Why is a small nut attracted to a straw whereas it should be repulsed?

[Lotto]

TL;DR Summary: I have a small nut tied to a string. I take a straw and generate free electrons on it by a handkerchief. Then I thouch the nut with the straw and the nut is to be repulsed by it, because of a transfer of part of its electrons. But it does not happen, the nut is attracted to a straw. Why?

The nut is to be repusled by a straw, not attracted. I have tried that with two different straws (both made of plastic) and it haven't worked yet. What do I do wrong?

 

[Drakkith]

My guess: the charges in the metal nut are redistributing themselves such that the electrons are further away from the straw, making the side of the nut nearest to the straw positively charged and the side further away negatively charged. Instead of a metal nut, try using an insulating material instead and see if that changes things.

 

[kuruman]

The plastic straw is not conducting. This means that the electrons in the straw are not free, as you claim, to move from the straw to the nut. @Drakkith's explanation would make more sense to you with this consideration.

 

[Vanadium 50]

First, it's not clear to me that when you rub surfaces toegther that know that charge moves from A to B and not from B to A (at least not without many more details).,

Second it's not at all clear why the nut should move at all - you have a conductor attached to an insulator by another insulator.

I think you need to rethink your system and what you are trying to accomplish.

 

[Rive]

The nut...

Please elaborate. What kind of 'nut' is that?

I guess it's about the 'pith ball experiment'
That experiment can have different outcomes depending on the actual rod and the 'nut'

 

[Lotto]

I was told at school that it should work. It is a normal small nut made of metal tied to a string. I tried that with a metal stick instead of the straw and with the handkerchief again and it didn't word either, the nut didn't even move. I need to generate free electrons on the straw or on something else, but how? If I had Van de Graaf generator, it would work, but I don't have it.

 

[Vanadium 50]

I was told at school

This is why we ask for references. We don't know if the person who told you misspoke, or you misunderstood or what. However, moving charge across an insulator defies the very purpose of insulators.

 

[kuruman]

I need to generate free electrons on the straw or on something else, but how?

You cannot generate free electrons on an insulator. If you rub the straw with a cloth, fur, etc. you will either rip electrons off its surface or deposit electrons on its surface and leave it, respectively. positively or negatively charged. Eventually, the net charge on the straw will be neutralized by ions in the air that will be attracted to it. Static electricity experiments are notoriously difficult to perform when the humidity in the ambient air is high.

I suggest the following experiment. You know that if you rub a balloon on your hair and you put it against a vertical wall, it will stick. Hang a balloon at the end of a string and suspend it vertically. Bring close to the hanging balloon a substantial piece of metal, e.g. a frying pan, and see if the balloon tips towards the metal. Be careful not to create air currents that might interfere with the experiment. This will demonstrate that there is an attractive force between a charged insulator and an uncharged conductor which is what your teacher asked you to do.

 

[Lotto]

You cannot generate free electrons on an insulator. If you rub the straw with a cloth, fur, etc. you will either rip electrons off its surface or deposit electrons on its surface and leave it, respectively. positively or negatively charged. Eventually, the net charge on the straw will be neutralized by ions in the air that will be attracted to it. Static electricity experiments are notoriously difficult to perform when the humidity in the ambient air is high.

I suggest the following experiment. You know that if you rub a balloon on your hair and you put it against a vertical wall, it will stick. Hang a balloon at the end of a string and suspend it vertically. Bring close to the hanging balloon a substantial piece of metal, e.g. a frying pan, and see if the balloon tips towards the metal. Be careful not to create air currents that might interfere with the experiment. This will demonstrate that there is an attractive force between a charged insulator and an uncharged conductor which is what your teacher asked you to do.

I am supposed to do the experiment to measure air conductivity and I can do it by determinig the distance between the nut and the straw, when I know this formula $Q(t)=Q_0 {\mathrm e}^{-\frac{\sigma}{\epsilon} t}$ in addition. I can calculate $Q$ by using maths. Times $t$ I am to measure.

However, I tried the experiment with two tied nuts, because I think that they should repulse each other now, but it didn't work again. I don't know how to measure the conductivity now.

 

[Rive]

It is a normal small nut made of metal tied to a string.

