Charged insulator touches a conductor

[conscience]

Ques : If a neutral metal sphere placed on an insulating stand is touched by a charged plastic rod , does the metal sphere acquire any charge ?

I think that by simply touching the sphere by insulating rod , the metal should not acquire any charge as the charges on the rod are immobile unlike a conductor .

But if the rod is rubbed against the metal sphere , then by friction sphere might acquire charge .

So , is the above text correct when it states that the metal sphere acquires a net charge on being touched by charged plastic rod ?

Thanks

 

[.Scott]

Although the plastic rod is not a conductor, the static charge is carries has a very high potential, thousands of volts) relative to the neutral sphere. Under these conditions, simply touching the plastic rod to the sphere will deposit some of that charge. Moving the rod across the metal sphere can deposit more of the charge - not just because of the friction (which may or may not work), but simply because, as you noted, the electrons on the plastic are immobile except at the very high potentials.

 

[Merlin3189]

I think I'll vote no. I guess a little bit of the charge could transfer at the point of contact, so strictly they could be correct. But if it doesn't transfer enough charge to pick up a bit of paper, then they've not fulfilled their whole claim.

It's not how we get a decent charge on a conductor using a charged insulator.

 

[conscience]

Thanks for replying .

I think I'll vote no.

Do you think what I have written in OP is theoretically correct ?

It's not how we get a decent charge on a conductor using a charged insulator.

By induction ?

I guess a little bit of the charge could transfer at the point of contact, so strictly they could be correct

Is it because rod might not be a perfect insulator ?

 

[Merlin3189]

Not because the rod is not a perfect insulator.
In order to become charged in the first place, it must be possible to move charges to or from the insulator. I think that only happens over a very short range - points of contact. Charging an insulator usually involves rubbing a soft insulator over the surface of the other insulator, so that many different points do come in contact.
So touching the metal sphere to the insulator would collect charges only from the point of contact and very near by. I don't know how near by. I expect, as Scott said, it depends on the potential. A highly charged insulator would induce a big charge on the surface of the sphere, so that there was a strong electric field between them capable of attracting charges from a longer range.

Induction captures all of the induced charge, so must charge the sphere more than just contact.here. Charging by contact must reduce the field and make it no longer strong enough long before the induced charge has been neutralised.

BTW I'm not expert in this area.

 

[sophiecentaur]

By induction ?

That's the key word in this business. Read up on Induction and see how you can charge the metal ball most effectively. If you try just 'wiping' charge from the rod onto the metal ball, your results will not be impressive.

 

[conscience]

Although the plastic rod is not a conductor, the static charge is carries has a very high potential, thousands of volts) relative to the neutral sphere.

Do you mind explaining why do you think charges on rod have very high potential ?

 

[sophiecentaur]

A highly charged insulator would induce a big charge on the surface of the sphere,

Not an awful lot, because the Potential on the sphere would soon equal that of the local piece of the rod and no more charge would be transferred 'physically'.

Do you mind explaining why do you think charges on rod have very high potential ?

Because the charges are very localised; they cannot move around. Small parts of the surface are like very tiny Capacitors and the Formula V=Q/C applies (Q is charge, C is Capacitance to Earth and V is the Potential with respect to Earth.
If you just touch the metal ball, some charge will transfer but the Capacitance of the ball is relatively high - so V will be correspondingly Low.
I suggest you read about Inductive Charging and that will tell you how you can get a much higher potential on the ball. I remember having the Electrophorus being demonstrated to us at School - pretty impressive demo.

 

[Merlin3189]

A highly charged insulator would induce a big charge on the surface of the sphere,

Not an awful lot, because the Potential on the sphere would soon equal that of the local piece of the rod and no more charge would be transferred 'physically'.

I remember having the Electrophorus being demonstrated to us at School - pretty impressive demo.

I think you're reading my comment differently than I intended.
Before the sphere contacts the rod, the field from all the charges on the rod is influencing the electrons in the sphere and causing a charge separation. With say a 10cm sphere held say a mm from the rod, the charges on the opposite side of the sphere are much further from the rod than the attracted charges on the near side.
If the sphere were earthed, I'd expect the charge on the near side to balance the field from the rod. You won't get the same charge as the total charge on the rod, but maybe towards half? If the sphere is not earthed, then the opposite charges on the opposite side can't escape. So they will compete to attract the charges away from the rod. But not much, because they are further away.

