Are capacitors a viable option for storing charge in electronic circuits?

[cpatel23]

I just finished my last physics class (e&m) and I was pretty intrigued by capacitors and their ability to store charge. I decided to take an electronics class next quarter so I bought a few things including breadboard, capacitor, resistor, diodes, motors, and so on.
Now...I have a 50V 1000uF capacitor. I filled(?) it with 24V (the motor I have is 3-24V range) and connected it to my motor, the motor spins for like 1/10th of a second. Then the capacitor goes down to like ~1V.
I did a little research on capacitor tanks, and even then, from what people write, capacitors don't last longer then 10 seconds. What gives? Why even use capacitors to store charge. Only useful application I can see is to implement them into circuits with a transformer to make the current a little less choppy.

I had my hopes set out on harnessing solar energy using photovoltaics and storing the energy in a capacitor, but now that seems like it isn't going to work.
Are rechargeable batteries a better option to store charge?

If someone can give me a reason on why capacitors are useful for storing charge that would be appreciated, along with answering the above. Thank you for your help (sorry for the wall of text).

 

[LordVader88]

Batteries are used for long term storage of electrical energy as you know.

I'm a near noob, but caps are used on small time scales, they are always getting charged/discharged, and they can be used to do a pile of things, but they are mostly always charging/discharging when taking part in whatever

1 basic thing is, to use a water analogy, say you where using a hose but the water pressure kept changing at random, but you wanted a smooth steady flow, a capacitor acts sort of like a little tank that the random hose would fill, but at the bottom had a steady steam coming out of it's tap.

So for a voltage that jumps up and down a bit, a capacitor is used to smooth it to a steadier level

 

[cpatel23]

Batteries are used for long term storage of electrical energy as you know.

I'm a near noob, but caps are used on small time scales, they are always getting charged/discharged, and they can be used to do a pile of things, but they are mostly always charging/discharging when taking part in whatever

Right. A camera flash circuit uses a capacitor to release a high amount of energy in a short amount of time. So that means capacitors are only good for short bursts of energy?

 

[nsaspook]

Right. A camera flash circuit uses a capacitor to release a high amount of energy in a short amount of time. So that means capacitors are only good for short bursts of energy?

No, it means that capacitors obey the rules of electrical charge movement. Batteries and capacitors can complement each other in a energy storage system. Batteries usually have a limited number of charge/discharge cycles before they start to fail and most batteries are not very efficient in storing energy when almost full. By adding a large capacitor bank that's discharged first instead of the batteries you can reduce the wear on the batteries and because the storage efficiency of a capacitor is usually the same at any point it can increase system efficiency if the load consists of a many short bursts like in a EV stuck in stop and go traffic with a regeneration system that captures the braking energy.

 

[Ratch]

cpatel23,

Why even use capacitors to store charge.

Are rechargeable batteries a better option to store charge?

If someone can give me a reason on why capacitors are useful for storing charge

Rechargable batteries and capacitors do not store charge, they both store energy. Knowing that, what is your question?

Ratch

 

[cpatel23]

cpatel23,

Rechargable batteries and capacitors do not store charge, they both store energy. Knowing that, what is your question?
Ratch

In terms of storing back up energy (for blackouts/being self sufficient), would it be better to use a battery or capacitor. From what I've learned, batteries would be more efficient even though they have their limitations.
This may seem stupid, but is it possible to store energy in a generator and then withdraw it when needed? Or is that the same as a battery?

No, it means that capacitors obey the rules of electrical charge movement. Batteries and capacitors can complement each other in a energy storage system. Batteries usually have a limited number of charge/discharge cycles before they start to fail and most batteries are not very efficient in storing energy when almost full. By adding a large capacitor bank that's discharged first instead of the batteries you can reduce the wear on the batteries and because the storage efficiency of a capacitor is usually the same at any point it can increase system efficiency if the load consists of a many short bursts like in a EV stuck in stop and go traffic with a regeneration system that captures the braking energy.

So from what I understand, if I have a breadboard whose primary function is to rotate a motor, it would be more efficient to have a capacitor give the initial push, and then use the battery to keep it going. Is this true?

thanks for your responses.

 

[Ratch]

cpatel23,

In terms of storing back up energy (for blackouts/being self sufficient), would it be better to use a battery or capacitor.

Several factors need to be considered, physicsl size, voltage, cost, and energy capacity.

From what I've learned, batteries would be more efficient even though they have their limitations.

Efficiency is not a consideration. Other factors listed above are more important,

This may seem stupid, but is it possible to store energy in a generator and then withdraw it when needed? Or is that the same as a battery

No, batteries can store energy chemically, capacitors can store energy in a electrostatic field, but a generator converts mechanical energy into electrical energy. So unless you can spin a flywheel connected to a generator, you cannot store energy in a generator.

