Capacitor DC charging question.

[jeffrey samuel]

dc maybe -ve or +ve but it must not vary

that is true but there is nothing called negative dc power supply we just connect the circuit in the reverse starting from the gnd to the supply

 

[BradtheRad]

Positive varying voltages/currents in the first half of the cycle and non-varying second half are also called DC?
That's what it is called pulsating?
Thanks
Jack

Pulsed DC is as good a name as any I've seen for it.

 

[D.A.(Tony)Stewart]

Each scope reading for Source, Cap, Resistor show Voltage and Current. Arbitrary diode chosen with 0.7V drop @ 1Amp

5Hz

50Hz

 

[jeffrey samuel]

if you are still not clear i will have to say that you are trying to imagine beyond and getting all tangled up with your ideas

 

[lucky6969b]

Sorry, I have come back.
In physics, I remember there is one thing called displacement current, where energy stored in magnetic fields are transformed into electric fields in between the cap and transformed back into magnetic fields on the other side of the cap for continuous currents. So my question is if the energy has 'passed' thru the cap, that means no energy is stored in the capacitor during the positive cycle. To have the smoothing out effect, during the positive cycle must have some stored energy inside the cap, so that there is some to release during the negative cycle. So where has the energy gone?

Hope you understand my question

 

[jeffrey samuel]

I am not able to get your question clearly and I am sure that the Capacitor never has any effect due to magnetic field. Only an Inductor plays around with the magnetic field

 

[Ratch]

lucky6969b,

Sorry, I have come back.
In physics, I remember there is one thing called displacement current, where energy stored in magnetic fields are transformed into electric fields in between the cap and transformed back into magnetic fields on the other side of the cap for continuous currents. So my question is if the energy has 'passed' thru the cap, that means no energy is stored in the capacitor during the positive cycle. To have the smoothing out effect, during the positive cycle must have some stored energy inside the cap, so that there is some to release during the negative cycle. So where has the energy gone?

Your question is too involved and large to be handled in a forum thread like this. Therefore, you should let a professor from MIT teach you what you want to know. View the first part of his lecture. None of us in this forum can compete with his knowledge and teaching aids, such a chalk board.

Ratch

https://videolectures.net/mit802s02_lewin_lec18/

 

[BradtheRad]

To have the smoothing out effect, during the positive cycle must have some stored energy inside the cap, so that there is some to release during the negative cycle. So where has the energy gone?

There are people who say the energy is stored in the plates.
And there are people who say it is stored between the plates.

The best I can manage to explain it in my mind is that one plate acquires a surplus of electrons, as compared to the other plate. Hence it is charged.

The charging occurs in a brief burst of current, during the time that incoming voltage is greater than capacitor charge.

Then for a while afterward, the capacitor powers the load.

Its electrons leave the negative plate to go around the loop.
At the same time electrons enter the opposite plate.

Thus the capacitor discharges through the load.

A smaller capacitor allows the volt level to drop more quickly.

I have videos on Youtube that portray capacitor behavior. They depict how a capacitor charges and discharges. They show current bundles moving within wires.

Capacitor behavior (Animated)

www.youtube.com/watch?v=eIWEU4pObJw

Equal time constants (Animated)

www.youtube.com/watch?v=q2FRCUuE0d0

Full-wave diode bridge power supply (Animated)

https://www.youtube.com/watch?v=quqBv_IqtuE

 

[Ratch]

BradtheRad,

There are people who say the energy is stored in the plates.

The energy is stored in the electric field, which resides mainly in the dielectric.

The best I can manage to explain it in my mind is that one plate acquires a surplus of electrons, as compared to the other plate. Hence it is charged.

One plate acquires a surplus of electrons, the opposite plate loses the same number of electrons. It takes energy supplied by a voltage to imbalance the plates like that. That causes an electric field to form, which stores the energy. A capacitor has the same net charge when the plates are imbalanced as when they had when there was no voltage across them, specifically zero. For that reason, a capacitor is never "charged", it is energized. I explained this many times before in other threads, so there should not be any wonderment in your mind about what happens. Do you have any questions or doubts about what I avered?