 

[sophiecentaur]

If you take an electroscope (gold leaf) and bring a charged rod to it, rubbing some charge off. The leaves will part because the scope is polarised; top cap + (say) and leaves -. They will fall almost together when the rod is removed. There is not much charge left on the scope. If you bring the rod near (touching or not - it doesn't make much difference), the leaves will part (polarisation). You then Earth the scope and the leaves collapse. Removing the rod sends the leaves wide apart again - showing that the scope has been left with a lot of charge. So the charging by direct transfer is not very effective but the charging by induction is impressive.
This link shows what happens. Not sure if this is in context with your comments Merlin but it's relevant, I think.

 

[Daniel Sellers]

If you take an electroscope (gold leaf) and bring a charged rod to it, rubbing some charge off. The leaves will part because the scope is polarised; top cap + (say) and leaves -. They will fall almost together when the rod is removed. There is not much charge left on the scope. If you bring the rod near (touching or not - it doesn't make much difference), the leaves will part (polarisation). You then Earth the scope and the leaves collapse. Removing the rod sends the leaves wide apart again - showing that the scope has been left with a lot of charge. So the charging by direct transfer is not very effective but the charging by induction is impressive.
This link shows what happens. Not sure if this is in context with your comments Merlin but it's relevant, I think.

You seem to be implying that you cannot appreciably charge an electroscope by conduction. I don't think this is the case. Charging by induction may be the more effective method, but I remember charging an electroscope by conduction just this last year in a lab class, and my textbook agrees with me on that point.

So while I'm not arguing that charging by induction can (potentially) leave more charge on a conductor, if the OP is asking whether a conductor can be charged by a charged insulator, yes it can.

 

[sophiecentaur]

You seem to be implying that you cannot appreciably charge an electroscope by conduction. I don't think this is the case.

I have not said that no charge is transferred. It's just a small amount and I gave a reason for that - I repeat:
If you have an insulator with charge all over it, the only way that any charge can transfer will be at a local level. A small area of the insulator that will be in contact with the ball will be at a high potential. It has a minuscule Capacitance. Touching it against the ball (much greater Capacitance) will share the charges between the two (C's in parallel, effectively) That will charge the ball with more or less all the charge on that patch of the insulator. But how much charge will that be and what Potential will the ball acquire? Measurable but not very significant. Charge on other parts of the insulator surface doesn't get to the conductor.
Thing is that you are actually doing no work on the system and the only energy available is the Potential of the original tiny bit of charged insulator. When you use Induction, the initial polarisation of the conductor requires very little energy. But separating the discharged conductor from the insulator involves Energy input from the experimenter. So you get a large, induced charge at a high potential as you increase the distance.
Google will give you many videos examples to support what I say and I have done it myself many times with a school electroscope.
No "appreciable" charge transfer.

 

[Daniel Sellers]

I have not said that no charge is transferred. It's just a small amount and I gave a reason for that - I repeat:
If you have an insulator with charge all over it, the only way that any charge can transfer will be at a local level. A small area of the insulator that will be in contact with the ball will be at a high potential. It has a minuscule Capacitance. Touching it against the ball (much greater Capacitance) will share the charges between the two (C's in parallel, effectively) That will charge the ball with more or less all the charge on that patch of the insulator. But how much charge will that be and what Potential will the ball acquire? Measurable but not very significant. Charge on other parts of the insulator surface doesn't get to the conductor.
Thing is that you are actually doing no work on the system and the only energy available is the Potential of the original tiny bit of charged insulator. When you use Induction, the initial polarisation of the conductor requires very little energy. But separating the discharged conductor from the insulator involves Energy input from the experimenter. So you get a large, induced charge at a high potential as you increase the distance.
Google will give you many videos examples to support what I say and I have done it myself many times with a school electroscope.
No "appreciable" charge transfer.

I understand what you're saying about induction being able to create a much more significant amount of potential on the conductor. But you can also charge an electroscope enough for the leaves to visibly separate and remain apart by touching it with one relatively small part of a charged insulator. Perhaps we're misunderstanding each other using terms like appreciable and significant.

