increase. That means that the absolute rate of change will be the sum of the absolute rates of change of the two fields. Given similar equipment and capabilities (essential if the question is to be meaningful), and the only reason for the reduced field being another dipole, if Tom can detect a change as he walks, so can John.

I suspect that the questioner has in mind a monopole and a ground that creates an opposite sign image of the monopole. However, in order to maintain the relative attenuation at distance from the monopole, the ground either has to be vertically large or shaped so as to separate the protagonists; it will be interesting to see whether the questioner has appreciated this.
Be that as it may, it is why I came up with the thought that the cause might be that John is in a conductive tunnel created by metal reinforcements in the ground and wiring in the ceiling (equivalent to a metallic waveguide at a frequency below cut-off).

Fyz

BTW, if it is at DC, the "dipole" is probably a monopole above a negative image created by the (adequately conductive) physical ground. So we already have an inverse cube law. If the screen that shields John were large vertically, that would mean that Tom sees a quadrupole, which has an inverse 6th field. That means the screen must not be very high, so it must effectively enclose the region where John is. A grounded ceiling covering John's end looks increasingly likely to me.

Physicist..

Since the original question wording specifically says "electric field" I am inclined to think that he/she is referring to a DC electrostatic field and not an electromagnetic field (as many others are assuming).

I am thinking of a situation as in the "electric field" (volts per meter) in the dielectric area of a capacitor). This could satisfy the problem if the voltage gradient varied as the Cube of distance but I am not sure of this relationship.

It the charged plate was reasonably small and on the ceiling, then as John approached the high field area, the change in distance would vary as a complex function (R + JX). Could this become a cube function?

Sounds like you are current in this area, do you think this is a probability?

Been out of school too long.

Also thank you Rose

Snakers

1/d³ is the classical formula for the magnitude of a quasi-static field due to a dipole. As the question says "electric field", that would be the field due to equal and opposite charges separated by a small distance.

If we assume the ground to be conducting (it usually is near DC), that field would be produced by a monopole, plus its equal and opposite image vertically opposite it under the ground (the image is a simple way of representing what the charge distribution on the ground needs to produce to keep the ground at zero Volts (BTW, that would result in the field Tom sees being approximately vertical, except right by the monopole). Clearly, there can't be any screening that affects the field distribution near to Tom, so the screen either needs to be low in height or very local to John.

So, as far as I can see, it's either a fairly low grounded conductive ceiling in John's area but not in Tom's, or the screen moves with John. One possibility could be that John is wearing antistatic clothing and carrying a large conducting umbrella (it would need to be very large, because the basic field rises very rapidly as you approach the centre of the room, so it would need provide very effective screening).

I'm sure there are other possibilities, but I would anticipate increasing complexity...

"that field would be produced by a monopole"

What is the difference between a monopole and a conductive plate or sphere used as an electrode connected to a voltage source?

Isn't what I proposed in #4 basically the same thing as what you are describing, except you are relying on the natural conductivitiy of the ground, and I am saying that the entire chamber is designed to be conductive, except that the two halves which the men occupy are electrically isolated. Tom's half is oppositely charged (by the voltage source), generating an electric field between the electrode and the chamber and the other half is grounded to the electrode, so no field is generated. John won't need any screening then to read no field strength, will he? Am I missing something?

I'm failing to follow your configuration (and yes, I'm probably being dense). Maybe a diagram?

Please!

Fyz

On which subject, here is my (feeble) attempt at one. I've arbitrarily chosen a specific environment where this could happen.
'O' represents the ball of a Van de Graaff generator, and horizontal lines represent grounded conductive floor or ceiling (I tried to make them even as well as continuous, but would have needed to use a drawing package). The full stops are needed to create gaps, and should be ignored.

. . . . . . . . . . . . . . . . . O
. . . . . . . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . |
_____________________|
John in protected area_________________Tom in open lab

The protected area may just be created by a metal network, or it could be offices under a conductive false ceiling.
BTW, there could alternatively be loads of fountains in an open area where John is making his measurements, or he could be a hypochondriac who always carries a balanced air ioniser, and don't forget that large conductive parasol (perhaps John's there to photograph the experiment), or ...

Fyz

How do things change if the tank if full of water (or some liquid) and Tom anf John are FROG-MEN?

Depends on the frequency of the near-field source and the conductivity of the water. The purely technical side would work if Tom is on dry land and John is under water. But isn't it pushing things for John-the-frog to be "walking" under the water. (Don't bother suggesting full underwater gear - it would still be an awfully poor challenge...)

The dude with the field has got fillings. The other dude is a perfect example of dental hygene.

There is charged bubble in the chember.Tom is outside bubble and John is inside buble.

If Tom sees a dipole, that implies a second and opposing charge (or charge image). John would see that without cancellation by the bubble, and so observe a much larger field and an inverse square law.