Ratch

 

[yungman]

Storing charges is only one of the many use of capacitors. Powering a motor or any circuit is not exactly an important use of capacitor. There are so many things involve capacitance that battery cannot do.

1) Cap don't pass DC current and voltage, so it is use to block DC from reaching from one side to the other. But at the same time, it can pass AC signals, so it can couple the AC signal from one stage to another.

2) In electronics, we use capacitor and resistors to form poles and zeros to create low pass and high pass networks.

3) Read up transmission line theory, it's all about L and C.

4) filter out ripple of the un regulated voltage source.

These are just few comes to mind, there are a lot more use that basic EM...or even advance Electrodynamics don't get into. You really need to take a class in basic electronics.

 

[the_emi_guy]

Why even use capacitors to store charge. Only useful application I can see is to implement them into circuits with a transformer to make the current a little less choppy.

There are many, many other applications where capacitors are uniquely qualified. For example capacitors can be made to charge and discharge *very* rapidly, much more rapidly than any battery. Capacitors used tune radio antennas are charged and discharged *billions* of times per second continuously.

 

[cpatel23]

Several factors need to be considered, physicsl size, voltage, cost, and energy capacity.

I was thinking of a small scale setup (until I really get the hang of things). My initial idea was to by a dozen or so 1.2V/9V rechargeable batteries. Putting them in series accordingly to achieve a needed voltage, recharging them using solar panels. As for cost, I understand they could get a little pricey but in my opinion, it's worth the experience/knowledge.

When you said energy capacity, do you mean Watts or amp hour?
Since Watts have been brought up, let's say I have something that requires a minimum amount of Watts, but I only supply 70% of the minimum. Would my appliance work?

 

[cpatel23]

Storing charges is only one of the many use of capacitors. Powering a motor or any circuit is not exactly an important use of capacitor. There are so many things involve capacitance that battery cannot do.

1) Cap don't pass DC current and voltage, so it is use to block DC from reaching from one side to the other. But at the same time, it can pass AC signals, so it can couple the AC signal from one stage to another.

What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.

 

[yungman]

I am kind of taken back by the example and the question! I don't mean to be sarcastic...I don't mean to. But this is like saying a TV can light up a dark room. But it is not that bright and it flicker, so what is the use of a TV! I am just kind of lost of words. Using in motor is only one of the many many...many use of a cap.

Yes, capacitor is that useful and that important.

 

[yungman]

What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.

You put 5V on one side, and if you connect a very high value resistor from the other side to ground, you won't measure any voltage on the side with the resistor to ground. The cap block the DC voltage. But if you put a high frequency signal on one side of the cap, you can see the signal on the other side, it passes the AC signal.

 

[cpatel23]

I am kind of taken back by the example and the question! I don't mean to be sarcastic...I don't mean to. But this is like saying a TV can light up a dark room. But it is not that bright and it flicker, so what is the use of a TV! I am just kind of lost of words. Using in motor is only one of the many many...many use of a cap.

Yes, capacitor is that useful and that important.

Lol, please forgive me, only education I have on electronics is from my physics classes. Taking an introduction to electronics this upcoming quarter. Hopefully it can clear things up.
Any good electronics books in mind that I can read until then?Also, when I use a DC voltage regulator to regulate the voltage from my transformer (25V down to 15V) it gets really hot. Is this normal?

 

[yungman]

Lol, please forgive me, only education I have on electronics is from my physics classes. Taking an introduction to electronics this upcoming quarter. Hopefully it can clear things up.
Any good electronics books in mind that I can read until then?


Also, when I use a DC voltage regulator to regulate the voltage from my transformer (25V down to 15V) it gets really hot. Is this normal?

Someone need to give you a name of an introduction book, I don't have one right off my head. You are dropping to much voltage ( from 25 to 15V) across the regulator. Remember W=IV. if you drop 10V across the regulator, and if you even draw 100mA, it is 1W power dissipating with the regulator. If you don't have a heat sink, it can get hot. Some of the LM7805 get hot even when you are not drawing current.

I know a very good book that is a step above AC/DC circuits. It's the Electronic Principle by Malvino. I just bought a copy for under $10USD shipped. Go on Amazon and look for a used one.

 

[jim hardy]

cpatel -Your interest and enthusiasm are refreshing. continue experimenting.