In regards to the OPs question (I actually studied from the same text his picture is from), I would say that in fact charge is transferred, at least enough to pick up small pieces of paper or visibly part the leaves in an electroscope. I agree that only the charge which is localized to the portion of insulator in contact would transfer, but experimentally that can be enough to continue with an introductory level demonstration like the OP was asking about.

 

[conscience]

In regards to the OPs question (I actually studied from the same text his picture is from), I would say that in fact charge is transferred, at least enough to pick up small pieces of paper or visibly part the leaves in an electroscope. I agree that only the charge which is localized to the portion of insulator in contact would transfer, but experimentally that can be enough to continue with an introductory level demonstration like the OP was asking about.

Hi Daniel ,

Thanks for your comments . Glad to know that you have actually performed this same experiment in your lab class .In real world , some charge might transfer from rod onto the metal sphere .

My concern was with theoritical aspect of the experiment . Why should charge transfer from an insulator owing to its property that charges on it are immobile unlike a conductor ? The demo could have used a charged conductor with an insulating handle to show charging by contact .

Do you think that some very tiny charge is transferred from the rod as it might not be a perfect insulator ? I am going by the theoretical definition of insulators given in intro Physics texts where charges are considered immobile in an insulator .

 

[Daniel Sellers]

Hi Daniel ,

Thanks for your comments . Glad to know that you have actially performed this same experiment in your lab class .In real world , some charge might transfer from rod onto the metal sphere .

My concern was with theoritical aspect of the experiment . Why should charge transfer from an insulator owing to its property that charges on it are immobile unlike a conductor ? The demo could have used a charged conductor with an insulating handle to show charging by contact .

Do you think that some very tiny charge is transferred from the rod as it might not be a perfect insulator ? I am going by the theoretical definition of insulators given in intro Physics texts where charges are immobile in an insulator .

When I did this experiment last year we simply held insulators in our hands after charging them, both plastic and glass. As someone said above, it doesn't have to do with the object not being a perfect insulator. Being an insulator just means that electrons are not free to move within the solid structure, as they are in a conductor.

Because in this case the charge exists in the form of ionized molecules on the surface of the insulator, the electrons will be attracted to the neutral matter in the sphere (or electroscope). So the insulator is still not allowing any charged particles to move about within it's structure. The electrons on the surface that makes contact (or very nearly makes contact) are repelled by their like charge neighbors and move onto the neutral conductor.

You may be taking the statement "charges on an insulator are immobile" a bit to strictly. This does not mean that an insulator can never be discharged. Because charge is a conserved quantity and insulators can be discharged, it follows that it must be possible to transfer charge from an insulator to other objects.

 

[conscience]

OK .

Please note that I am bothered more with the theory of this demo rather than what happens practically .What should a student answer if the question asked in OP is given in a theory paper ?

Ques : If a neutral metal sphere placed on an insulating stand is touched by a charged plastic rod , does the metal sphere acquire any charge ?

 

[Daniel Sellers]

Ques : If a neutral metal sphere placed on an insulating stand is touched by a charged plastic rod , does the metal sphere acquire any charge ?

I would answer that yes, it would acquire some charge. That is, provided the piece of the rod that touches the sphere has charge on it. Insulators do not spread the charge uniformly over their surface like conductors too, so it is possible that an insulator be considered 'charged' but that the charge is not on the portion touching the metal sphere. If you're being asked specifically for a yes/no (which would be kinda lame) then I would still say yes.

 

[conscience]

I would answer that yes, it would acquire some charge. That is, provided the piece of the rod that touches the sphere has charge on it. Insulators do not spread the charge uniformly over their surface like conductors too, so it is possible that an insulator be considered 'charged' but that the charge is not on the portion touching the metal sphere. If you're being asked specifically for a yes/no (which would be kinda lame) then I would still say yes.

Alright .

Actually a similar question appeared in one of the Junior Science Olympiads of our country and the official answer key suggested d) as correct option . Please see the exact question in the attached image .

I was okay with the question and it's answer until I read this textbook

 

[conscience]

@Daniel Sellers , I see you have edited post 13 and posted a video showing charging by conduction . The text in the video states "Electrons transfer when conductors touching each other " . Are you sure the rod in the person's hand is an insulator ?

