Capacitor makes the circuit open?

[samieee]

We know charge is accumulated on the conductor plates of capacitor. Here is a circuit (image) with voltage source, resistor and capacitor. Now due to the capacitor the circuit is actually open so flow of charge aka current is zero. Then how can charge be accumulated on the plates of capacitor?

 

[DragonPetter]

We know charge is accumulated on the conductor plates of capacitor. Here is a circuit (image) with voltage source, resistor and capacitor. Now due to the capacitor the circuit is actually open so flow of charge aka current is zero. Then how can charge be accumulated on the plates of capacitor?

It is only open at steady state, meaning after the circuit is in this configuration for a long time so that only the DC component of your voltage source is present.

Before the circuit is in the state of your schematic, there is no charge accumulated on the plates and so there is no voltage across the capacitor, this is known as an initial condition. Once the voltage is applied, charge flows through the resistor and begins accumulating on the plate. Initially, since there is no charge on the plate and so no voltage, all of the voltage is dropped across the resistor. As the plates begin filling up with charge, their voltage increases and so less voltage is across the resistor and so less current is flowing. Once enough charge has been accumulated on the plates so that its voltage is equal to the voltage of the source, the voltage across the resistor will be 0, and so no more current will flow. This is when it is considered an open, and in stead state -- the charge is already accumulated.

So, you should know that the capacitor is only an open to DC voltage/current, and not to AC.

 

[samieee]

Thanks for your reply.

Once the voltage is applied, charge flows through the resistor and begins accumulating on the plate.

Though voltage is applied the circuit is in open condition so the current flowing through resistor should be zero isn't it?

 

[DragonPetter]

Thanks for your reply.

Though voltage is applied the circuit is in open condition so the current flowing through resistor should be zero isn't it?

The circuit is only in the open condition once enough charge has accumulated on a capacitor so that its voltage is equal to the DC voltage applied.

Remember the voltage on a capacitor is V = Q/C, so as more charge is added, its voltage increases.

To your question specifically, if the capacitor is already in the charged condition, then the current flow through the resistor will be 0.

 

[sophiecentaur]

Thanks for your reply.

Though voltage is applied the circuit is in open condition so the current flowing through resistor should be zero isn't it?

Eventually, yes, with DC applied.

 

[samieee]

The circuit is only in the open condition once enough charge has accumulated on a capacitor so that its voltage is equal to the DC voltage applied.

But before that? Isnt it open in all conditions due to the dielectric between the plates?

 

[DragonPetter]

But before that? Isnt it open in all conditions due to the dielectric between the plates?

No. If there was initially no charge on the capacitor, and then you put it in a circuit with current, the current coming into it would get "captured" by the capacitor, and the charge of the current will start to accumulate on the capacitor. This actually means that the capacitor is acting more like a short circuit rather than an open circuit in the very beginning. Once the capacitor has captured enough charge, its voltage increases til it cannot capture any more charge, and this happens over a long time. When it is finally filled with charge that it can't take anymore, it acts like an open circuit.

The dielectric simply determines how much charge the capacitor can hold at a given voltage. It determines what the capacitance value of the capacitor is, and so you can see from what I said before V = Q/C, if you rearrange it to how much charge it can hold per a given voltage C = Q/V. Its basically telling you how much energy the capacitor can store inside its dielectric.

 

[sophiecentaur]

But before that? Isnt it open in all conditions due to the dielectric between the plates?

I think you are using the word 'open' wrongly which is leading you to a false conclusion. Any 'open' circuit - even the gap between switch contacts or bare ends of two wires - has a finite Capacitance, which can allow signal leakage so the 'open-ness' is all relative to the frequency involved and the particular situation involved.
An Open circuit is a practical description of a situation in which any current which may be flowing is negligible in the context of the other, more significant, currents when the open circuit has been closed.

It is very easy to try to categorise things too rigidly and to become 'obsessed' with getting the names right. This is the beauty of using Maths to describe things in Science because it involves using actual Numbers and Quantities, rather than Absolute terms to 'classify' things.

 

[DragonPetter]

This might be a useful analogy. Imagine you are pumping a really small tire with a hand pump that is connected with a hose.

Charge is air, and the flow of air is current.

The diameter and length of the hose is the resistor, it determines how much air you can pass through it at a given pressure on the air.

The tire is the capacitor. When it is flat, it has no air in it, and when it is full, it has the most air you can fill it with. The elasticity of the tire rubber is like the dielectric, how much it can stretch determines how much air it can hold at a given pressure. If you fill it til the tire can't hold anymore and pops, its like breaking through the dielectric.

The pump applies pressure that is like the voltage. If you start pushing down the pump handle, it will create a pressure that pushes air through the hose and into the tire.

As you begin pushing down the handle, the tire is empty (capacitor is not charged) and so it takes all of the air into it with no resistance. In this sense, it is acting like a short circuit, because there is no pressure in it so it looks like the hose isn't even blocked by anything and freely flows air to the outside pressure. The only resistance you see is from the hose, since it can only pass a certain amount of air at a given pressure.

As the handle moves down, the air keeps filling up the tire, and so the pressure in the tire builds up. This means it is becoming harder to fill the tire with more air, and so the air flow slows down, and more pressure is in the tire and less in the hose.

Once the tire fills up completely, you won't be able to push down the handle anymore because you aren't strong enough. You don't have enough pressure to push down the handle, you can sit on the handle and it won't move. The air still won't move since the pressure in the tire is just as strong. In this way, it looks like an open circuit, because no more air will flow.

