Challenge Questions

That kind of depends.

First, where is the electric field in relation to the loop of tape and how is that field shaped?

Second, has anything been recorded on the tape?

What speed (could be anywhere from 4.76 to 76.2 cm/sec)?

Is the data audio of is this a computer reel of tape (digital)?

I assume the word "type" is a typo and really means "tape".

Audio tape works by magnetizing a strip of cellulose acetate or nylon backed ribbon coated with iron oxide. Regions of the tape are magnetized either at one polarity or another with varying intensities of magnitization.

If the recorded material was music, then the data recorded (assuming analog) varies pretty quickly along the tape. Bass would cover larger regions and may provide more areas of change along the loop due to its lower frequency.

If the irregularities are of enough potential and they are spaced far enough apart, it would be possible to distort the shape of the loop somewhat, but you have to consider the tensile strength of the tape backing and the magnitudes of the fields in play.

Further, if there is two (as in stereo) or more channels of audio on the tape, then the average magnetic density along the strip would be even more homogonous, making it harder to deform the tape.

Even if the tape were blank, there would be a net attraction between the field generator and the iron oxide of the tape. In a perfect world the tape would remain round if the loop was a perfect circle and friction with whatever surface was in contact with the tape was nil or perfectly balance. In reality things are seldom perfect, so the potential for loop distortion runs higher.

In the end it will depend on the friction in the system, the field strength, and the true shape and location of the tape loop proximal to the generated field. Also, we must consider the generated field and determine if it is perfectly symmetric in shape.

My best guess, regardless of what was recorded on the tape, the attraction of the iron oxide to the generated field would probably overcome any minute magnetic moments recorded on the tape, so the actual recording may not be relevant and in a perfect world with a perfect loop of perfect magnetic tape, there would be no distortion of the loop if the field was a point source in the very center of the loop.

However, who knows how this puzzle was really intended to be set up.

Good answer. I assumed the tape loop to be simply a loop of tape in space or sitting on an object, but not in a system where it would see friction and external mechanical forces. Also, don't overlook the adhesive for the iron oxide. Polyurethane TPU has a fair amount of polar functional groups along the chain which can interact with the radiation.

The shape of the tape remains the same as static electric charges do not interact with magnetic fields.

Sounds like you are saying it will pick up a static charge. Audio tape should be sufficiently conductive that the charge will be able to re-destribute itself on the tape. All parts of the tape would then repell all other parts. Assuming the tape is able to move freely, it will form a circle (with a circumference of 100 cm) (assuming that by "loop", you mean the ends are joined to each other).

It seems to me it does no matter whether the tape is audio or magnetic or the like. It is loop and one had picked up an electric charge (static). So due to the principle of minimum of potential energy it's shape will be ended up as circumference or ring from top view.

Of course here's considered as given an ideal conditions. If tape's is placed over horizontal flat surfuce(desktop) gravity and any friction forces should be disregarded.

"You have a loop magnetic audiotape 100 cm in length. Put the tape in a field where the tape will pick up electric charges. Does the tape change its shape? If so, what is the final shape? If the shape of the tape remains the same, why?"

There are far too many variables to answer fully, but here is my attempt at a start:

We don't know what other forces are imposed on the tape, and how the tape has been "set" prior to our use. However, if it is simply laid out (with standard gravity) on a smooth plain, it will probably lie in straightish flat sections with relatively short uprights or twists that provide most of the curvature.

Suppose we now charge the tape, and then remove any external electric field (inlcuding any due to charge on the plane). The tapes necessary antistatic properties will allow the charge to reach its equilibrium position in a relatively short time, so the tape will be self repelling. But, unless the supporting plane is a very thin insulator, the attraction to the ground will dominate (the general effect will be the same regardless of whether the ground is conductive or insulating with a dielectric constant >1).
Taking the two case separately:
On a very thin insulating support, the tape will tend to become more circular - the upright sections will become slightly longer and less curved.
On any other support, the flatter sections of the tape will cling closer to the ground, and parts of the curved sections will also cling to the ground. Eventually, each bend will become a fold.

Now suppose we add a uniform lateral field. With a smooth plane, the tape would accelerate down the direction of the field until air resistance caused equilibrium. Because the tape is conductive, opposite charges would accumulate at the points of maximum and minimum potential, so the tape would also straighten out along the direction of the field.

If the field is radial and repulsive of the tape, it would tend to become more circular.
If the field is radial and attractive, the closest section of the tape will be more attracted to the source, at which point the tape might recharge to the same potential as the source, and become repulsed.

I could go on, but I feel I've already outstayed my welcome here.

To the answer of "Physicist?" I do agree most. However, I've seen how electrostatically charged magnetic tape attracted itself and therefore I assume that it is possible to have locally separated electrostatic charges with different amount and thus causing attraction too. Distraction is caused by absolutely uniformly distributed surface charges, which is IMHO not existing in reality. Thus the tape loop will form circles at bends with small radii but will also have longer straigth sections which attract each other. At least my mag tape loop did so... 30 years ago...