 

[Daniel Sellers]

@Daniel Sellers , I see you have edited post 13 and posted a video showing charging by conduction . The text in the video states "Electrons transfer when conductors touching each other " . Are you sure the rod in the person's hand is an insulator ?

Hmm that is confusing. I assumed it was a conductor because, A) it appears to be plastic and more definitively B) he is holding an object in his hand which he uses to charge the electroscope by conduction, which would discharge the rod if it were a conductor. Perhaps part of the rod is a handle and the other end is a conductor? It appears to be a homogenous material. There is also this video.



Although she does do at least a minimal amount of rubbing in order to charge the electroscope. Other videos explain with diagrams the same concept; charging the electroscope by touching it with a charged insulator.

It's certainly possible I'm mistaken (and misremembering my class?) and, practically speaking, a simple touch of very limited area is not sufficient to conduct a significant charge to a conductor, but I'm not convinced that's the case. This second video demonstrates that, at least theoretically, some charge is transferred from the insulator to conductor.

Edited "...touching it with a charged conductor" to "...with a charged insulator" above. Whoops, typos.

 

[conscience]

Thanks everyone for your contribution in this thread .

 

[sophiecentaur]

Please note that I am bothered more with the theory of this demo rather than what happens practically .What should a student answer if the question asked in OP is given in a theory paper ?

If the demo is part of a School or College course then the Syllabus will tell one whether or not Induction will be tested. One should always teach to the syllabus when in doubt because timetables plus interruptions make it hard to squeeze everything in.
I still feel that, whether or not the pupils are likely to be tested, it would be criminal not to show the full demo. The effect on the leaves is very striking and kids like striking effects.
Hearts and minds.

 

[conscience]

If the demo is part of a School or College course then the Syllabus will tell one whether or not Induction will be tested.

In case you have missed it please see post 18

 

[sophiecentaur]

In case you have missed it please see post 18

I read about the Junior Olympiad but I didn't think that would necessarily be tied to a local syllabus. Contestants in an Olympiad would be expected to have a wider knowledge than you might find in a School Syllabus. The sort of people who organise such events are probably better informed than educational politicians etc who tend to set the educational agenda. (Just look at the misguided decisions that are made in the UK

 

[ZapperZ]

OK .

Please note that I am bothered more with the theory of this demo rather than what happens practically .What should a student answer if the question asked in OP is given in a theory paper ?

Ques : If a neutral metal sphere placed on an insulating stand is touched by a charged plastic rod , does the metal sphere acquire any charge ?

Answer: Yes, both in theory and in practice.

Assuming that this is at the level of first year undergraduate intro physics, then I will quote from Randall Knight text "Physics for Scientists and Engineers", 3rd Edition, Pg. 727 when he discussed charged insulating rod:

Insulators are often charged by rubbing... The charge can be transferred to another object upon contact, but it doesn't move around on the rod.

Zz.

 

[lesaid]

I notice that in the video posted earlier, the lady first ensured the rod was neutral by holding it in her hands, and moving her hands around to ensure that they touched all parts of the rod. This has to require that charge was transferred (by touching) from the insulating rod to a conductor (her hands) - and, it seems to me, not by the same mechanism that charges the rod by rubbing it with another insulator.

I have also found in practical experiment that demonstrations involving some 'insulators' can be misleading - ordinary paper for example is damp enough in my experience, even in a 'dry' atmosphere, to act like a poor conductor. There is another demonstration I recall from my schooldays where something (I forget what) was initially picked up by a static charge on a handheld comb, and then immediately repelled again due to picking up a like charge. That also has to be a demonstration of charge transfer from an insulator (the comb that retained its charge even though handheld) to something else that was probably a very poor conductor? I imagine that charge transfer of this kind is probably not enough to move the leaves of an electroscope though.

I can attest from personal experience that it is quite possible to get an electric shock from an insulator (piece of plastic) that has a significant surface charge - albeit from a corona discharge from a power supply rather than by rubbing it. Surface charges can be transferred over short distances via sparks if the voltage gradients are high enough - including along a substantial length of insulator.

So might it be correct to say that charges are not transferred directly by conduction from an insulator to a conductor, other perhaps than from those molecules specifically in 'contact' with each other, but can be transferred locally by conduction via ions in the air along the surface of the insulator due to sparking or corona (or UV light). And that this effect could be on a microscopic scale so that the nature of the charge transfer might not be apparent.