It is the same way in a capacitor. Your voltage source begins pushing current through, and the capacitor is empty, so it looks like a short circuit. Once you have filled the capacitor with charge, its voltage is equal to the voltage you use to push the charge, and so you can no longer push anymore through, and it looks like an open.

 

[sophiecentaur]

Umm.
That is a very long winded analogy and I wonder just how useful it is for explaining things simply in terms of the actual electrical Capacitance across a gap in a wire vs the capacity of a high value Capacitor.
Sometimes less is more. But then, you really needed to included the case then the hose is blocked off before the tyre, in order to show the quantitative difference between a Capacitor and an open circuit.

 

[DragonPetter]

Umm.
That is a very long winded analogy and I wonder just how useful it is for explaining things simply in terms of the actual electrical Capacitance across a gap in a wire vs the capacity of a high value Capacitor.
Sometimes less is more. But then, you really needed to included the case then the hose is blocked off before the tyre, in order to show the quantitative difference between a Capacitor and an open circuit.

The point is that a capacitor does act exactly as an open circuit or a short circuit in specific conditions, and not in all conditions (t = infinity/~5 time constants and t= 0).

And I think the analogy serves to give the concept of "capacitance" pretty well since the OP seems to be confused with the relationship of charge accumulation and capacitance, and how this leads to the short/open conditions. It shows that you only reach an open after charge has been pumped, not in all conditions, and it also shows that in fact it is acting like a short when you initially begin pumping.

 

[sophiecentaur]

You actually made my point for me because that first sentence of yours expresses the whole idea perfectly in just over one line of writing.

PS I am very cranky about analogies because they so often let you down in the end! I guess I should get over it!

 

[DragonPetter]

You actually made my point for me because that first sentence of yours expresses the whole idea perfectly in just over one line of writing.

PS I am very cranky about analogies because they so often let you down in the end! I guess I should get over it!

Yes, but explaining it directly did not seem to help a lot and I'm still not sure that it would until a better idea of what is going on is there. That's why I decided to make an analogy.

 

[DragonPetter]

PS I am very cranky about analogies because they so often let you down in the end! I guess I should get over it!

I know analogies are bad, but they help make pictures/animations in your head that give you the intuitive feelings of things.

Anyway, if an analogy is mathematically equivalent to what it is being an analogy of, is it still an analogy or at least a bad analogy? Differential equations for mechanical and electrical systems can have identical relationships.

 

[sophiecentaur]

I take your point but every analogy needs it own Caveat if you want to prevent problems. I so often read responses that have grabbed the wrong bit of the analogy and ignored the important message. Having said that, I just lurve the mass on spring analogy for almost every oscillator description.

 

[sophiecentaur]

I know analogies are bad, but they help make pictures/animations in your head that give you the intuitive feelings of things.

Anyway, if an analogy is mathematically equivalent to what it is being an analogy of, is it still an analogy or at least a bad analogy? Differential equations for mechanical and electrical systems can have identical relationships.

AH yes - once you introduce the maths model and the correspondence can be spelled out, you get my vote.

 

[Averagesupernova]

PS I am very cranky about analogies because they so often let you down in the end! I guess I should get over it!

I think that it is generally understood that ALL analogies only go so far. An analogy is a good place to start from. The fault lies within the person who expects it to solve all of the problems, not within the analogy. Analogies have a tendency to put a persons mind in a certain mode that better prepares them the real thing rather than jumping right into the real thing. I have seen the question asked about how computers know how to do stuff. My reply to that is: How do mousetraps know how to catch mice? It is such a brief reply that it can hardly be considered an analogy but it activates a certain mode of thinking which is important.

 

[psparky]

I liked the analogy with the tire and air pump...good stuff.

Don't mind Sophie...he always gets grumpy with those...haha!

Let's face it...an interesting analogy AND a mathematical model work best for learning.

If you have only one of the two...you are typically missing something!

My firehose and water analogy leading into P=IV was a perfect example of this!

 

[psparky]

Because I'm pickin on Sophie today...let me sum up you capacitor question:

i(t)=C*dv/d(t)

A perfect mathematical model.

Any questions?

Alright...I'll throw in a small analogy as well...what the heck.

That statements says...the change in voltage in respect to time multiplied by the capacitance...equals the current.

So when you first put your voltage source across your uncharged zero volt capacitor...the change in voltage is large compared to time...so you get a fairly large current...as it charges...the voltage across the cap starts to climb and the change is much less...so the current gets to be less. Then when the cap reaches its full charge...there is no change in voltage...there fore there is no current!

Any questions?

 

[psparky]

Might as well explain inductors as well...the work exactly the opposite. They resist current at start up and short in steady state DC.

v(t)=L*di/dt

Before they get energized...they have zero current thru them. When you turn the switch on...the change in current is fairly large...then a fairly large voltage is across them...to have a voltage...I must have a resistance. As time passes...the change in current is much less...so the voltage across is much less...at steady state there is no change in current...therefore no voltage...and the inductor acts like a short.