Tinkering with capacitors can give you that intuitive "feel" for the meanings of

charge, , Q, Coulombs, a coulomb being around 6E18 charge carriers(usually electrons in our business)

current, I, Amperes, amount of charge moving past a point per unit time, one ampere being one coulomb per second

voltage , Joules per coulomb, energy per unit of charge, one volt being one joule per coulomb

energy , joules or watt seconds, amount of work that can be done, one joule being one Newton-meter

capacitance, Farads,, C a measure of the ease with which a capacitor stores charge. Units of capacitance are curiously same as length because it's area (of plates) divided by distance separating them, more later

Capacitors do store charge, in the amount Q = C X V
in other words one coulomb will charge one farad to one volt

That leads to concept of DC current into a capacitor-
one ampere into one farad will raise its voltage one volt per second
but it's not practical for that to go on for very long, voltage becoes too high.


Capacitors, by storing charge, also store the energy which pushed the charge into the capacitor, in amount 1/2 C X V^2
in other words one farad charged to one volt holds a half-joule
charged to two volts it holds two joules
charged to three volts it holds four and a half joules, etc
it's analogous to compressing a spring

The capacitor stores the energy in the dielectric separating the plates. There is an attraction between the plates because of their charge, so pulling the plates apart would add energy to the capacitor and this would show up as increased voltage.
The formula for capacitabce is C= ε X A / D
A and D being area and distance between plates, and ε is the dielectric property of the material between the plates. Doubling the distance would halve the capacitance. Practical capacitors have small separation between the plates and dielectrics with high value.

Remain curious my friend, and enjoy electronics.
www dot discovercircuits dot com has a forum that is popular with hobbyists and students alike, check it out.

EDIT to your original question

Capacitors are universally used in converting wall power AC into direct current for electronics.
AC runs through a rectifier which turns it into a series of positive (or negative) pulses. The capacitor stores charge to be delivered between pulses, thereby providing a fairly constant flow of charge(current) instead of pulses.

 

[nsaspook]

So from what I understand, if I have a breadboard whose primary function is to rotate a motor, it would be more efficient to have a capacitor give the initial push, and then use the battery to keep it going. Is this true?

With proper design that's true if you plan to recapture energy with a lead-acid battery system as most of the efficiency problems I talked about are mainly due to it's poor charge efficiency above 80% SOC and limited charge/discharge cycles times.

http://web.mit.edu/first/kart/#concept
http://web.mit.edu/first/kart/circuit.jpg

 

[mheslep]

Capacitors used for bulk energy storage are in the range of thousands or tens of thousands of Farads, not microfarads.
http://www.tecategroup.com/store/index.php?main_page=product_info&products_id=1259&gclid=CKCa2sX-q7QCFYZM4Aod3CoAUA&zenid=0eltghpum8v6g1ctjuteom0ma0

 

[cpatel23]

Capacitors used for bulk energy storage are in the range of thousands or tens of thousands of Farads, not microfarads.
http://www.tecategroup.com/store/index.php?main_page=product_info&products_id=1259&gclid=CKCa2sX-q7QCFYZM4Aod3CoAUA&zenid=0eltghpum8v6g1ctjuteom0ma0

wow. Is there a reason why all the capacitor tanks have low voltage (sub 9V)?

You are dropping to much voltage ( from 25 to 15V) across the regulator. Remember W=IV. if you drop 10V across the regulator, and if you even draw 100mA, it is 1W power dissipating with the regulator. If you don't have a heat sink, it can get hot. Some of the LM7805 get hot even when you are not drawing current.

Are these any good? >>>Heat Sink It says 1W, is that how much it dissipates or how much energy it requires to operate?

AC runs through a rectifier which turns it into a series of positive (or negative) pulses. The capacitor stores charge to be delivered between pulses, thereby providing a fairly constant flow of charge(current) instead of pulses.

So a rectifier doesn't completely convert AC to DC? It just creates pulses that flow in one direction?

http://web.mit.edu/first/kart/#concept
http://web.mit.edu/first/kart/circuit.jpg

That's some awesome stuff.

A question about putties batteries in series with capacitor.
This would be putting batteries in series with a capacitor to light an LED:
http://i.imgur.com/7BK9C.jpg

and this would be destructive
http://i.imgur.com/S4mWt.jpg

right?

and does the order matter? for instance battery, battery capacitor would be the same in terms of output as capacitor, battery, battery?
Sorry for all the questions, had to many, what seemed to be, close calls yesterday.

 

[pantaz]

... Sorry for all the questions, had to many, what seemed to be, close calls yesterday.

Before you hurt yourself, start here = Introduction to Electronics

 

[nsaspook]

A question about putties batteries in series with capacitor.
This would be putting batteries in series with a capacitor to light an LED:

The series capacitor in the CAP CART is switched out of the circuit when discharged and reconnected during regeneration charging. Just putting a series capacitor in a led circuit won't work. I think you need to step back and study electronics first to get an understanding of how circuits actually work.