 

[Asymptotic]

I can attest from personal experience that it is quite possible to get an electric shock from an insulator (piece of plastic) that has a significant surface charge - albeit from a corona discharge from a power supply rather than by rubbing it. Surface charges can be transferred over short distances via sparks if the voltage gradients are high enough - including along a substantial length of insulator.

On dry winter days, I've see bright blue discharges of more than a foot emanating from PET polymer as it exits the cooling rolls in the haul-off section in an extrusion sheet line.

 

[sophiecentaur]

On dry winter days, I've see bright blue discharges of more than a foot emanating from PET polymer as it exits the cooling rolls in the haul-off section in an extrusion sheet line.

We've all seen that to a lesser extent when pulling cling film off the roll at home. It would be interesting to know what the actual charging mechanism is. Is it due to the original heating, stretching and rolling that one face of the film gets one charge and the other face gets the other? Or is it to do with the curvature?
Those long sparks are a great demonstration of Q = CV. The C between faces drops drastically as they separate and so the V has to increase accordingly. The user supplies the energy in this case (and the roll does, in fact, feel sticky due to the attraction)

 

[lesaid]

We've all seen that to a lesser extent when pulling cling film off the roll at home. It would be interesting to know what the actual charging mechanism is. Is it due to the original heating, stretching and rolling that one face of the film gets one charge and the other face gets the other? Or is it to do with the curvature?

Are you suggesting that the charge might have been laid down during the manufacturing/rolling process and 'locked' into the consumer roll until someone unrolls it? Given the elapsed times involved, would such a charge be that stable for long enough?

Those long sparks are a great demonstration of Q = CV. The C between faces drops drastically as they separate and so the V has to increase accordingly.

Never thought of it that way before! However, the long sparks I referred to earlier was, I think, a different mechanism. There was a fairly uniform and high surface charge over a large area. When a grounded conductor was close enough, an arc appeared to jump a distance of over two feet along the surface of the insulator, starting from 'nowhere' - just a point somewhere along the insulator. Photographing it revealed a network of smaller sparks in a cascade along its length.

I was wondering whether a microscopic version of this process might account for some of the apparent demonstrations of charge transfer by conduction between a 'charged' insulator and a conductor, where the sparks are too small to be noticed.

 

[sophiecentaur]

Are you suggesting that the charge might have been laid down during the manufacturing/rolling process and 'locked' into the consumer roll until someone unrolls

I'm suggesting that the molecules may have been polarised whilst warm and then held the molecular distortion when it cooled. It seems that one side of the roll has a different charge from the other side. If it's not that then could it be triboelectric due to distortion of the plastic as it's unrolled and the radius of curvature changed? With a film that thin, the stretching would be very small.
It seems to be a bit of a one-shot effect, though. The stuff sticks to itself very well the first time but re-doing it never sticks so well.

 

[lesaid]

I'm suggesting that the molecules may have been polarised whilst warm and then held the molecular distortion when it cooled. It seems that one side of the roll has a different charge from the other side. If it's not that then could it be triboelectric due to distortion of the plastic as it's unrolled and the radius of curvature changed? With a film that thin, the stretching would be very small.
It seems to be a bit of a one-shot effect, though. The stuff sticks to itself very well the first time but re-doing it never sticks so well.

I'm no expert here, but if a molecular distortion was 'locked in' as the film cooled, would it then be capable of causing a discharge? Would it not instead be like an electret material, where the charge itself is 'captive' and the polarisation (and 'stickiness') would remain, until perhaps, the film was warmed again?

I feel a little experimenting coming on!

 

[sophiecentaur]

Yes, it would remain because the potential would not be too high but, once the separation increased (unrolling the film) and the Capacitance Decreased accordingly, the Potential would be greatly increased. The work done in the experiment can easily be disregarded when explaining a number of electrostatic phenomena.

 

[Asymptotic]

We've all seen that to a lesser extent when pulling cling film off the roll at home. It would be interesting to know what the actual charging mechanism is. Is it due to the original heating, stretching and rolling that one face of the film gets one charge and the other face gets the other? Or is it to do with the curvature?
Those long sparks are a great demonstration of Q = CV. The C between faces drops drastically as they separate and so the V has to increase accordingly. The user supplies the energy in this case (and the roll does, in fact, feel sticky due to the attraction)

To set up the scene requires a lot of verbiage.