 

[samieee]

According to boylestad's "Introductory Circuit Analysis", chap-10(capacitors) section 10.3:

Before continuing it is important to note that the entire flow of charge is through the battery and resistor- not through the region between the plates. In every sense of definition there is an open circuit between the plates of capacitor

http://books.google.com.bd/books?id=cH0KitWACEcC&pg=PA274&dq=boylestad+eleventh+edition&hl=en&sa=X&ei=wBx2T9uLOcrKrAfWiqWLDQ&redir_esc=y#v=onepage&q&f=truepage 398

Those are the words that made me confused. I thought if a circuit is open then how can charge be accumulated or flowed? Or 'flow of charge and accumulation of charge is different thing'-is the point I am not getting?

 

[psparky]

According to boylestad's "Introductory Circuit Analysis", chap-10(capacitors) section 10.3:http://books.google.com.bd/books?id=cH0KitWACEcC&pg=PA274&dq=boylestad+eleventh+edition&hl=en&sa=X&ei=wBx2T9uLOcrKrAfWiqWLDQ&redir_esc=y#v=onepage&q&f=truepage 398

Those are the words that made me confused. I thought if a circuit is open then how can charge be accumulated or flowed? Or 'flow of charge and accumulation of charge is different thing'-is the point I am not getting?

i(t)=C*dv/dt

It's impossible to sum it up any better than this :)

 

[DragonPetter]

According to boylestad's "Introductory Circuit Analysis", chap-10(capacitors) section 10.3:http://books.google.com.bd/books?id=cH0KitWACEcC&pg=PA274&dq=boylestad+eleventh+edition&hl=en&sa=X&ei=wBx2T9uLOcrKrAfWiqWLDQ&redir_esc=y#v=onepage&q&f=truepage 398

Those are the words that made me confused. I thought if a circuit is open then how can charge be accumulated or flowed? Or 'flow of charge and accumulation of charge is different thing'-is the point I am not getting?

I think his point is that charge does not physically travel through the capacitor. This does not mean that charge on both ends of the capacitor cannot change, which a change in charge over time is current. Remember that the separation of charge over a distance defines an electric field, and so there is a voltage and current across the capacitor when the charge transfers between the positive and negative plates of a capacitor, even if charge does not pass through the capacitor. This is only in AC, transient conditions.

 

[sheshank]

Hello friend
I found myself with the same question some time around three years back...I managed to find the answer. I think I can explain what I understood...
First let's understand the concept of Capacitors. Capacitors are nothing on their plates. Everything lies in the di-electric layer between them. The Chemical composition of few di-electrics in such a way that they transfer the force (EMF) but refuse to transfer electrons (Current). So, how does the force gets transmitted to the other end? This process is common in almost all the conduction phenomena. The charge of opposite polarity develops on either sides of the material. Now the electrons on the negative terminal of the batter respond to move through the material. It depends on the type of solid how much current flows. In case of di-electrics, it doesn't flow. The force gets transmitted, but electrons aren't allowed to pass. As a result the faces of the di-electric facing the positive and negative terminal of the batter stay the way as they are - after the force is transmitted. That is such that they have charge deposited on its surfaces. We use this property of the dielectric. We interface the surface of the capacitor with conductive plates.
As the plates are conductive they also place themselves with the same charge as on the surface of the dielectric. So, after a DC voltage is applied the charge is developed in the process as just described. Now, if the voltage is so heavy, then dielectric break down occurs and the current flows. This means that the chemical properties of the dielectric in between the capacitor plates has been chemically remodified. This is what we term as 'burning of capacitors'. Usually its these capacitors and transformers that burn in any circuit causing a very bad smell.

Supposing that the voltage applied is in the range of the capacitor, then what happens? The charge develops and this we call as the charged capacitor. The amount of charge i.e.. capacity of a capacitor is mainly based on the surface area the conducting plates are in contact with and the type of dielectric material between the plates. Once the dielectric is charged it has some energy stored (inside the dielectric because of the charge accumulation on the surface) its ready to be dissipated. Now, when something like a closed circuit with a combination of resistors is attached to the capacitor, the capacitor immediately discharges exponentially (You can compute this in any way using integral calculus.).

As it decreases exponentially, there is some time lag between the charge and slow discharge.(Even though it immediately charges but slowly discharges). This is used in RC and ringing circuits to produce frequencies of various scales. This is the base for the present Nanotechnology and Wireless communications.

I think I solved your question. If you have a query about how the AC travels through the capacitor post another question and I'll try to explain in the best way I can.

 

[vk6kro]

There is a simple experiment which usually settles questions like this one.

You get a large electrolytic capacitor and a flashlight bulb and put them in series across a power supply.
Observe polarity for the electro.

When you connect the circuit, the bulb will light up briefly and then gradually dim and then go dark.

Remove the power source and connect the two leads that went to the power source, together.

The flashlight bulb will again light up briefly while the capacitor discharges, and then it will go dark.

I avoid plumbing analogies because the discussion then becomes about plumbing.
These analogies don't work anyway, because household plumbing mostly involves water leaving the system, not going in circles like electrical circuits.

