Basic questions on Static Electricity

[Take me to the Pilot]

Hi, I'm new here. An absolute beginner who intends to learn about electronics with a bottom up approach... hence my starting with static electricity.

1) Firstly, I've a question about charging by contact. I get that different objects hold on to their electrons with different degrees of attraction, but when it comes to the cloth and rod I still don't get why a cloth can gain/lose electrons assuming both the cloth and rod are neutral in the first place. Assuming both items are neutral to start out with, why does a rod (whether it be cellulose acetate or polythene) become charged. I get why it would happen if the cloth already had a charge, but the cloth doesn't have any charge. Is it just due to the friction?

2) And Why is the cellulose acetate rod the one that becomes positive, with the polythene rod becoming negative?

3) The first image attached here shows charging an object by induction to make it positively charged. I'm wondering how would you draw the same diagram to charge the sphere negatively? It's hard to imagine What would happen when the finger would touch it because it's only negative charge can actually move? In this case the finger will touch the side of the sphere with the positive charge! The positive charge can't move in this case. Only the negative charge can move, and I don't see why it would want to given that the +ve charge would be in it's way.



4) I have the same confusion regarding the middle diagram in the second attached image. It shows the charging of a gold leaf electroscope. But how do the electrons get through the positive charge at the metal cap?



5) What if you charge a gold leaf electroscope, and then touch the cap of it with an insulator (that has the opposite charged to have the opposite charge to the gold leaf electroscope)? What happens?

6) And the last question - how do you charge an insulator? If electrons cannot move within insulators then how can they be charged? How is it possible to induce a piece of paper (with pen) if it's an insulator?

 

[timof]

These are very good questions!

Remember, the pictures you show are very simplistic, and the books that have these pictures are giving some very basic, preliminary information on the subject.

Some of the answers to your questions are very simple and straightforward, but some others are not simple (or not possible) at all.

Let me try to answer your questions.

1. The effect of "electrization" (i.e. charging) of objects or materials by rubbing them against each other has been known for millennia, and is called triboelectricity, You can read about it on Wikipedia, but the name does not explain the effect:

Triboelectric effect - Wikipedia

en.wikipedia.org

Similar to questions "what is charge", what is electron", and others, I view triboelectricityas an empirical, experimental fact, and do not bother to understand "why". I am sure there is a microscopic explanation to triboelectricity, but I am also sure it is not trivial, not elementary. Physics does not explain the basics, you should accept them as is, as experimental fact, like axioms in mathematics.

Three basic things are required for triboelectricity - two materials, and their close proximity (rubbing one against the other).

2. I do not know, and I do not think that (for majority of the people) that matters.

3. To charge the sphere negatively, you can touch the positively charged surface, to add electrons to neutralize the positive charge, or you can bring the sphere in proximity with a positively charged object, and touch the opposite side, to add electrons.

In general, in solids, the current is transported by electrons only (I am not going to talk about (positively charged) holes in semiconductors, that behave like electronc with positive sign - you can learn about that once you study the basic electricity first). The positive charge is attributed to positively charged ions, that "lose" electrons, and that are fixed in the crystalline (or amorphous) structure of the solid. So, while "+" and "-" charges in your pictures look kind of similar to each other, heir nature is very different.

4. The second picture seems to be wrong. To remove negative charge, you would have to touch the negatively charged part of this device.

5. The charges will be redistributed between these two objects, to equilibrate the voltage (or to equilibrate the Fermi levels).

6. It's a good question. I think charge s"sit" on the surface of the insulator (they are also sitting on the surface of a charged conductor, in equlibrium).

 

[c_mitra]

1) Firstly, I've a question about charging by contact. I get that different objects hold on to their electrons with different degrees of attraction, but when it comes to the cloth and rod I still don't get why a cloth can gain/lose electrons assuming both the cloth and rod are neutral in the first place. Assuming both items are neutral to start out with, why does a rod (whether it be cellulose acetate or polythene) become charged. I get why it would happen if the cloth already had a charge, but the cloth doesn't have any charge. Is it just due to the friction?