 

[cpatel23]

Last question,
I have a circuit like so
http://i.imgur.com/WxSzV.jpg
When I let the capacitor fully charge it comes out to ~7.2V (Battery is at 8.7V).
The resistors are 180ohms each

Now when I remove the resistors and leave the LED and capacitor in series, the voltage on the capacitor will read 7.2V.

Why doesn't the voltage supplied to the capacitor reduce when I include the resistors?

 

[Drakkith]

Why doesn't the voltage supplied to the capacitor reduce when I include the resistors?

Once the capacitor is charged you will be reading the applied voltage from your voltage source across it. Think about it. When you apply a voltage to your circuit, current flows through it. As current flows the charges build up on each plate of the capacitor, with the negative charges on the negative side and the positive on the positive side. Since current doesn't flow through the dielectric in the capacitor these charges cannot go anywhere and instead build up until they match the applied voltage. Once this happens current has ceased to flow in the circuit. Adding resistance reduces current flow and will increase the time it takes the capacitor to charge or discharge, but since the current cannot get through the capacitor you will wind up with charges built up on either side of the capacitor still. The capacitor is "acting" like an open in the circuit at this point.

As for why your measured voltage doesn't seem to match the applied voltage, I don't really know.

 

[cpatel23]

Once the capacitor is charged you will be reading the applied voltage from your voltage source across it. Think about it. When you apply a voltage to your circuit, current flows through it. As current flows the charges build up on each plate of the capacitor, with the negative charges on the negative side and the positive on the positive side. Since current doesn't flow through the dielectric in the capacitor these charges cannot go anywhere and instead build up until they match the applied voltage. Once this happens current has ceased to flow in the circuit. Adding resistance reduces current flow and will increase the time it takes the capacitor to charge or discharge, but since the current cannot get through the capacitor you will wind up with charges built up on either side of the capacitor still. The capacitor is "acting" like an open in the circuit at this point.

As for why your measured voltage doesn't seem to match the applied voltage, I don't really know.

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

 

[Drakkith]

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

Before I answer this, I want you to do something for me.

Explain to me what Voltage, Current, and Resistance are. In whatever words you want to use, it doesn't have to be exact. I just want to get a feel for what you already know.

 

[flatmaster]

Someone has recently been developing a battery/capacator hybrid called an ultracapacitor

http://en.wikipedia.org/wiki/Electric_double-layer_capacitor
http://www.technologyreview.com/news/417053/a-battery-ultracapacitor-hybrid/

 

[cpatel23]

Before I answer this, I want you to do something for me.

Explain to me what Voltage, Current, and Resistance are. In whatever words you want to use, it doesn't have to be exact. I just want to get a feel for what you already know.

in my own words...
Voltage - from what I've learned in e&m, it is the magnitude of an EMF.
Current - the rate at which voltage is transported
Resistance (in terms of circuits/electricity) - something that is against the flow of current

 

[Drakkith]

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

If you measured the voltage in your conductor right before it connects to the terminal of your battery you would read nearly 0 volts. This is because the wire itself has very little resistance and you have almost no loss in voltage (voltage drop) across it. When you measure the voltage of something in a circuit you are measuring the voltage drop across it. In a simple series circuit the voltage drop of all components must add up to the applied voltage.

in my own words...
Voltage - from what I've learned in e&m, it is the magnitude of an EMF.
Current - the rate at which voltage is transported
Resistance (in terms of circuits/electricity) - something that is against the flow of current

Hmm. Ok. I could spend an hour or so typing up what those three things mean, but your best bet is to grab a basic electronics book and study up. A few things though.

Voltage is a difference in electric potential between two points. Electrical potential exists any time you have an imbalance of charges between two points, or when you have a conductor in a changing magnetic field.
Current is the flow of electrical charges past a point. It is not the flow of voltage. Voltage does not flow.

 

[sophiecentaur]

in my own words...
Voltage - from what I've learned in e&m, it is the magnitude of an EMF.
Current - the rate at which voltage is transported
Resistance (in terms of circuits/electricity) - something that is against the flow of current

Those definitions are all pretty flawed, I'm afraid.
This may sound very pernickety but it is really important to get the definitions of these terms right in your head from the start. "Near enough ain't good enough' when it comes to these things or you are starting off on the wrong foot. Don't go by what you read on a Forum, which has been written by someone non-academic who wants to make it 'friendly' and accessible. There are a mixture of definitions. even on this thread and some of them are not 'quite right' and some of them are dodgy personal interpretations. If you aren't in a position to distinguish (which you aren't, because you are asking about them) find the right way of saying and writing things by looking at a reputable site. I always recommend the Hyperphysics site, which you should be able to rely on. (Or a decent textbook, of course)

Do not take offense at this - just think of what an eejit someone (an aging adult, for instance) can sound when they try to talk about the things in young persons' lives. It's cringeworthy - and so are the things some people say about Electricity.