 

[sophiecentaur]

Hello friend
I found myself with the same question some time around three years back...I managed to find the answer. I think I can explain what I understood...
First let's understand the concept of Capacitors. Capacitors are nothing on their plates. Everything lies in the di-electric layer between them. The Chemical composition of few di-electrics in such a way that they transfer the force (EMF) but refuse to transfer electrons (Current). So, how does the force gets transmitted to the other end? This process is common in almost all the conduction phenomena. The charge of opposite polarity develops on either sides of the material. Now the electrons on the negative terminal of the batter respond to move through the material. It depends on the type of solid how much current flows. In case of di-electrics, it doesn't flow. The force gets transmitted, but electrons aren't allowed to pass. As a result the faces of the di-electric facing the positive and negative terminal of the batter stay the way as they are - after the force is transmitted. That is such that they have charge deposited on its surfaces. We use this property of the dielectric. We interface the surface of the capacitor with conductive plates.
As the plates are conductive they also place themselves with the same charge as on the surface of the dielectric. So, after a DC voltage is applied the charge is developed in the process as just described. Now, if the voltage is so heavy, then dielectric break down occurs and the current flows. This means that the chemical properties of the dielectric in between the capacitor plates has been chemically remodified. This is what we term as 'burning of capacitors'. Usually its these capacitors and transformers that burn in any circuit causing a very bad smell.

Supposing that the voltage applied is in the range of the capacitor, then what happens? The charge develops and this we call as the charged capacitor. The amount of charge i.e.. capacity of a capacitor is mainly based on the surface area the conducting plates are in contact with and the type of dielectric material between the plates. Once the dielectric is charged it has some energy stored (inside the dielectric because of the charge accumulation on the surface) its ready to be dissipated. Now, when something like a closed circuit with a combination of resistors is attached to the capacitor, the capacitor immediately discharges exponentially (You can compute this in any way using integral calculus.).

As it decreases exponentially, there is some time lag between the charge and slow discharge.(Even though it immediately charges but slowly discharges). This is used in RC and ringing circuits to produce frequencies of various scales. This is the base for the present Nanotechnology and Wireless communications.

I think I solved your question. If you have a query about how the AC travels through the capacitor post another question and I'll try to explain in the best way I can.

Unfortunately, what you write about dielectrics is not relevant to the basic function of a capacitor because two plates IN A VACUUM still behave as a Capacitor.

Also, as you introduced the idea of exponential decay, why carry on with an extra verbal description when the Maths has done it all for you?

 

[sophiecentaur]

I avoid plumbing analogies because the discussion then becomes about plumbing.
These analogies don't work anyway, because household plumbing mostly involves water leaving the system, not going in circles like electrical circuits.

Hear Hear.

 

[Averagesupernova]

I avoid plumbing analogies because the discussion then becomes about plumbing.
These analogies don't work anyway, because household plumbing mostly involves water leaving the system, not going in circles like electrical circuits.

But there gets to be a point where we just assume that certain things are happening within a circuit that 99.9% of us will never prove or disprove, or even need to understand. It is faith. A designer who specializes in voltage controlled oscillators for instance doesn't worry about the battery chemistry in the battery of the portable receivers that the VCOs will go into. He may be concerned about the things about that battery that affect the VCO but these things are spec'd and someone besides the VCO designer deals with it. The same thing can be said about the water outside of the building. In the big picture, the water does go round and round in our environment, but obviously not with the same tight controls within our electrical circuits. But for plumbig analogies, we could just was well assume that there is a magic pump outside of the house that collects all spilled water, filters it and pumps it back around. In this analogy, the magic pump is the battery that the VCO designer could not care any less about. It just doesn't concern him beyond the specs that are required. To the VCO designer the battery is a magic pump.
-
In analogies, the question is always asked that says: "What if this or that happens? This doesn't follow what happens in the electrical circuit!" And it may well be true, but at that point, the analogy ends. There will likely be a day when how we think of electricity, semiconductors, or matter in general will probably no longer work. We will then have determined that how we thought of it up until then was just an analogy. To an electrician who installs wiring in new houses the wires he installs and hooks up just as well be carring invisible magic green marbles. It will work fine for him. But when that electrician becomes a semiconductor designer, well, the marbles will fail him.
-
I don't see why people rip on analogies. It is like someone selling me a set of combination wrenches and I suddenly expect them to be able to do machine work on engine internals. The failure isn't with the wrenches, it is with my unreasonable expectations of a simple set of tools.

 

[psparky]

But there gets to be a point where we just assume that certain things are happening within a circuit that 99.9% of us will never prove or disprove, or even need to understand. It is faith. A designer who specializes in voltage controlled oscillators for instance doesn't worry about the battery chemistry in the battery of the portable receivers that the VCOs will go into. He may be concerned about the things about that battery that affect the VCO but these things are spec'd and someone besides the VCO designer deals with it. The same thing can be said about the water outside of the building. In the big picture, the water does go round and round in our environment, but obviously not with the same tight controls within our electrical circuits. But for plumbig analogies, we could just was well assume that there is a magic pump outside of the house that collects all spilled water, filters it and pumps it back around. In this analogy, the magic pump is the battery that the VCO designer could not care any less about. It just doesn't concern him beyond the specs that are required. To the VCO designer the battery is a magic pump.
-
In analogies, the question is always asked that says: "What if this or that happens? This doesn't follow what happens in the electrical circuit!" And it may well be true, but at that point, the analogy ends. There will likely be a day when how we think of electricity, semiconductors, or matter in general will probably no longer work. We will then have determined that how we thought of it up until then was just an analogy. To an electrician who installs wiring in new houses the wires he installs and hooks up just as well be carring invisible magic green marbles. It will work fine for him. But when that electrician becomes a semiconductor designer, well, the marbles will fail him.
-
I don't see why people rip on analogies. It is like someone selling me a set of combination wrenches and I suddenly expect them to be able to do machine work on engine internals. The failure isn't with the wrenches, it is with my unreasonable expectations of a simple set of tools.

Word up Homey Claus.

Where's the "Like" button on this thing?