We could set up a pair of intersecting bubbles with opposite charges, and John inside both bubbles - but I suspect that might be just a little bit artificial?

John is electric charged body and Tom is not

Tom is a robot.

John is 150 meters behind Tom and has yet to reach the position in which John read a weak field.

The idea of being 150 meters behind or on the side away from the source is likely, but add on to that, if John was in a corner of the room, this would compound the affect.

Tom is in dimension x while John is in dimension y.

Cuznmonkey was on the right lines, Tom has hair, John is bald! Simple detector.

Or he could be wearing a circa '74 Elvis-style jumpsuit (the polyest of polyesters w/obligatory metal backed sequins) and a silk scarf.

I agree with OZZB. The relative locations of Tom and John aren't given in relation to the center of the room. John could be behind Tom by 150 meters and since the field will also fall off "rather quickly" becasue it's still inversely porportional to the cube of the distance, The signal is too weak to read at Johns distance from the center of the chamber if they're both using the same equipment.

Electrical stuff is all "smoke and mirrors" to me, so in my simplistic understanding of electrical fields. I would say one of two things are going on.

1. John is grounded to the chamber and Tom is not. (or vise versa).

2. There is an electrical shield somewhat along the center on John's side of the chamber.

I'll let the "sparkys" argue about Dipole, monopole whatevers and the like.

"It is often better to be silent, letting others think you the fool, than to open your mouth and confirm it." (not verbatim) Abe Lincoln... Oh #$%@ I opened my mouth again.

"It is often better to be silent, letting others think you the fool, than to open your mouth and confirm it." (not verbatim) Abe Lincoln... Oh #$%@ I opened my mouth again.

I thought that was Mark Twain (Sam Clemens)?

STL, Both of us are correct. After a quick internet search it appears that President Lincoln and Mr. Twain are given credit for the quote. So that brings up the question of who said it first... Regardless, the origin of the quote is from the book of Proverbs, chapter 17, verse 28, "Even a fool is counted wise when he holds his peace; when he shuts his lips, he is considered perceptive."

A preacher, a politician and an engineer were led to the guillotine. The preacher was asked if he wanted to be face up or face down when he met his fate. The preacher said that he would like to lie face up so he would be looking toward heaven when he died. The blade of the guillotine was raised and released. It came speeding down and suddenly stopped, just centimetres from his neck. The authorities took this as divine intervention and released the preacher.

Next, the politician came to the guillotine, and he also decided to die facing up, hoping he would be as fortunate as the preacher. So the blade of the guillotine was raised again and released. It came speeding down and suddenly stopped just centimetres from his neck, so he was released as well.

The engineer was next, and he too decided to die facing up. The executioner slowly raised the blade of the guillotine, when suddenly the engineer said, "Hey, I see what the problem is…"

Just adding my two eV to the fray. Nowhere in the puzzle does it mention anything about the configuration of the chamber. All we know is its is a "large" space (how big is big?) and our two techs are 150 meters apart. It could be the Astrodome or a JumboJet Maintenance Hangar for all we know.

I get the feeling that Tom is inside a large Faraday cage with our electric field source and John is somewhere outside.

Neither of our techs or the source need to be anywhere near the centre of the chamber.

Mostly I agree the principles. But if the source is inside the Faraday cage, the cage has to be very large compared to the distances that Tom is moving for the field distribution to look like an inverse cube. So I think it's more likely that Tom is outside the Faraday cage, and John is inside it. And the cage needs to be of a shape that does not disturb the field. The question also says they are in the same chamber; I know that John could be in a chamber within the chamber, but that's moving into trick question territory, which really is not what a CR4 discussion group should be about. So it really should be an area that is effectively screened but without physically separating John and Tom

BTW, I think the wording of the question implies that the 'distance' for which the inverse cube applies is measured from the centre of the chamber; again, this is not explicitly stated, but otherwise we are back in trickquestionland.

Fyz

Hmmmmm, maybe John's meter is dead. No batteries.

I remember seeing that for linemen dealing with huge voltages (15,000 volts)they found a way to make the lineman part of the 'float' high voltage. In other words if the lineman is completely insulated from the ground he can touch the line and become part of the circuit without injury. Birds do this all the time as they roost on the high voltage line. (I will never try this). In this way they can work high up on the high voltage lines and as long as they are careful to never touch anything that would complete a circuit to ground they are ok. When they come down they reverse the procedure (insulate from high voltage and then touch ground).

IF Tom were insulated from the mild electric field (rubber boots or however), and john were not.. wouldn't that account for the results. The reason the voltage increases exponentially is because any electric field is like that? Similar to *Radar Range equaition is an exponential drop off with distance away from the radar transmitting antannae/dish.

The reason the voltage increases exponentially is because any electric field is like that?

Exponentially, yes. But for a normal electric field the exponent is 2 (squared), not 3 (cubed) as stated in the challenge.