I have assumed the tape is sufficiently conductive along its surface to avoid the effect you describe. 30+ years ago when I last handled magnetic tape this was not commonplace. However, I believe the advent of thinner tapes for longer playing times required such conductivity, and that once the technology was available it was introduced to all tape types as it improved the transport properties. I could very easily be wrong about this, as I have no direct experience - but that is the assumption of my posting.

I also assumed that there was no conduction between the tape and the supporting plane. If we have a conducting support plane without a surface insulating layer, that would be incorrect. In that case the charge on the tape would be reversed once it contacted the support plane. If the field was great enough to raise the tape bodily, the result would depend on the distance between the plane and the source of the field. For large separation >50-cm) the tape would whip back and forth between the plane and the source; for small separation, the tape would form a bridge between the source and the ground (I'm unsure as to the length at which the dynamics of the situation would allow this to happen, however)

Well assuming that the tape picks up exaggerated amount of charge and that it is indeed connected at the ends, then yes the tape will form a circle with a circumference of 100 cm or a radius of 15.909 cm. the tape would be standing on its one side, kind of forming a ring.

Now if the tape is on a flat ground, then wouldn't the tape lose its charge to the ground and hence owing to gravity fall flat on the ground with there being a twist in the tape in either one, two or three places (generally).

Although while the tape is aliging itself into a ring, the earths magnetic field would interact as well wouldnt it and cause the ring to be pulled flat onto the ground.

So a flattened kind of circular shape with twists in it (i would say 2to 4 twists).

the open ring would get charges and if uniformly spread would tend to form it into a ring if it were on a friction free surface. If not uniformely spread it could bulge. Charges will not distribute as it is a non conductor of charge

If the tape is the Ferro Oxide (I think it what it is) then its possibly conductive.

I guess what your getting at with the tape being joined at the end, it will make a loop, if a changing magnetic field is bought near the tape, then it will become a "shorted-turn" with a fairly high resistance, if too much current is induced in the tape, then it will heat, and if there is too much heating, the tape will affect the base material by shrivveling, I'm not sure what will happen then, but I guess the resistance will drop making it conduct even more current, and get hotter.....

I do apologize as my yesterdays' post had been done at deep night. I can only add that tape is dielectric so it can keep electrical energy. All pieces of tape have the same sign uniformly distributed el. charge so there'll be mech Coulumb repulsive forces making it circle-shaped.

the only things that will cause the tape to move are physical contact or wind. The tape is non-conductive and is unable to store either a magnetic or electric charge strong enough to have any physical effect on the tape. It's shape will not change.

Insufficient data due to poor construct of question variables.Unable to process.ID 10T error.

ROFL

So far, most of the responses (the serious ones, anyway) have addressed whether the loop would change its shape. The question asks if the shape of the tape would change, and to that, I think the answer is "no".

I assume that you are referring to permanent changes of shape, rather than temporary ones?

Even there, that depends on the field. Assuming a very strong electric field, the source of which charges the tape, if the tape lay on a >1 dielectric block (or a conductive ground that did not discharge it), I think it would be so strongly attracted to the surface of the block that it would acquire permanent folds. If, on the other hand the surface were to discharge the loop, it would likely become deformed by the stretching and whipping as it oscillated back and forth between the plane and the source of the field.

I was thinking along the lines of a DVD or CD disc, which is pitted by a laser and thus actually would change its shape. Magnetic tape merely is charged here and there so would not be dimensionally changed in that way. But perhaps I'm being too simplistic there...

Perhaps it's a communications issue. My reading was that magnetic tape was only specified so we would know that the tape would be mildly electrically conducting (just sufficient for antistatic purposes), and to give the general shape. I take the magnetic properties to be irrelevant. Now we put it into an electric field that charges it up (it has to be a strong electric field to impart electric charges to the tape). So the question is: what are the effects of the forces of the electric field on the charged tape? As it's a strong field, it'll clearly result in some movement, which likely implies a change in the immediate shape of the tape. If the field is too strong, it will probably cause permanent mechanical damage, even before the onset of breakdown

BTW, at least some rewritable discs use phase change materials. The change in physical dimension can be comparably as small as the effects of changing the magnetisation of a tape...

If the tape is put in a microwave oven, then it could possibly move/warp/flash

My guess, and note that I said guess, is that the LENGTH of the tape would not deformduring recording (e.g., change length) because the tape drive moves the tape across the heads at a constant speed (15 IPS or whatever). The motor speed does not change to compensate for the amount of data on the tape, so if the tape length was deformed during recording, when it was played back, the information on the tape would be corrupted; if music it would sound funny.

Please reread the question - no recording mentioned, just the type of tape.
Although ambiguously written in most respects, the one thing that is clear is that this is not intended to be about normal use - the tape is just a vehicle to explore the effects of electrostatic forces.

Sorry, I thought that recording would expose the tape to electrostatic forces. Hey, wait, isn't that the way it works?