 

[sophiecentaur]

But there gets to be a point where we just assume that certain things are happening within a circuit that 99.9% of us will never prove or disprove, or even need to understand. It is faith. A designer who specializes in voltage controlled oscillators for instance doesn't worry about the battery chemistry in the battery of the portable receivers that the VCOs will go into. He may be concerned about the things about that battery that affect the VCO but these things are spec'd and someone besides the VCO designer deals with it. The same thing can be said about the water outside of the building. In the big picture, the water does go round and round in our environment, but obviously not with the same tight controls within our electrical circuits. But for plumbig analogies, we could just was well assume that there is a magic pump outside of the house that collects all spilled water, filters it and pumps it back around. In this analogy, the magic pump is the battery that the VCO designer could not care any less about. It just doesn't concern him beyond the specs that are required. To the VCO designer the battery is a magic pump.
-
In analogies, the question is always asked that says: "What if this or that happens? This doesn't follow what happens in the electrical circuit!" And it may well be true, but at that point, the analogy ends. There will likely be a day when how we think of electricity, semiconductors, or matter in general will probably no longer work. We will then have determined that how we thought of it up until then was just an analogy. To an electrician who installs wiring in new houses the wires he installs and hooks up just as well be carring invisible magic green marbles. It will work fine for him. But when that electrician becomes a semiconductor designer, well, the marbles will fail him.
-
I don't see why people rip on analogies. It is like someone selling me a set of combination wrenches and I suddenly expect them to be able to do machine work on engine internals. The failure isn't with the wrenches, it is with my unreasonable expectations of a simple set of tools.

Analogies are just fine - in retrospect. When you actually 'know' the real thing, then the analogy makes perfect sense. But that's because you can see where the analogy does and doesn't fit.
The water-in-pipes analogy is fine as long as no one imagines the Kinetic Energy is involved in 'Electricity'. From what I read on these pages, the MAJORITY of people trying to get a better understanding of Electricity have KE at the heart of their picture. They can't help wanting the electrons to be like drops of water hurtling through a pipe and doing their work of heating things up and moving things. For them, it has been two steps forward and three steps back. And they don't realize it.
Likewise, with Photons, everyone starts off believing that the word "particle" implies all sorts of things about size, shape and even frequency. Talk to anyone who 'really' knows their stuff about photons as 'particles' and you can see that they have had to shed the 'particle analogy' way back in their learning process.
Analogies are best kept to within one's own mind or in one-to-one private conversations where the 'teacher' can maintain just where and when the analogy is valid and can help steer the 'learner' from massive pitfalls.
And in reply to the wrench 'analogy'. There are many people who keep using a wrench as a hammer because they don't know any better. They are not even aware that the chair they have built with it will soon fall to pieces. How were they being "unreasonable" to use a wrench if the 'expert' gave them the wrench in the first place?

 

[Averagesupernova]

And in reply to the wrench 'analogy'. There are many people who keep using a wrench as a hammer because they don't know any better. They are not even aware that the chair they have built with it will soon fall to pieces. How were they being "unreasonable" to use a wrench if the 'expert' gave them the wrench in the first place?

Ummmmm, not sure what to say to this. So all of a sudden someone who was given a simple tool by an expert cannot be held accountable to stupid things they do with those simple tools? It's not quite like giving guns to 4 year olds. Maybe I'm not reading you right. I guess I just think of things on a different level than most people.

 

[sophiecentaur]

If that 'ignorant' person believes what the 'expert' has told him then he will be using the wrench in good faith. Of course you can't hold him accountable because he believed his expert. I would feel really bad if I gave someone a wrench for his carpentry but he thought he could use it for the wrong purpose. Likewise, I would not want to give him an analogy which he could well use inappropriately.
I think one would need to second, third and fourth guess what any analogy may be used for before scattering it around, willy nilly for Tom Dick and Harry to pick up and run with in any old direction. Sorry about the mixed metaphors but I think you will get my meaning.

 

[Averagesupernova]

If that 'ignorant' person believes what the 'expert' has told him then he will be using the wrench in good faith. Of course you can't hold him accountable because he believed his expert. I would feel really bad if I gave someone a wrench for his carpentry but he thought he could use it for the wrong purpose. Likewise, I would not want to give him an analogy which he could well use inappropriately.
I think one would need to second, third and fourth guess what any analogy may be used for before scattering it around, willy nilly for Tom Dick and Harry to pick up and run with in any old direction. Sorry about the mixed metaphors but I think you will get my meaning.

Ok. I get the idea. But on this side of the pond I would think most people would say that as long as the borrower doesn't damage the tool it is their own stupid fault if they misuse it and wreck something. I cannot and will not save a population of people from themselves.

 

[sophiecentaur]

But, if you are the 'expert', do you take no responsibility for what you tell people? Why would you give advice that could be harmful? Why post on a forum like this one?

 

[Averagesupernova]

Since when did I tell anyone to misuse a tool? I admit I use analogies all the time but I am also the first to admit that they only go so far. If you come to my garage to borrow a 9/16 inch wrench to tighten the head bolts on your lawn mower and twist the bolt off, don't blame me. I really don't think there is any more to say about it.