 

[Drakkith]

Sophie's right. It is extremely important that you understand the proper meaning of those terms. Otherwise most of electronics just doesn't make much sense if you try to think about it.

 

[sophiecentaur]

Sophie's right. It is extremely important that you understand the proper meaning of those terms. Otherwise most of electronics just doesn't make much sense if you try to think about it.

Sophie is a grumpy old git but he is right!

 

[Ratch]

cpatel23,

When you said energy capacity, do you mean Watts or amp hour?

Energy is measured in joules, power in watts, and charge in amp-hrs. Notice those units are not capitalized.

Since Watts have been brought up, let's say I have something that requires a minimum amount of Watts, but I only supply 70% of the minimum. Would my appliance work

Ask your appliance if it is happy to run at reduced power.

What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.

That is a misleading description. A voltage needs two points to be defined. So if the voltage increases across a capacitor, what does passing voltage mean? Current never goes through an ideal capacitor. The charge can accumulate on one plate and deplete on the opposite plate for a transient period of time, thereby allowing a current in the branch where the cap is located. Anytime the voltage changes across the capacitor, a current will exist for a transient time until the shift of charge between the plates of the cap is complete.

When I let the capacitor fully charge it comes out to ~7.2V (Battery is at 8.7V).

Remember what I said in post #5. Caps don't charge, they energize. The cap has the same net charge at 7.2 volts that it had at 0 volts. It does contains more energy, however, at 7.2 volts.

Now when I remove the resistors and leave the LED and capacitor in series, the voltage on the capacitor will read 7.2V.

Why doesn't the voltage supplied to the capacitor reduce when I include the resistors?

I would like to see a schemat, not a dimly lit breadboard.

So if I had a circuit with a battery giving 20V and current flows through a few resistors. If I measure the voltage right before the current re-enters the battery, the voltage would be less then 20V, correct?

Charge flows, not current. Again, a schemat will work wonders.

Voltage - from what I've learned in e&m, it is the magnitude of an EMF.

What is EMF? What does it mean?

Current - the rate at which voltage is transported

Wrong.

Resistance (in terms of circuits/electricity) - something that is against the flow of current

Other things inhibit current, but they are not resistive.

sophiecentaur,

Those definitions are all pretty flawed, I'm afraid.

That is for sure.

Ratch

 

[jim hardy]

Echo previous sentiments. Even though i am guilty of making some assertions a few posts back... i hope they painted a reasonable mental picture, but I'm not a trained educator as are some others here.

You need to go into electronics with a clear mental picture of those fundamental terms
and that comes from working problems.

It's confusing at first because everything is named after dead scientists instead of a word that takes your mind straight to the concept.

And the water analogies, while they can be useful, can easily mislead you
because water flows easily through air which charge does not
and water can be pumped copiously out of the ground
which leads to a mistaken concept of "ground" in circuits.

The basic units are probably well defined in your physics book...

i was hoping to stimulate your curiosity further, as i said

Good luck in your electronics course. Become fluent in laws of Kirchoff and Ohm.

old jim

 

[jim hardy]

Here's an illustrated explanation of what Ratch was saying about capacitors, from Sophie's suggested site:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html

a picture is worth a thousand words.

 

[sophiecentaur]

cpatel23,


Remember what I said in post #5. Caps don't charge, they energize. The cap has the same net charge at 7.2 volts that it had at 0 volts. It does contains more energy, however, at 7.2 volts.



Ratch

I agree with the rest of your post. The above is a matter of your personal preference. The Charge that is 'in' a capacitor can be made to flow around a circuit - the same as the Ahr that are 'in' battery. When the term "Battery Charger is officially replaced with "Battery Energiser" on every Charger you can buy in an Autospares Shop, I shall start to think your way.

Your obsession with this issue is spoiling the credibility of your otherwise good ideas, I feel. You will not change the World on this. Useage is a very powerful badge of authority in language.

 

[Ratch]

sophiecentaur,

The Charge that is 'in' a capacitor can be made to flow around a circuit - the same as the Ahr that are 'in' battery.

Yes, and the amount of charge expelled from the capacitor equals the amount of charge taken in, for a net change of zero. The amp-hour rating of a battery determines how much charge can be pumped around the circuit, not how much charge is in a battery.

Your obsession with this issue is spoiling the credibility of your otherwise good ideas, I feel.

My obsession with the correct description in no way spoils the credibility of my statements. How can it?

Ratch

 

[jim hardy]

Perhaps you're coming from the world of electrostatics where net charge has significance.

But in electric circuits(electrodynamics) the phrase is "Charge a capacitor " .
http://micro.magnet.fsu.edu/electromag/electricity/capacitance.html

My 1901 Electric Machinery book describes "charging a condenser" not energizing it.