Every substance has electrons (because they are made of atoms). And these electrons are attached to the atoms or molecules or the solid by electrostatic attraction. For solids, these energy levels are called bands. We have conduction band, valence band etc. Tightly bound inner electrons do not form bands. The electrons at the top of the band are loosely bound and can be removed if we supply some extra energy (ionization potential for atoms; band gap energy for solids). Just touching won't work, you need to rub vigorously. This is possible only if these two materials are different; because then one will accept electrons and the other will donate electrons. The body that can give up electrons easily will be positively charged and the other solid that can accept electrons easily will be negatively charged.

2) And Why is the cellulose acetate rod the one that becomes positive, with the polythene rod becoming negative?

I do not know the energy band structures of these two materials but we can guess from the results. It is because the solid 1 can give up electrons easily when rubbed with solid 2. It is possible that solid 1 can take up electrons and become negative if rubbed with a different solid 3 that can give up electrons more easily (compared to solid 1).

3) The first image attached here shows charging an object by induction to make it positively charged. I'm wondering how would you draw the same diagram to charge the sphere negatively? It's hard to imagine What would happen when the finger would touch it because it's only negative charge can actually move? In this case the finger will touch the side of the sphere with the positive charge! The positive charge can't move in this case. Only the negative charge can move, and I don't see why it would want to given that the +ve charge would be in it's way.

This is basic polarization. The charge further away from the source of the electric field (electric field is created by the inducing substance) is more labile (can move easily) compared to the charge closer to the inducing charge (that sees a stronger electric field). This experiment is difficult to perform with an insulator (because the insulator cannot be discharged if you just touch it). When you touch any part of the solid the labile charge moves to the earth (the point with the lowest electric field) but the charge close to the inducing field stays. I hope this is clear; if not come back for more.

4) I have the same confusion regarding the middle diagram in the second attached image. It shows the charging of a gold leaf electroscope. But how do the electrons get through the positive charge at the metal cap?

Now this is made of metal (electrons are free to move). In absence of the electric field (inducing charge), the metal has same potential everywhere (else electrons can move from higher potential to lower potential). When the electric field is present and you touch the fellow at any point, the labile charge will move to the earth (your body in this example). That will not depend on where you touch the fellow. The opposite charge is tightly held by the electric field (this works only if the electric field is strong at one point and weak at the other end).

5) What if you charge a gold leaf electroscope, and then touch the cap of it with an insulator (that has the opposite charged to have the opposite charge to the gold leaf electroscope)? What happens?

It will have effect because there will be the electric field but touching will have negligible effect (unless the electric field is very strong). You cannot discharge a charged body by touching it with an insulator.

6) how do you charge an insulator?

Very simple: spray it with electrons like you paint a surface. More simple way: rub it with another insulator (cloth; wool; silk etc etc). The charge will stay permanently (it will be messy to discharge this body). It will produce an electric field but just by touching will not discharge it. To discharge this object, you need to rub the surface with a damp cloth (damp cloth is conducting). Remember that the electrons will stay on the surface (but you can use Gauss's theorem) and will associated with a electric field.

 

[wwfeldman]

Assuming the Bohr Model of the atom:
since we can only move the electrons around, things are negatively charged if they have
an excess of electrons, and are positively charged if they lack electrons
charge is conserved, so we can only move charge around

3) to charge the sphere negatively, touch the negatively charged rod to the sphere.
sliding the rod along the sphere may transfer more charge

touching the sphere with a finger (or a wire) provides a path for the
electrons to move further away from the negatively charged rod

4) the electrons do not "get through" the + charged cap
the + charged cap is + because the electrons have moved as
far away from the - rod as they can
remember we can only move the charge around - the parts of the electroscope
become + or - because we moved the electrons around

in the second picture, the finger provides a path for the electrons to move
the diagram shows the electrons moving away

i think this explanation works better:
this link uses a wire to ground and shows the electrons moving toward
the positivly charged top of the electroscope from far away

Charging electroscope

In above picture. When near positive rod to the electroscope, electrons go to top of electroscope. then connect electroscope to ground. Why electrons go from ground to electroscope? in top of

physics.stackexchange.com

6) you charge an insulator by rubbing it with another material,
as you described in 1)
rubber rod with fur (or wool) yields a - charged rubber rod, and + charged fur (wool)
glass rod with silk yields a + charged glass and - charged silk (requires quite a dry atmosphere)

 

[Easy peasy]

equilibrate = equalise ...