 

[jim hardy]

Before continuing it is important to note that the entire flow of charge is through the battery and resistor- not through the region between the plates. In every sense of definition there is an open circuit between the plates of capacitor

Those are the words that made me confused. I thought if a circuit is open then how can charge be accumulated or flowed? Or 'flow of charge and accumulation of charge is different thing'-is the point I am not getting?

okay saimee, try this not-analogy

Charge flows along conductors we agree.
Whatever it is carrying that charge, usually electrons, moves slowly.
But the propagation of charge is quick - when a charge enters one end of the wire an indentical one comes out other end with almost no delay. But it's riding on a different electron .
Point of this paragraph - the wire neither stores nor absorbs energy, just passes it through. And charge is not energy.

Next paragraph
Cut 1mm out of the middle of wire and insert two plates parallel (like a capacitor symbol) of area A.
Now you have a capacitor of value εA/1mm .
Push one unit of charge into the wire. Where does it go? It moves infinitesimally into the wire. That increases the charge contained in the volume of the wire and its capacitor plate, which increases the electric field at its surface.
Point of this paragraph - the sea of electrons in a conductor can be made very slightly more dense. Electric field results.

Next paragraph
The second capacitor plate is now in presence of electric field from first plate. Charge in that plate is repelled by field from like charge in first plate. So some of it moves off the plate and down the wire, emerging at far end just as if it had flowed through the space between the plates (which it did not)
Point of this paragraph: That's how charge gives the illusion of moving through a capacitor.

Next paragraph
So now we have two plates , one slightly overpopuated with charge and the other slightly underpopulated. There should exist between them a physical attractive force .
our mind leaps immediately to $F=\frac{Q_1Q_2}{r^2}$
but that's the trouble with analogies, we sometimes jump ahead.
Force between two charged parallel plate is constant with distance not inverse square. That's because the charge on each plate is a sheet not a point. The electric field between the sheets depends only on the charge density and nature of dielectric between them.
Look right here:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html#c2

E field, $E = \frac{\sigma}{\epsilon_0}$. No distance term.

And $F = QE$
Which means the mechanical force between parallel plates of a capacitor aren't like a spring, there's no increase of force with distance.
Point of this paragraph: you'll hear of spring analogies to capacitor. They are just that, analogies describing mathematical not mechanical similarities.

Next paragraph:
Unlike in the wire, Energy is stored in the electric field between the plates. In a good dielectric polar molecules realign with the E field and the work it took to make them do that is mostly recoverable. In that sense the dielectric resembles a spring. And that's why the equations for a capacitor's electical behavior are so similar to those describiing a spring's mechanical behavior.
Point of this paragraph: our "mental pictures" and analogies must be constantly refined as we learn more. And that's a lifelong process. I learned a lot writing this .

I was taught in high school by a remarkable man whose analogies have stood the test of time. I realize now how fortunate i was. He taught us boys about that mechanical behavior of capacitor plates without calculus. See if these links help. I stumbled across them while trying to write something similar to first one.

http://www.haverford.edu/physics-astro/course_materials/phys102b/phys102_S99/notes/capnote2.html

Then this one for e field between sheet charges (it's constant - no distance term !)
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html#c2

and some others for good measure

See https://www.physicsforums.com/showthread.php?t=99544

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elewor.html#c2

http://www.schoolphysics.co.uk/age1...t/Force_between_two_charged_plates/index.html
and http://www.regentsprep.org/Regents/physics/phys03/aparplate/
Would anyone who sees them please point out any mistakes in this ? I need to get better in this area.. corrections welcome.

respectfuly,

old jim himself

 

[sophiecentaur]

Since when did I tell anyone to misuse a tool? I admit I use analogies all the time but I am also the first to admit that they only go so far. If you come to my garage to borrow a 9/16 inch wrench to tighten the head bolts on your lawn mower and twist the bolt off, don't blame me. I really don't think there is any more to say about it.

The analogy you give is like a tool without the Complete instruction guide. The 'proper' model or tool comes complete with instructions in the form of all the other resources in books etc. You cannot always be sure that you have included all the necessary caveats when you introduce a home made analogy.
At the start of this thread I pointed out the shortcomings of a post that involved a very long story as a substitute for about two lines of the real thing. How much sub-editing had been done on it to avoid misconceptions?

 

[sophiecentaur]

Am I being too heavy in introducing the subject of personal responsibility here?

 

[sheshank]

Unfortunately, what you write about dielectrics is not relevant to the basic function of a capacitor because two plates IN A VACUUM still behave as a Capacitor.

Also, as you introduced the idea of exponential decay, why carry on with an extra verbal description when the Maths has done it all for you?

Yes..Vaccum still behaves as a dielectric. I accept, and I did want to make it sweet and simple, so avoided such complexities. For your information, Capacitors are never made with 100% Vaccum. They still have some medium in between the plates (gas) which is pressurized to very low where Air can act as Ions and develop charges (get polarized). I hope you remember the Thomson's experiment of discovery of electrons : Gas conducts at very low pressures i.e.. almost vaccum.

Mathematics is just a tool not the soul. Mathematics just describes something quantitatively, not qualitatively. Qualitative analysis requires Imaginations and Imaginations are baseless without Idea. I just tried to create an Idea. I want everyone to grow everything on their own rather than depend on equations somebody else developed. Develop the same, or similar equation, on your self based on the ideas rather than somebody else's equations. That's, I feel, is the best way to learn rather than just looking at some equation as it can never be remembered with exact preciseness.

This is not in vengeance, this is just my view. That's all.

 

[sophiecentaur]

Frankly, I'm quite happy with Maxwell's work. If you can do what he did, all on your own then well done.