My high school electronics textbooks used "charge" as did my Freshman physics text, Sears and Zemansky.

10.Ben Franklin's "single fluid theory" showed that a given body possessing a normal amount of electric fluid was called neutral. During the process of charging, the fluid was transferred from one body to the other; the body with the deficiency being charged minus and the body with the excess charged plus . But no fluid is lost.
Ben's "single fluid theory" led to the electron theory in 1900: electrons move about conductors much as a fluid might move. Nobel Prize winner and physicist, Robert A. Millikan, called Ben's experiment that led to this theory "probably the most fundamental thing ever done in the field of electricity".


11.Ben Franklin had to invent electricity terminology as he went along in his experiments. A scholar who traced Ben's vocabulary found at least twenty-five electrical terms which he was the first to use: examples -- armature, battery, brush, charged, condense, conductor, plus and minus, positively and negatively .

http://www.franklinbusybody.com/facts.asp

Ben also redesigned the Leyden jar as today's flat plate capacitor.

You're trying to rewrite history.

 

[sophiecentaur]

sophiecentaur,



Yes, and the amount of charge expelled from the capacitor equals the amount of charge taken in, for a net change of zero. The amp-hour rating of a battery determines how much charge can be pumped around the circuit, not how much charge is in a battery.



My obsession with the correct description in no way spoils the credibility of my statements. How can it?

Ratch

You have managed to conflate two things here. Whether we talk of Energy of Charge is entirely a matter of which suits the particular circumstance more. For instance, the energy in two capacitors doesn't give you any clue as to which direction the power will flow if they are connected. PD would be appropriate there. If you were designing a filter then charge / current are the relevant quantities. If you are making an electronic flash gun, it's the Energy stored that counts. Your choice of description would be totally acceptable in some circumstances but not all.

The issue of 'no charge on a capacitor' is a separate thing altogether. If you would accept that one 'charges' the ball on a Van Der Graaff generator then that must hold for any other form of capacitor. There are no extra charges available on the Earth when you 'charge a sphere' and there are none when you charge a 1μF capacitor. When one lead of a capacitor is connected to Earth, the situation is exactly the same. As Jim says, you are trying to re-write history. You have as much chance of doing that as changing the sign of charge on electrons so that the 'current direction' thing doesn't confuse students.

 

[Runei]

In regards to the battery versus capacitor question, another extremely important point is that the if the capacitor is used to power some kind of network, the voltage it supplies will drop as a decaying exponential. So the voltage will drop rather quickly.

A battery, however, is designed to deliver an almost constant voltage, even though it is half empty or fully charged. This property of the battery makes it superior to a capacitor for powering circuits, as the voltage in most applications needs to be constant. A capacitor cannot do this.

Capacitors are very important in circuits.

The three main uses of capacitors, I would say, are:

1. Power supply stabilization - You use them to remove noise on the powersupply and protect against voltage spikes.

2. Filters - Highpass, Bandpass, Lowpass, Bandstop. All these filters are used in analog signal processing in communications systems to look at the frequencies containing all the information.

3. Coupling capacitors - In amplifiers, the capacitors are used to block a DC signal and only allow the AC signal to pass. The bigger the capacitor, the lower frequencies can be passed without distortion.

 

[mheslep]

wow. Is there a reason why all the capacitor tanks have low voltage (sub 9V)?

The material & methods used for making the new super caps have a fairly low voltage breakdown, so far. The manufacturers recognize this so many of them offer pre-assembled higher voltage series capacitor arrays and associated circuitry.

http://www.tecategroup.com/store/index.php?main_page=product_info&products_id=1298&gclid=CK6P24fTsLQCFQyk4AodLAUAsw

 

[Ratch]

jim hardy,

Perhaps you're coming from the world of electrostatics where net charge has significance.

Physics is the same everywhere.

But in electric circuits(electrodynamics) the phrase is "Charge a capacitor " .
http://micro.magnet.fsu.edu/electrom...pacitance.html

The first two sentences of that line say, "Capacitance is the property of an electric conductor that characterizes its ability to store an electric charge. An electronic device called a capacitor is designed to provide capacitance in an electric circuit by providing a means for storing energy in an electric field between two conducting bodies."

First it says capacitance is the ability to store charge, and then it says it is the ability to store energy. Well, it certainly stores energy, but a cap does not store charge for the reason I already gave many times. Shifting electrons from one place to another is not "charging".

My 1901 Electric Machinery book describes "charging a condenser" not energizing it.

My high school electronics textbooks used "charge" as did my Freshman physics text, Sears and Zemansky.

Lots of textual material and people call energizing a capacitor as charging it. A consensus of errors does not make it correct.

http://www.franklinbusybody.com/facts.asp

Ben also redesigned the Leyden jar as today's flat plate capacitor.