I don't think you'll see that experiment anywhere with nuts. Try some of the tings on the videos: a small Al-foil ball or table tennis balls.
As 'straw' you can use a plastic pipe or ruler.
Nuts are just too heavy, and I don't really know what is that 'straw'.

Also, try to imitate those videos first (including touching with the flat part or the tip).
Try to do a systematic work for all combinations.

 

[kuruman]

I am supposed to do the experiment to measure air conductivity and I can do it by determinig the distance between the nut and the straw, when I know this formula $Q(t)=Q_0 {\mathrm e}^{-\frac{\sigma}{\epsilon} t}$ in addition. I can calculate $Q$ by using maths. Times $t$ I am to measure.

Can you post the assignment exactly as was given to you? Is the straw & nut idea yours or is it required by the assignment?

 

[Lotto]

Can you post the assignment exactly as was given to you? Is the straw & nut idea yours or is it required by the assignment?

I just need to make the nuts repulsing from the straw, the rest won't be any problem.

 

[kuruman]

I just need to make the nuts repulsing from the straw, the rest won't be any problem.

Thank you for posting the statement. I wonder if the person who suggested it has actually done it and collected useful data. I note the casual statement " . . . then transfer some of the charges to the object." That's your sticky point. Since the "objects" are conductors, the most reliable way to transfer charge to them is by induction. Try it.

 

[Vanadium 50]

Well, at least we are starting to get somewhere. Posting the exact question n- which, granted, means admitting it was schoolwork and belongs there, but that can't be helped - would have been a good start.

How big is your nut? You want an itty bitty one. If it;s easy to thread the string, it is almost certainly too big. This won't work with a truck lug nut.

 

[Vanadium 50]

As a PS. Since it's an assignment (which you should have told us) you probably can't change anything, but using a nut is, well, nutty. Use a piece of down. The down will be initially attracted to the charge, and once it tuches it, repelled. No string required,

Don't be a nut!

 

[haruspex]

There are some puzzles in the question statement.

First, why two nuts? I suggest it meant to say "begin to repel each other".
Second, how does the hint relate to the task? Is the idea to do this twice, waiting different times before charging the nuts?

As for how to charge the nuts, yes, induction is likely to be more effective, but the word "transfer" implies contact. That would transfer some charge, but you would need to slide the straw around so that the nut gets to touch a lot of the straw's surface.
Note also that induction will lead to the nuts being positively charged, so attracted to the straw but maybe repelling each other.

I say "maybe" because as @Drakkith points out in post #2 the charges in the nut will redistribute when exposed to a field. Indeed, like-charged conductors can attract! To minimise that effect, you want the nuts to be thin (washers, even) but not allowed to rotate much. You need them to remain facing each other square-on. Tie each with two strings that form a wide angle.
If you set it up with the string pairs suspended from the same two points, so that initially the washers are flat against each other, you could then use induction to charge both and watch them separate. You will have just reinvented the electroscope.

 

[kuruman]

First, why two nuts? I suggest it meant to say "begin to repel each other".
Second, how does the hint relate to the task? Is the idea to do this twice, waiting different times before charging the nuts?

I think the idea is to suspend the charged objects from a common support and measure the decay of the distance between them as a function of time. The geometry can be used to find the charge and hence its decay in time. I think that using charged conductors for that purpose is not a very good idea. Charged toy balloons would serve the same purpose, will be easier to handle and hold more charge than metal nuts.

 

[vela]

A Fun Fly Stick is an easy way to charge metallic objects quickly.

https://www.amazon.com/dp/B07L3F29VM/?tag=pfamazon01-20

 

[haruspex]

Charged toy balloons would serve the same purpose, will be easier to handle and hold more charge than metal nuts.

Yes, that could be much better, but air currents might be problematic.

 

[kuruman]

Yes, that could be much better, but air currents might be problematic.

I used to show the two charged balloons as a classroom demonstration in a big lecture hall. If I poked them with a meter stick, they rotated about each other, however the distance (~30 cm) between them stayed pretty constant. Oscillations were not a problem probably because of the high air resistance. I discharged them by using the smoke from a blown out match.