Free space has a Permittivity and has no dipoles. If a vacuum can conduct then I wonder how a thermionic diode is supposed to work. (Do you remember them?)

 

[sophiecentaur]

There are a few more details which you may need to sort out, such as the fact that the relative permittivity of a vacuum of 10^-7 Torr (εr is pretty much =1) so how much 'polarisation' of the gas molecules does this represent? (There may be some QM issues here, to consider, I think.)
Pictures in your mind are very useful but they need to agree with observation and well established theory.

 

[samieee]

Thanks to all folks for helping me understand the concept. For the last two days I was busy with it, studied from several sources. Now I have managed to solve my confusion.

 

[sheshank]

The conductivity depends on the applied voltage and the pressure... At some point the gas acts as a dielectric, similar to the ones applied in the conductor, and at some point it acts as a conductor and at some point it acts as a semi-conductor. There is a very narrow boundary between them.

Thermionic, or Vaccum tubes, involves in emission of electrons i.e..., diffusion not drifting. When there is diffusion, it doesn't require any media to travel. Do Earth required any media to travel? NO. Similarly, an electron doesn't require media to travel from cathode to anode of the diode. Its diffusion not drifting. The work is done by allowing the Cathode part to emit electrons which exhibits the exponential increase. Hence its used as a diode.

Its waste of time doing what Maxwell already did. I believe that there is an underlying philosophy, idea of every thing proposed till date. If we get to crack and understand that idea, then we can march forward rather than struggling to understand what it already said.

 

[sophiecentaur]

The conductivity depends on the applied voltage and the pressure... At some point the gas acts as a dielectric, similar to the ones applied in the conductor, and at some point it acts as a conductor and at some point it acts as a semi-conductor. There is a very narrow boundary between them.

Thermionic, or Vaccum tubes, involves in emission of electrons i.e..., diffusion not drifting. When there is diffusion, it doesn't require any media to travel. Do Earth required any media to travel? NO. Similarly, an electron doesn't require media to travel from cathode to anode of the diode. Its diffusion not drifting. The work is done by allowing the Cathode part to emit electrons which exhibits the exponential increase. Hence its used as a diode.

Its waste of time doing what Maxwell already did. I believe that there is an underlying philosophy, idea of every thing proposed till date. If we get to crack and understand that idea, then we can march forward rather than struggling to understand what it already said.

So a gas varies from a conductor, through being a semi-conductor to being a dielectric? Is this for all applied voltages? This is a novel idea.

That is when electrons have been released from the Cathode there is no conduction from the Anode when reverse biased - because the near-vacuum doesn't conduct on its own, once the thermionic electrons have reached an electrode. (You know that, surely?)
And, if the gas molecules in a vacuum capacitor (Pressure 10^-7Torr) contribute to the capacity significantly, how come the capacity is not very much more than the value with just 'free space' inside it? Look at some values of ε_r for some gases - then reduce the density to near-zero (i.e. for a low vacuum). Where do your "dipoles" come into the model, under those conditions. Some dodgy Science at work in what you say about this, I think.

Is it a "waste of time" to get down to it and understand what Maxwell tell us? Have you actually sussed it all out and do you actually have a better model that accounts for all the phenomena that Maxwell does? I'm very impressed if you have; the Maths is non-trivial and can't be ignored.

And have you "cracked it" ?

 

[sheshank]

So a gas varies from a conductor, through being a semi-conductor to being a dielectric? Is this for all applied voltages? This is a novel idea.

That is when electrons have been released from the Cathode there is no conduction from the Anode when reverse biased - because the near-vacuum doesn't conduct on its own, once the thermionic electrons have reached an electrode. (You know that, surely?)
And, if the gas molecules in a vacuum capacitor (Pressure 10^-7Torr) contribute to the capacity significantly, how come the capacity is not very much more than the value with just 'free space' inside it? Look at some values of ε_r for some gases - then reduce the density to near-zero (i.e. for a low vacuum). Where do your "dipoles" come into the model, under those conditions. Some dodgy Science at work in what you say about this, I think.

Is it a "waste of time" to get down to it and understand what Maxwell tell us? Have you actually sussed it all out and do you actually have a better model that accounts for all the phenomena that Maxwell does? I'm very impressed if you have; the Maths is non-trivial and can't be ignored.

And have you "cracked it" ?

My intention behind saying "waste of time" is something of the kind of " Why always try to hang on it? Why don't we try some of them ourselves." It doesn't mean to replace any of them or of any such sort, but just to try them on our own. Maths, as I always feel, is secondary not the primary one. Primary one's the idea giving rise to imagination, put down on paper in the form of diagrams and described precisely using a universally recognized language called 'mathematics'. You just can't catch anything and everything with mathematics. The best example : vital error which maths assumes i.e.. everything is ideal and it turns out way far from practical where nothing is ideal ; there will always be error which can never tend to Zero. Mathematical models are just models and have and ought to be reconstructed.

Well, coming to the emission of electrons from cathode it so happens and I stated. I think you have mistaken a little. I suggest you recheck your basics because I already did it before I am posting this reply and I am sure what I typed was right. Electrons can never be emitted if they aren't in excess. Voltage only pulls up electrons - with a uniform speed- which are excess at the other side, not the electrons of the material itself. If it pulls, then the whole thing blasts up- because the charges need to get accelerated not drifted which means change in energy-and the wires burn up and light's emitted etc.. I guess you know the capacitor failures and bad odor they emit. (di electric failures where the di electric turns)

Not all the gases act as dielectric. It depends on the composition.