Bully for Ben. I don't deny that he was a smart, clever, and inventive man. But his fluid theory of electron motion leaves much to be desired. Fluids are not easily compressible, whereas electrons can be crowded together. Fluids have pressure, whereas electrons do not. Fluids do not have a polarity, whereas electrons do. Fluids are governed by hydraulic principles, whereas electrons are governed by quantum principles. Would you call a battery's voltage a pressure?

You're trying to rewrite history.

Nonsense, correcting a falsehood is not rewriting history.

Ratch

 

[jim hardy]

Would you call a battery's voltage a pressure?

If you read British textbooks as recent as 1960's you'll see they use the term "Pressure" instead of voltage, so in some circles yes. Tubes were called valves over there too.

Can you provide a scholarly reference for your assertion?

Go ahead, the last word is yours.

Adieu to you and Rocinante .

 

[OmCheeto]

... Fluids are not easily compressible...

You can blame this correction on Russ Waters if you like.

In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.

I subscribed to this thread to add my infinite knowledge of capacitors. But seeing the high quality of knowledgeable posts*, I deemed my input unnecessary.

---------------------------------
*you know who you are. ;)

 

[Ratch]

sophiecentaur,

You have managed to conflate two things here. Whether we talk of Energy of Charge is entirely a matter of which suits the particular circumstance more. For instance, the energy in two capacitors doesn't give you any clue as to which direction the power will flow if they are connected. PD would be appropriate there. If you were designing a filter then charge / current are the relevant quantities. If you are making an electronic flash gun, it's the Energy stored that counts. Your choice of description would be totally acceptable in some circumstances but not all.

I don't understand what you mean. If you know the energy of a cap and its capacitance, then you know its voltage or the charge difference between its plates. What more do you need?

The issue of 'no charge on a capacitor' is a separate thing altogether. If you would accept that one 'charges' the ball on a Van Der Graaff generator then that must hold for any other form of capacitor. There are no extra charges available on the Earth when you 'charge a sphere' and there are none when you charge a 1μF capacitor. When one lead of a capacitor is connected to Earth, the situation is exactly the same. As Jim says, you are trying to re-write history. You have as much chance of doing that as changing the sign of charge on electrons so that the 'current direction' thing doesn't confuse students.

A Van De Graff is not a capacitor. It is a high voltage generator which separates charge by induction rather than the conduction method of a battery and capacitor. You can talk all you want about providing a conduction path between the separated charges of the ball and ground, but charges are not created or destroyed.

I am not trying to rewrite history, I am trying to correct misleading and false jargon. If everyone decided to call a electron positive, then ions and holes would have the "wrong" charge.

Ratch

 

[Ratch]

jim hardy,

If you read British textbooks as recent as 1960's you'll see they use the term "Pressure" instead of voltage, so in some circles yes. Tubes were called valves over there too.

Yes, I have also come across the words "tension" and "EMF", whatever that means. I don't care how scholarly the article is, if they equate pascals to volts, then they need to be corrected. I think valves are a better description than tubes. Valves describe the function, whereas tubes describe the shape of the electrical component.

Can you provide a scholarly reference for your assertion?

I know of no scholarly article on common sense.

Ratch

 

[Drakkith]

Capacitors may or may not store charge depending on what you mean by "storing charge". It just depends on your point of view. Seeing as how each plate has a buildup of one type of charge I personally see no problem saying they do in fact store charge.

 

[sophiecentaur]

sophiecentaur,



I don't understand what you mean. If you know the energy of a cap and its capacitance, then you know its voltage or the charge difference between its plates. What more do you need?



A Van De Graff is not a capacitor. It is a high voltage generator which separates charge by induction rather than the conduction method of a battery and capacitor. You can talk all you want about providing a conduction path between the separated charges of the ball and ground, but charges are not created or destroyed.

I am not trying to rewrite history, I am trying to correct misleading and false jargon. If everyone decided to call a electron positive, then ions and holes would have the "wrong" charge.

Ratch

Q=CV
and E=CV²/2
Are written in a totally arbitrary form and can be re-arranged in any way you choose. They say nothing about the relative significances of the quantities involved. That is just in your head.

The reason that a VDG generator works is that the sphere has Capacitance wrt Earth and a charge builds up on it. No sphere, no build up of charge. And where does this charge come from? By displacing an equal and opposite amount of charge to earth. You could charge a paper capacitor in a similar way, with a rolling belt but the voltage you could achieve would be much less - that's the only difference. You clearly don't want to accept anything that goes against your argument about no charge being stored but, as Jim says, there are very very few instances where a charge is built up on something without a similar opposite charge ending up somewhere else. Can you think of one? An Ion Drive, perhaps?