I looked at the videos, and found them confusing to say the least.

Ratch

 

[pebe]

Ratch,

I don’t know why anyone should be so pedantic over ‘charged’ and ‘energised’.

The SI unit of quantity of charge is the coulomb; it is the charge transported by a steady current of one ampere in one second.

If a voltage source were applied to a capacitor that had no voltage across its terminals, a current would flow into the capacitor for such time as it took for the current to reduce to zero. If then the voltage source were disconnected, the capacitor’s terminal voltage would be the same as the source voltage.

A charge equal to the product of current flow in amperes and time in seconds would have passed from the source to the capacitor – so the capacitor could be said to have been ‘charged’.

Would you disagree with that?

 

[BradtheRad]

One plate acquires a surplus of electrons, the opposite plate loses the same number of electrons. It takes energy supplied by a voltage to imbalance the plates like that. That causes an electric field to form, which stores the energy. A capacitor has the same net charge when the plates are imbalanced as when they had when there was no voltage across them, specifically zero. For that reason, a capacitor is never "charged", it is energized. I explained this many times before in other threads, so there should not be any wonderment in your mind about what happens. Do you have any questions or doubts about what I avered?

Hmmm, okay, I knew I had to be careful what I say. Not to be too definite. Want to account for all situations.

Such as if someone were to bring up a situation where one plate is charged to 10V and the other plate to 20V. How do we sum that one up?

Or both plates charged to 10V? How do we sum that one up?

Or both to 1000V? We might think of that as being a static charge. But I read that the nature of static electricity is really no different from low voltage. It's just that static is known for odd behavior such as making hair stand on end, and making sparks, etc.

electric field

energized

No argument from my end. After all, we can't call it a magnetic field. Magnetism is not present.

So it must be something else. But is the electric field of the same nature as a static charge? If not then which situation should be called which?

Suppose we energize a capacitor's plates to 10V and 0V? Then we separate the plates to a distance of 1 inch? Is it still a capacitor?

To a separation of 1 foot?

When does it stop being a capacitor? How do we sum up the situation at that stage?

Fortunately we do not necessarily have to know the exact terminology of how a capacitor works, in order to use one in a project.

I looked at the videos, and found them confusing to say the least.

It's no surprise that the ideal simulator doesn't exist.

To enhance our understanding, the ideal simulator ought to let us FEEL the forces at work, the voltage, the current flow, etc.

I don't know of an easy way to do this. The closest thing is to play with the values, frequency, simulation speed, etc., and see what affects what. Participation, in other words. However my simulator is not yet ready for distribution.

Watching an animated simulation (my videos) is a step removed from participation. It's second best, no doubt about it. I will have to work further on perfecting my simulator.

 

[Ratch]

pebe,

I don’t know why anyone should be so pedantic over ‘charged’ and ‘energised’.

Because the "energized" way is the correct way to understand what happens, and the "charged" way is not.

Would you disagree with that?

Yes, I would. Now, let me tell you why.

The SI unit of quantity of charge is the coulomb; it is the charge transported by a steady current of one ampere in one second.

Yes, I have learned that a long time ago.

If a voltage source were applied to a capacitor that had no voltage across its terminals, a current would flow into the capacitor for such time as it took for the current to reduce to zero.

Actually you stated that wrong. A charge, not a current would accumulate on one plate and deplete on the opposite plate. Current does not flow. Current is charge flow, so saying current flow is like saying "charge flow flow". The total net accumulation charge difference between the time the voltage was applied until the capacitor voltage equalized to the source voltage is zero. So if the charge difference between the the energized capacitor and its initial condition is zero, then it is wrong to say a capacitor is "charged". True, it is charged with energy, not coulombs. But then you might as well say it is energized.

A charge equal to the product of current flow in amperes and time in seconds would have passed from the source to the capacitor – so the capacitor could be said to have been ‘charged’.

The voltage source received as much charge back from the capacitor as it supplied. All you can say about that senario is that the plate charges are imbalanced. You can say one plate is charged, but then you have to also say the opposite plate is discharged. So what is the status of the capacitor? How do you resolve that ambiguity? Since there is energy present in the capacitor, and a voltage difference across the capacitor when the plates are imbalanced, then energized is the logical, sensible and correct word to describe a capacitor in that situation.