Nope, the record and/or playback heads for tape devices use electromagnetism, one to impose the signal on the tape, the other to detect the signal. The question doesn't say anything about magnetism except to identify the tape as magnetic.

To me, the ambiguous aspect of the question is: Is the tape subjected to electrostatic discharge, or to magnetic fields generated by electrostatic fields, or what? What is meant by electric charges? The term "electric charges" usually refers to electrostatic fields, but it could also mean ionic radiation.

Well, enough pot-stirring for now...

I suppose we could interpret this as a field of radiation. But I think that an electrostatic field that is strong enough either to create a modicum of ionic charging or to locally induce charges when the tape contacts a conducting surface looks to be the most direct interpretation.

The tape disintegrates in a flash of time travel to the present where HDD recording rules the day.

As others have commented, it depends on the field and the stength of the magnetic charges.

Put that loop in a field in central Florida during the summer thunderstorm season, and one lightning bolt could create 'electric charges' that would vaporize the tape. That would certainly change its shape!

Okay, I admit I'm slightly bending the meaning of 'field' and exagerating the sense of 'electric charges', but I'm doing this to make the point that the stated problem leaves too much to interpretation.

I don't understand/believe the answer. A diagram of whta's meant would have helped.

(with apologies to non-Brits)

Dear Victor

Yes, it does appear simplistic. Evidently, the challenge envisages a situation where the tape is initially charged up and then the source of charging is removed and the loop of tape floats in zero gravity. It also assumes that the rigidity of the tape is either negligible compared with the electrostatic forces (or that the tape is uniform). Assuming that it is inside some sort of enclosure (say the space station), that situation is unstable even in the short term - the slightest offset from the electrostatic centre and the tape will be attracted towards a conductive surface, where it will attempt to flatten itself against the wall until it is discharged. Once discharged, its rigidity will cause it to bounce off the wall. Eventually, and assuming there are no astronauts around to disturb the air: after multiple bounces it will settle to a near-circular form (i.e. modified according to the discontinuity at the joint).

Welfrid

Ahhhh, now I see ! Many thanks for the clarification - all I need now is someone to replace my flower-pot .

Due to language syntax problems, the offical answer needs illustration to make full sense. (The answer could be interpreted in more than one way.)

Circular in what dimension?

How oriented in field?

Did not answer...if tape remains same, why?

etc.

I think the answer is sufficient to tell us what the challenger meant by the original question - see #31. Clearly it's not even a very good answer to that interpretation of the challenge, but that is another matter*.
Regarding "if tape remains the same, why": (s)he clearly assumes that the tape has changed its shape, which means that this question doesn't need answering.

*This is an honestly considered carp.
____________________________________

An ounce of honest consideration is worth a ton of carping

What if the tape is a mobius strip,like the old(ancient) 8 track tapes of yesteryear?Would there actually be a difference in charges between the two sides of the tape? (in this case, only one side since a mobius is a only one dimensional )?

some tape conducts, being metal film based, but the old mag tape does not

Being a good conductor (or have I been here)

Wouldn't the tape get hot as it now forms a single turn shorted winding, increasing in temperature and affecting the base material in some way?

having hot spots would dent the circle your hoping to be left with?

Or would it have enough resistance as to not get hot enough?

well, the metal is thin and the head runs at a high frequency with a bias( ~200 to 400 Khz) and the shorted turn problem is probably not significant. The metal may not be a good conductor.

But your also trying to impart enough of a electric field so that the tape possibly changes shape.

If your running the tape over the R/W head(s) and allowing the free part of the tape to hand free, then it will have a kind of drifted drip shape (drifted towards the outgoing side of the mechanism)?

This is (presumably intended to be) an electrostatic problem. The challenge says the field is strong enough to charge the tape (either ionic transport through an atmosphere or field emission in vacuum). If we assume that this does not cause either breakdown of the atmosphere or substantial re-emission from the tape, there will only be current in the tape while it is charging - and the rates of charging under non-breakdown conditions are relatively low, and unlikely to cause damage.
Of course, there is nothing written in the challenge to say that extreme conditions are not experienced - so we can't exclude solutions that say the tape melts/chars and scatters as an array of fine beads.

Do you buy off on the "official" answer?

I could accept the original answer under very limited conditions - the loop needs to be:
. untwisted, and of modern materials (i.e. with a somewhat conductive surface);
. effectively in free space once it is charged, so that it can float away from the source of the charge without coming into contact with a conductive or high-dielectric surface;
. rigid enough across the width of the tape that the concentration of charge at the edges of the tape does not cause these to bow outwards excessively;
. the mechanical properties at the joint in the tape need to be reasonably similar to those of the rest of the tape;
etc.

Under practical earthly conditions, my original answers (post # 5) seem more reasonable.

BTW, assuming the tape material is in the region of 25um thick and > 1mm wide, redistribution of the charge on the conducting side can be expected to compensate any effect of moderately uneven distribution on the other side.

"...practical earthly conditions..."

That was the part that galled at me, too...