 

[Lotto]

I think the idea is to suspend the charged objects from a common support and measure the decay of the distance between them as a function of time. The geometry can be used to find the charge and hence its decay in time. I think that using charged conductors for that purpose is not a very good idea. Charged toy balloons would serve the same purpose, will be easier to handle and hold more charge than metal nuts.

Well, I tried that using induction, I rubbed the straw with the handkerchief and brought the straw close to the nuts and then took a metal stick and touched them with it. Then I removed the straw and the nuts (both have mass under 1 gram) were to repel each other, but that didn't happen. I wonder whether it is the best way to measure it at home.

I don't have to measure the conductivity exactly by doing this particular experiment, but according to the creator of the assignment it seems to be the easiest way to do so. And I don't have balloons, which would be hard to do the measurement with, I think at least.

 

[haruspex]

Well, I tried that using induction, I rubbed the straw with the handkerchief and brought the straw close to the nuts and then took a metal stick and touched them with it. Then I removed the straw and the nuts (both have mass under 1 gram) were to repel each other, but that didn't happen. I wonder whether it is the best way to measure it at home.

I don't have to measure the conductivity exactly by doing this particular experiment, but according to the creator of the assignment it seems to be the easiest way to do so. And I don't have balloons, which would be hard to do the measurement with, I think at least.

The nuts may be too heavy and thick. As I wrote, you will have more chance of success with washers, hung as I described.
Balloons would be better. You only need to measure the angle between the strings from which they hang.

 

[Lotto]

The nuts may be too heavy and thick. As I wrote, you will have more chance of success with washers, hung as I described.
Balloons would be better. You only need to measure the angle between the strings from which they hang.

Yes, I have already tried that with washers, they were moving more, but the problem persists, they were moving towards the straw.

And wouldn't I have the same problem with balloons? If not, couldn't I use anything else instead of the nuts/washers? But what material?

 

[haruspex]

tried that with washers, … they were moving towards the straw.

Sounds like you had not succeeded in transferring much charge to them. Can you compare with how much an uncharged washer moves towards the straw?

wouldn't I have the same problem with balloons?

Two balloons, charged the same way, will definitely repel.

 

[Lotto]

Sounds like you had not succeeded in transferring much charge to them. Can you compare with how much an uncharged washer moves towards the straw?

Two balloons, charged the same way, will definitely repel.

Well, it just doesn't work. I put the charged straw between the washers, then touched them with a metal rod and put the straw away and either nothing happens or the washers move towards the straw, but mostly nothing happens. I think this is the electrostatic induction, but it doesn't work for me.

 

[haruspex]

Well, it just doesn't work. I put the charged straw between the washers, then touched them with a metal rod and put the straw away and either nothing happens or the washers move towards the straw, but mostly nothing happens. I think this is the electrostatic induction, but it doesn't work for me.

How about not bothering to charge the washers? Just put the straw near a hanging washer, observe that the washer is attracted, then plot how the attraction decays.

 

[Lotto]

How about not bothering to charge the washers? Just put the straw near a hanging washer, observe that the washer is attracted, then plot how the attraction decays.

Yes, that has occured to me as well, but the attractive force is so weak, that the washer can't stay deflected and returns back immediately to its original position. But I will try it with balloons.

 

[Lotto]

How about not bothering to charge the washers? Just put the straw near a hanging washer, observe that the washer is attracted, then plot how the attraction decays.

So, I have tried to rub two tied balloons against my hair and they were repeling each other, but the problem is that I need them to have the same charges, so that I can say $F_\mathrm e=k\frac{Q^2}{r^2}$. Now I can only say that it is more or less the same. Isn't there any way to arrange it? And another big problem is that when I bring a rule closer to measure a distance of the balloons, they are attracted to it, so I can't measure it. I think that balloons aren't the right way.

 

[nasu]

How do you make sure you have the same charge for the case of two nuts?

 

[Lotto]

How do you make sure you have the same charge for the case of two nuts?

Well, if I transfer some electrons of the straw to the nuts by conduction, then they should have the same charges. Or if I do it by induction, they should have them the same as well. Or not? Of course, it won't be totally the same, but is should be, in the case of the balloons, there is no reason why the charges should be the same. Or am I wrong?