Read this -an extract from a website

In 1896, Thomson visited America to give a course of four lectures, which summarised his current researches, at Princeton. These lectures were subsequently published as Discharge of Electricity through Gases (1897). On his return from America, he achieved the most brilliant work of his life - an original study of cathode rays culminating in the discovery of the electron, which was announced during the course of his evening lecture to the Royal Institution on Friday, April 30, 1897. His book, Conduction of Electricity through Gases, published in 1903 was described by Lord Rayleigh as a review of "Thomson's great days at the Cavendish Laboratory". A later edition, written in collaboration with his son, George, appeared in two volumes (1928 and 1933).

Happily, I was able to read both the volumes and I am confident as long as they hold their truth value, I hold mine. At the same pressure, Hydrogen conducts in the discharge tube while normal air acts as a dielectric and oxygen acts as a semi-conductor with a function of temperature and applied voltage. If its not for pressure then why aren't we filling the incandescent bulb with some other gas and why only Noble gases? Got the clue?

And there can't be capacitance with just "free-space" in it. We, humans, can never attain 100% vacuum. I bet, you'll be clear with all your doubts (confusions) once you read the books mentioned above. Just revise them once and you'll confusion will be solved.

 

[sheshank]

There are a few more details which you may need to sort out, such as the fact that the relative permittivity of a vacuum of 10^-7 Torr (εr is pretty much =1) so how much 'polarisation' of the gas molecules does this represent? (There may be some QM issues here, to consider, I think.)
Pictures in your mind are very useful but they need to agree with observation and well established theory.

Here is where all my friends, my lecturers, my colleagues fail. They fail to understand the basic underlying concept. First of all the permittivity is not naturally set one. We take it as one fundamental concepts. It varies with everything around you. Moreover, Permittivity is only compared not measured. Comparisions can never be depended upon unless you have a complete control of it. As you, yourself mentioned, its relative permittivity so recheck it again. Its relative, not absolute or anything of that sort.

Pictures are clue to the understanding. Theories come and go. They should fall apart one or the other day, when a better theory comes. Not every theory stands still. Not even Newton's theories stood still. Leave about theories. Pictures need to agree with observations I agree, but there are many other factors which affect the observations one or the other way and can't be eliminated. The only choice we have is to picturize it. Picturization may give rise to the description of it, precisely and Quantitatively. That'll never agree with observation again. Its like dog chasing its own tail.

Gases never polarize. Polarization happens only in the molecules in which they themselves can't drift and only their electrons can drift (like in all solids). When the molecules are ready to move, like in all liquids and gases, they move for the conduction. And here, in case of capacitor they accumulate at the surfaces of the plates.

 

[jim hardy]

And there can't be capacitance with just "free-space" in it.

uh-oh. I hope that's not so else my belief in aether has to go away.


or were you speaking of tha capacitor plates having to be in it ?

 

[sophiecentaur]

@sheshank

I conclude, from what you have written, that you have not 'The Maths" to cope with Maxwell or you would appreciate how elegant it all is when stated in mathematical form.
I also conclude that you have not heard of Thermionic Emission or the details of how electrons were first discovered.
As Jim says, you are an adherent to the idea of Aether -a very outdated and pretty well discredited idea. You have not heard of the permittivity of free space? (To which all 'relative permittivities' are related and which is a basic quantity involved with the speed of light).

I think a little more learning and a little less ego could help you a lot with your Science.

Lecturers and Colleagues all fail. Well, that is always the problem for the true Genius. I wish you well with your (non-mathematical) Physics studies and look forward to finding your name in the Nobel Prizewinners list.

 

[sheshank]

@sheshank

I conclude, from what you have written, that you have not 'The Maths" to cope with Maxwell or you would appreciate how elegant it all is when stated in mathematical form.
I also conclude that you have not heard of Thermionic Emission or the details of how electrons were first discovered.
As Jim says, you are an adherent to the idea of Aether -a very outdated and pretty well discredited idea. You have not heard of the permittivity of free space? (To which all 'relative permittivities' are related and which is a basic quantity involved with the speed of light).

I think a little more learning and a little less ego could help you a lot with your Science.

Lecturers and Colleagues all fail. Well, that is always the problem for the true Genius. I wish you well with your (non-mathematical) Physics studies and look forward to finding your name in the Nobel Prizewinners list.

What you concluded is right. I am not a pure or atleast partial mathematics student. I don't like everything around me to be complex. I want them to be simple and easy. Mathematics assumes everything to be ideal, and expects the same with the period of time which is not ideal. So, It stays away from reality. That's what troubles me and makes me hard to accept Math in everything.

I definitely read and watched the visuals and did almost everything learning in the discovery of Atom and its structures. Perhaps, with your stress, I think I'll have to read them again. Permittivity is not perfectly defined just like a charge or mass or time. Its just relative and not absolute. We assume it be to be some quantity and compare everything with it. I doubt the whole scientific progression regarding this.

I should apologize If I meant to express anything of the sort of Ego here. I didn't really mean it. Many people around me say that I express ego and thanks for that. I don't know how or where I express it, but its repeated again and again. I am still learning and didn't stop it. Well, finding my name in Noble Prize winners ! That's really a very big way. I think its too much for me taking my age of 19 into consideration. I have to learn Mathematics, but even Mathematics is not providing exact answer for my ideas. Anyway, thanks for your concern.