It is difficult to measure static voltage at the die outlet (>600°F, and not very safe, being at the roll nip), but I never read anything higher than 200V per an older model Simco electrostatic field meter. However, web coming out of the roll stack could hit 150 kV to 200 kV before partially discharging. Surface charge dropped to 10 kV or less after the sheet passed through the next stage - a bath of diluted silicone release agent.

The sheet didn't rub against anything in between the roll stack, and silicone bath, so charge must have been generated within the roll stack itself. The one I have in mind is a conventional 3 roll vertical stack with 30" diameter chromed steel cooling rolls in a "downstack" configuration where melt enters the upstream nip between the middle and bottom roll, and sheet exits the stack from the top roll (moving downward, hence the "downstack" moniker). Haul-offs designed to process PET and PLA are generally in an "upstack" configuration (or a "J" stack variation, or for thin sheet, a horizontal roll arrangement may be used). I think this factors into it, because the only processing lines known for lightning bolts shooting across the sheet were both downstacks.

What makes a downstack problematic in PET/PLA processing is PET freezes very quick upon contacting the first chilled roll it touches. Gravity assures the 'bottom' side of the melt curtain hits the bottom roll first, whereas most of the cooling is done by the middle roll against the 'top' side of the sheet. Another issue is how much material 'bead' is at the roll nip. If there isn't enough bead (or it isn't uniform across the nip) the sheet will have flat spots and other surface defects. Polystyrene, and many other plastics are more forgiving in this regard, but with PET, too much bead demands considerably more drive torque than normal (or can stall the rolls outright), and creates an inordinate amount of roll deflection leading to a different form of surface defect. Finally, because the sheet has been shrinking all the time it has been in contact with the middle roll it adheres to it to a degree, and (with gravity assisting) tends to want to wrap around the middle roll rather than transfer to the upper roll before exiting the stack.

I suspect several mechanisms are at work, but my guess is most of the static generation occurs as the sheet peels away from the center roll. Static discharge appeared to be more severe when operators were running with a heavier-than-normal bead, but I don't know if this was a consequence of squeezing it in the nip, or some other effect.

 

[sophiecentaur]

@Asymptotic : you have definitely 'been there and got the T shirt'. Interest that you measured some very high voltages along the line.
I found this movie about the manufacture. The massive bubble system is interesting. The movie is more about pretty pictures than Physics and the section (around 2minutes +) about how it sticks is not satisfactory for me. The 'added glue' part explains it partly but the commentary is clearly wrong about it not involving Static Electricity because sheets are attracted over distances of 10cm and more. Glue can't do that.

but I never read anything higher than 200V per an older model Simco electrostatic field meter

You don't need many Volts. What you need is Polarisation / Distortion of the molecules - i.e. Charge Imbalance. I think this will be maintained whilst the surfaces are together (it's the lowest energy condition and I don't think leakage is a relevant concept). Pulling the sheets apart is what generates the High Volts.

 

[Asymptotic]

@Asymptotic : you have definitely 'been there and got the T shirt'. Interest that you measured some very high voltages along the line.
I found this movie about the manufacture. The massive bubble system is interesting. The movie is more about pretty pictures than Physics and the section (around 2minutes +) about how it sticks is not satisfactory for me. The 'added glue' part explains it partly but the commentary is clearly wrong about it not involving Static Electricity because sheets are attracted over distances of 10cm and more. Glue can't do that.

You don't need many Volts. What you need is Polarisation / Distortion of the molecules - i.e. Charge Imbalance. I think this will be maintained whilst the surfaces are together (it's the lowest energy condition and I don't think leakage is a relevant concept). Pulling the sheets apart is what generates the High Volts.

I was only briefly and peripherally involved with film manufacturing (helped install an extruder & pelletizer in a film development lab), but you are right - the mention about 'glue' being involved is a head-scratcher. My understanding is that once it has cooled enough the bubble is fed through a slitter and branches off to feed one or more take-up rolls.

If there was a unique mark on the roll it would be possible to estimate it's speed (it appears fast to me), but this video snippet shows film being taken up onto a roll, and generating discharges similar in character to those I had observed with thicker, 12.5 mil sheet at 85 feet per minute.