 

[nasu]

You transfer charge from straw to both nuts at the same time or one after the other? But either way, I don't see why would you expect the nuts to end with the same charge.

 

[Lotto]

You transfer charge from straw to both nuts at the same time or one after the other? But either way, I don't see why would you expect the nuts to end with the same charge.

Let's say I have only one nut and after rubbing the straw, the nut is attracted to it, so if I put the straw far enough so that the nut can't touch it, what would be their distance $r$, when I then want to use formula $F_\mathrm e=k\frac{Q^2}{r^2}$? The distance between the center of mass of the charged part of the straw and the center of mass of the nut? And will their charges have the same value? I think yes, but I am no sure. I think I know how to conduct the measurement now, but I need to make sure I understand some crucial points.

 

[haruspex]

So, I have tried to rub two tied balloons against my hair and they were repeling each other, but the problem is that I need them to have the same charges, so that I can say $F_\mathrm e=k\frac{Q^2}{r^2}$.

Why do you need to be able to say that?
If the initial charges are q₀ and Q₀, what is the repulsive force at distance r, time t?

 

[Lotto]

Why do you need to be able to say that?
If the initial charges are q₀ and Q₀, what is the repulsive force at distance r, time t?

Since I must determine $Q$ of the straw. I must use this equation $Q(t)=Q_0 \mathrm{e} ^{-\frac{\sigma}{\epsilon}t}$. So when I calculate $Q$ (by using geometry) and know its $t$, I can use fitting with an amount of these datas and determine $\sigma$.

 

[haruspex]

Since I must determine $Q$ of the straw.

Do you? Please answer my question in post #33.

 

[Lotto]

Why do you need to be able to say that?
If the initial charges are q₀ and Q₀, what is the repulsive force at distance r, time t?

The repulsive force is $F_\mathrm e =k\frac {q_0 Q_0}{r^2}$ in this case, but I must be able to calculate $Q$ of the straw dependent on time, see post #12. I can do so by measuring the distance $r$, and then by using geometry, I can determine $F_\mathrm e$ thanks to the deflected nut/washer. Then I calculate $Q$ as $Q=2r\sqrt{\pi \epsilon F_\mathrm e}$.

 

[haruspex]

The repulsive force is $F_\mathrm e =k\frac {q_0 Q_0}{r^2}$ in this case, but I must be able to calculate $Q$ of the straw dependent on time, see post #12. I can do so by measuring the distance $r$, and then by using geometry, I can determine $F_\mathrm e$ thanks to the deflected nut/washer. Then I calculate $Q$ as $Q=2r\sqrt{\pi \epsilon F_\mathrm e}$.

You did not answer the question I asked. What is the force at time t? What is the ratio between that and the force at time 0?

 

[Lotto]

You did not answer the question I asked. What is the force at time t? What is the ratio between that and the force at time 0?

I am not sure whether I understand, but $$F_\mathrm e (t)=\frac{Q_0 k q}{r^2} {\mathrm e}^{-\frac{\sigma}{\epsilon} t}=F_\mathrm{e_0} \left(\frac{r_0 }{r }\right )^2 \frac{q}{q_0} {\mathrm e}^{-\frac{\sigma}{\epsilon} t}.$$ So
$$ \frac{F_\mathrm e (t)}{F_\mathrm {e_0}}=\left(\frac{r_0 }{r }\right )^2 \frac{q}{q_0} {\mathrm e}^{-\frac{\sigma}{\epsilon} t}$$
But why do we need it?

 

[nasu]

You don't need the actual values of Q and Qo. And it would be very difficult to find them. You just need to find how the ratio $Q/Q_o$ depend on some distance or angle in your specific setup. Then you will plot this (distance or angle) versus time.

 

[Lotto]

You don't need the actual values of Q and Qo. And it would be very difficult to find them. You just need to find how the ratio $Q/Q_o$ depend on some distance or angle in your specific setup. Then you will plot this (distance or angle) versus time.

Well, I am actually supposed to determine $Q$ dependent on time, see post #12.