Ratch

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BradtheRad,

Hmmm, okay, I knew I had to be careful what I say.

That is always a good policy.

Such as if someone were to bring up a situation where one plate is charged to 10V and the other plate to 20V. How do we sum that one up?

Or both plates charged to 10V? How do we sum that one up?

Or both to 1000V?

Ask yourself. What is the voltage difference across the plates? If both plates are equal at X volts measured from some reference, then there is no charge imbalance, and the cap is not energized. You can figure what the energy is in the other examples you presented.

We might think of that as being a static charge. But I read that the nature of static electricity is really no different from low voltage. It's just that static is known for odd behavior such as making hair stand on end, and making sparks, etc.

Irrelevant to the subject being discussed.

No argument from my end. After all, we can't call it a magnetic field. Magnetism is not present.

Did you see the beginning of the lecture from MIT I posted previously?

So it must be something else. But is the electric field of the same nature as a static charge? If not then which situation should be called which?

Suppose we energize a capacitor's plates to 10V and 0V? Then we separate the plates to a distance of 1 inch? Is it still a capacitor?

Certainly, see link https://en.wikipedia.org/wiki/Capacitor, look at the parallel-plate model paragraph and see how capacitance varies as plate separation distance.

To a separation of 1 foot?

When does it stop being a capacitor? How do we sum up the situation at that stage?

It never does, although its capacitance is very small at large plate separation.

Fortunately we do not necessarily have to know the exact terminology of how a capacitor works, in order to use one in a project.

Not "exact terminology", "exact principle". You do have to know what a cap does, however.

Watching an animated simulation (my videos) is a step removed from participation. It's second best, no doubt about it. I will have to work further on perfecting my simulator.

To each his own. I never used a simulator to understand how a cap works.

Ratch

 

[pebe]

The voltage source received as much charge back from the capacitor as it supplied. All you can say about that senario is that the plate charges are imbalanced. You can say one plate is charged, but then you have to also say the opposite plate is discharged. So what is the status of the capacitor? How do you resolve that ambiguity? Since there is energy present in the capacitor, and a voltage difference across the capacitor when the plates are imbalanced, then energized is the logical, sensible and correct word to describe a capacitor in that situation.

You could also say that as there is a voltage difference between the two plates of the capacitor, then one plate has a acquired a charge relative to the other one (C = F x V).

All but the most pedantic (and I consider myself among the ‘all’ group) might then consider the capacitor to be ‘charged’.

But I can understand your attitude – because I abhor the current use of the terms ‘positive’ and ‘negative’ as an upmarket replacement for ‘good’ and ‘bad’!

 

[Ratch]

pepe,

You could also say that as there is a voltage difference between the two plates of the capacitor, then one plate has a acquired a charge relative to the other one (C = F x V).

What is F in the equation? It is not descriptive enough to say that one plate acquired a charge relative to another. Do you mean that one plate acquired 4 coulombs of charge, and the opposite plate did not get any? So the cap would then have a net charge of 4 coulombs? It doesn't happen that way. One plate acquires 4 coulombs, and the other plate loses 4 coulombs for a net charge change of zero. How would you put some coulombs on one plate and not remove the same amount on the opposite plate?

All but the most pedantic (and I consider myself among the ‘all’ group) might then consider the capacitor to be ‘charged’.

Nope, the net charge would still be zero.

But I can understand your attitude – because I abhor the current use of the terms ‘positive’ and ‘negative’ as an upmarket replacement for ‘good’ and ‘bad’!

Don't try to understand my attitude. Try to understand my reasoning. Speaking of reasoning, I don't understand how your statement about positive/negative vs good/bad ties into what we are discussing.

Ratch

 

[pebe]

Ratch,

pepe,

I’m pebe, not pepe.

What is F in the equation?

The SI unit of capacitance, of course.