 

[Take me to the Pilot]

Every substance has electrons (because they are made of atoms). And these electrons are attached to the atoms or molecules or the solid by electrostatic attraction. For solids, these energy levels are called bands. We have conduction band, valence band etc. Tightly bound inner electrons do not form bands. The electrons at the top of the band are loosely bound and can be removed if we supply some extra energy (ionization potential for atoms; band gap energy for solids). Just touching won't work, you need to rub vigorously. This is possible only if these two materials are different; because then one will accept electrons and the other will donate electrons.

Thanks very much for your reply.

When you say bands (energy levels), is that the same thing as orbits? As in the orbits that the electrons have? As I haven't got my head around the way electrons orbit the nucleus, I like to think of it as if there's 2 electrons in the first shell, and 8 in every shell outside that. Don't tightly bound inner electrons belong to an energy level too (n=1)? So why wouldn't they form bands?

And the other question I have is; what's the difference between a conduction energy level and a valence energy level?

--- Updated Aug 27, 2020 ---

Every substance has electrons (because they are made of atoms). And these electrons are attached to the atoms or molecules or the solid by electrostatic attraction. For solids, these energy levels are called bands. We have conduction band, valence band etc. Tightly bound inner electrons do not form bands. The electrons at the top of the band are loosely bound and can be removed if we supply some extra energy (ionization potential for atoms; band gap energy for solids).

How is the band structure of an element represented? What's the format? Is it simply a matter of listing all the possible E levels that an electron may have, as well as all possible E levels that an electron may not have? The idea of listing out all the E levels that an electron cannot not have seems unusual to me at first: I mean how many levels do you list before you decide to stop listing?

 

[c_mitra]

This is not the right place to deliver a class room lecture here, but I shall try to explain very briefly.

Consider the energy levels of an atom. These are the atomic orbitals. Now consider two atoms (say of oxygen): these two atoms are brought closer to each other (starting at infinity). As they come closer, the electrons interact with each other and the energy levels are modified. The outer electrons interact strongly (just because they are outer) and the inner electrons to a much smaller extent (it will be wrong to say that they do NOT interact). The net result will be an attraction at large distance and repulsion at short distance. The interaction will result in the formation of the oxygen molecule. The two atoms have a defined distance from each other. The attraction results from the exchange process and the repulsion results from the coulomb effect. If we can push the two atoms closer still, there will be roles of the inner electrons also (like what happens in nuclear fusion).

Because the electrons are interacting strongly, the individual atomic orbitals lose their identity and fuse to become a molecular orbital. For oxygen, six atomic orbitals (forget the two inner electrons now) each for the two atoms will be now 12 molecular orbitals. The rule is simple: each corresponding orbital will interact and produce one bonding and one antibonding orbitals. In reality, the empty levels will interact too ...

In solid, there are a large number of interacting atoms: say N identical atoms. Each of these orbitals will now produce N molecular orbitals (yes, a solid may be considered a giant molecule) and these are so close to each other that they form a band. Bonding and antibonding energy levels are now called valence and conduction bands. But now the description starts getting messy but I hope your question has been answered. Any text book will tell you more details.

How is the band structure of an element represented? What's the format? Is it simply a matter of listing all the possible E levels that an electron may have, as well as all possible E levels that an electron may not have? The idea of listing out all the E levels that an electron cannot not have seems unusual to me at first: I mean how many levels do you list before you decide to stop listing?

You are right, we can actually go on to infinity. You stop listing when you think that this is sufficient for the current purpose. For example, at the current room temp, the electron may not occupy a very high energy level. But if you are considering a very high temp, then you may wish to add a few more energy levels. You stop when the electron energy becomes positive (free electron) because these states are not interesting (the electron is ionized). For hydrogen atom, this value is 13.6eV (the energy of the bound electron is -13.6eV). That is what gives rise to thermoionic emission! In practice, it is not useful to consider 1-2 higher empty levels (if you are studying photoelectric effect).




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As for putting a lens near the transmitting led... In my tests the type that works is large diameter, long focal length. Short focal length causes a vastly enlarged image at the receiving end, with the effect of dimming whatever illuminates the photodiode.
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