 

[Lotto]

However, let's say I have only one nut and after rubbing the straw, the nut is attracted to it, so if I put the straw far enough so that the nut can't touch it, what would be their distance $r$, when I then want to use equation $Q=2r\sqrt{\pi \epsilon F_\mathrm e}$? The distance between the center of mass of the charged part of the straw and the center of mass of the nut? And will their charges have the same value? I think yes, but I am no sure.

 

[haruspex]

Well, I am actually supposed to determine $Q$ dependent on time, see post #12.

No, it says find the product of the charges, qQ. But if the purpose is to find $\sigma$ then you do not even need to find that.

 

[nasu]

Well, I am actually supposed to determine $Q$ dependent on time, see post #12.

There is no such requirement in the post #12. Using that formula does not mean that you need to know Q.

 

[haruspex]

I am not sure whether I understand, but $$F_\mathrm e (t)=\frac{Q_0 k q}{r^2} {\mathrm e}^{-\frac{\sigma}{\epsilon} t}=F_\mathrm{e_0} \left(\frac{r_0 }{r }\right )^2 \frac{q}{q_0} {\mathrm e}^{-\frac{\sigma}{\epsilon} t}.$$ So
$$ \frac{F_\mathrm e (t)}{F_\mathrm {e_0}}=\left(\frac{r_0 }{r }\right )^2 \frac{q}{q_0} {\mathrm e}^{-\frac{\sigma}{\epsilon} t}$$
But why do we need it?

Since you are working with two balloons, both discharging, the equations are $$F_\mathrm e (t)=k\frac{Q(t)q(t)}{r^2} =k\frac{Q_0q_0}{r^2} {\mathrm e}^{-2\frac{\sigma}{\epsilon} t} =
F_\mathrm{e_0} \left(\frac{r_0 }{r }\right )^2 {\mathrm e}^{-2\frac{\sigma}{\epsilon} t}$$.
Using the angle, you will measure F(t) and r(t), so you can plot.. what against what to find $\sigma$?

 

[Lotto]

Since you are working with two balloons, both discharging, the equations are $$F_\mathrm e (t)=k\frac{Q(t)q(t)}{r^2} =k\frac{Q_0q_0}{r^2} {\mathrm e}^{-2\frac{\sigma}{\epsilon} t} =
F_\mathrm{e_0} \left(\frac{r_0 }{r }\right )^2 {\mathrm e}^{-2\frac{\sigma}{\epsilon} t}$$.
Using the angle, you will measure F(t) and r(t), so you can plot.. what against what to find $\sigma$?

I understand, but I won't do it with balloons, I will do it this way: I will rub the straw and then made the washer deflect because it will be attracted to the straw (but the straw will be far enough so that it won't touch it). Then I will measure the distance between the washer and the straw. What can I say about their charges? If the straw has a charge $Q$, what charge has the washer? It is attracted, so it must have a charge, maybe only a partial one, but still some. Isn't its charge the same as $Q$ (in value)? Or are their charges different? I know it doesn't matter, but it interests me.

And I still think that when I am to measure the dependence of the straw's self-discharge on time, I am to measure its $Q$ dependent on time, but that is only my opinion.

 

[haruspex]

Or are their charges different? I know it doesn't matter, but it interests me.

And I still think that when I am to measure the dependence of the straw's self-discharge on time, I am to measure its $Q$ dependent on time, but that is only my opinion.

Since the problem statement only expects you to be able to find the product of the charges, the question setter clearly understood that you cannot find the individual charges in the asymmetric (washer and straw) model.
Different discharge rates is a problem, though. You can only find the sum of the $\sigma$ values.
With two balloons you can assume the discharge rates are the same, but I am not sure about washer and straw.

what charge has the washer? It is attracted, so it must have a charge,

No, even an uncharged conductor will be attracted because of the induced charge distribution.

 

[Lotto]

No, even an uncharged conductor will be attracted because of the induced charge distribution.

And can I say that the washer has a partial charge, that it has on one side a negative charge and on the other side a positive charge?

 

[haruspex]

And can I say that the washer has a partial charge, that it has on one side a negative charge and on the other side a positive charge?

If this is in the case where you touched the straw against the washer, both will have a negative charge overall, and the side of the washer away from the straw will be negative.
On cursory analysis, the side of the washer near the straw may be positive or negative. But maybe a more detailed view would resolve that.