It is not descriptive enough to say that one plate acquired a charge relative to another. Do you mean that one plate acquired 4 coulombs of charge, and the opposite plate did not get any? So the cap would then have a net charge of 4 coulombs? It doesn't happen that way. One plate acquires 4 coulombs, and the other plate loses 4 coulombs for a net charge change of zero.

It is pointless to talk about a ‘net charge’ of zero, just as it would be pointless to say the ‘net voltage’ of a sinusoidal waveform is zero.
Quote from Wikipedia: “Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them”.

Hence F = C/V. If calculated for a net charge of zero, then the capacitance, F, would be infinite.

Don't try to understand my attitude. Try to understand my reasoning.

I cannot. I understand that a capacitor can hold a charge on its plates, between its plates, or wherever. And I consider a capacitor with a voltage between its terminals to be 'charged'

Speaking of reasoning, I don't understand how your statement about positive/negative vs good/bad ties into what we are discussing.

It doesn’t. It’s about attitude.

 

[jeffrey samuel]

This is hard core Argument here and each and every post is having many things to ponder on. Come on guys do tell What is the actual thing happening in a Capacitor?

PS: Where is the thread starter here ?

 

[Ratch]

pebe,

I’m pebe, not pepe.

Sorry for that typo.

What is F in the equation?
The SI unit of capacitance, of course.

Yes, I should have realized that. I am not used to seeing it written that way.

It is pointless to talk about a ‘net charge’ of zero, just as it would be pointless to say the ‘net voltage’ of a sinusoidal waveform is zero.

Well, charge is a quantity, and voltage is the energy density of the charge. So, if it is pointless to talk about density (voltage), it is not necessarily pointless to talk about quantity (charge). They are two different entities. Therefore, what applies to one does not automatically apply to the other.

Quote from Wikipedia: “Energy is stored in the electrostatic field. An ideal capacitor is characterized by a single constant value, capacitance, measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them”.

Wikipedia is known for being wrong at times. That is because they allow people who are not expert in their field to post articles, and I don't know if there is any editor who checks the accuracy of what they post. Every sentence you quoted above is correct except the last. Here's why. Referring to my last example, if a cap had +4 coulombs of excess charge on one plate, and -4 coulombs of deficient charge on the other plate, then the ratio would be ±1. Wikipedia should say something like "..the imbalance of charge from its value when the leads are shorted. . . ." According to Wikipedia, the calculation would be C=1/V, whereas it should be C=4/V.

Hence F = C/V. If calculated for a net charge of zero, then the capacitance, F, would be infinite.

That would be true if the above formula represented the net charge, but it doesn't. C represents the charge imbalance. Caps operate by charge imbalance. If they did not permit their plate charge to imbalance, they they would not be able to store energy or allow a voltage to exist across their terminals. But no matter how imbalanced their charge is, they still have a net charge of zero.

I cannot. I understand that a capacitor can hold a charge on its plates, between its plates, or wherever. And I consider a capacitor with a voltage between its terminals to be 'charged'

"Imbalanced" is a much better word that "charged" to describe what is happening. When you say charge, do you mean charge imbalance, net charge, plate charge (which plate?), or energy charged? "Energized" is the best word of all because it requires no description and is completely unambiguous. In addition, energized refers to the cap as a whole, not just to its parts, such as the plates or dielectric.

Ratch

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jeffrey samuel.

This is hard core Argument here and each and every post is having many things to ponder on. Come on guys do tell What is the actual thing happening in a Capacitor?

Energy is being stored in the form of an electrostatic field. Haven't you deduced that by now? By the way, why do you capriciously capitalize words within your sentences?

Ratch

 

[pebe]

Ratch,

Wikipedia is known for being wrong at times. That is because they allow people who are not expert in their field to post articles, and I don't know if there is any editor who checks the accuracy of what they post.....

Do you accept the formula "Charge = Capacity x Voltage" to be true?

 

[Ratch]

pebe,

Do you accept the formula "Charge = Capacity x Voltage" to be true?

Certainly, provided that charge means charge imbalance.

Ratch

 

[pebe]

Ratch,

If, by 'charge imbalance', you mean the two plates have different charges, would not that be implied by their voltage difference?

I am just trying to get some solid information out of all this verbiage.