I don;t think you can know what "conflating" means - I mean that the Charge on a capacitor and the Charging vs Energising argument are totally different issues and you are treating them as one.

Just reiterating that you are somehow right about the Energise thing gets you nowhere. The only references you have given have been non-learned web pages which have been associated, mainly with Power Engineering situations.
You clearly didn't catch on to what I was getting at but every student I have ever talked to would have understood immediately what I am referring to and I think all other readers would, too.

I know of no scholarly article on common sense.

That's no argument at all. Science is not "common sense". Much more is needed than common sense. Common sense tells us that things fall to the ground (no Science there). It tells us that "Nature abhors a vacuum" (common 'experience' but no explanation of what's happening).

If Einstein had ended his paper on Special Relativity with the argument that it's "common sense", people would have laughted because that is the last thing it is. If you don't want to relate what you write to scholarly articles then you are wasting your time on PF. Read the guidelines. It is one of the strong points of PF that we don't hold forth about unsupported theories.

 

[Ratch]

Drakkith,

Capacitors may or may not store charge depending on what you mean by "storing charge". It just depends on your point of view.

I explained many times that capacitors do not store a net charge. They only shift the charge from one plate to another. That shift of charge either takes or gives energy. Shifting a charge is not "charging", it is energizing.

Seeing as how each plate has a buildup of one type of charge I personally see no problem saying they do in fact store charge.

The opposite plate has a depletion of charge, so it could just as well be called "discharging". Therefore, the description is vague and ambiguous. Only energizing describes exactly what is going on.

Ratch

 

[Drakkith]

Drakkith,
I explained many times that capacitors do not store a net charge. They only shift the charge from one plate to another. That shift of charge either takes or gives energy. Shifting a charge is not "charging", it is energizing.

I disagree. Each plate has charged being moved onto/off of it. AKA charging.

The opposite plate has a depletion of charge, so it could just as well be called "discharging". Therefore, the description is vague and ambiguous. Only energizing describes exactly what is going on.

Ratch

Net charge builds up on each plate through the movement of charges. I'd say "charging" is fine. That's the final thing I'll say on it, as it's not on topic and it's all personal opinion anyways.

 

[Ratch]

sophiecentaur,

Q=CV
and E=CV2/2
Are written in a totally arbitrary form and can be re-arranged in any way you choose. They say nothing about the relative significances of the quantities involved. That is just in your head.

Those formulas say everything about the quantitative significances of the quantities involved.

The reason that a VDG generator works is that the sphere has Capacitance wrt Earth and a charge builds up on it. No sphere, no build up of charge. And where does this charge come from? By displacing an equal and opposite amount of charge to earth. You could charge a paper capacitor in a similar way, with a rolling belt but the voltage you could achieve would be much less - that's the only difference. You clearly don't want to accept anything that goes against your argument about no charge being stored but, as Jim says, there are very very few instances where a charge is built up on something without a similar opposite charge ending up somewhere else. Can you think of one? An Ion Drive, perhaps?

I keep repeating that I said the net charge does not change. If a charge builds up in one place and an opposite charge builds up in another place, then the net charge is zero, isn't it?

I don;t think you can know what "conflating" means - I mean that the Charge on a capacitor and the Charging vs Energising argument are totally different issues and you are treating them as one.

Yes, I do know what conflate means. If I did not, I could look it up in a dictionary. They are not totally different. The first above is what the charge separation is, and the second is how it got that way. So what is the point?

Just reiterating that you are somehow right about the Energise thing gets you nowhere. The only references you have given have been non-learned web pages which have been associated, mainly with Power Engineering situations.
You clearly didn't catch on to what I was getting at but every student I have ever talked to would have understood immediately what I am referring to and I think all other readers would, too.

I never said that describing a capacitor as being energized doesn't get you anywhere. Energizing is correct no matter what the application of a capacitor is.

That's no argument at all.

It is not an argument, it is a statement of fact.

Science is not "common sense". Much more is needed than common sense.

No one said common sense was everything in science.

Common sense tells us that things fall to the ground (no Science there). It tells us that "Nature abhors a vacuum" (common 'experience' but no explanation of what's happening).

Common sense does not tell us those two things. Those are observable phenomena. Common sense is how one arranges and relates facts into a coherent principle.

If Einstein had ended his paper on Special Relativity with the argument that it's "common sense", people would have laughted because that is the last thing it is. If you don't want to relate what you write to scholarly articles then you are wasting your time on PF. Read the guidelines. It is one of the strong points of PF that we don't hold forth about unsupported theories.

Not unless Einstein showed how common sense applied to his theories, which he did mathematically and descriptively. But I did not propose a theory, I proposed a change in nomenclature, and explained why it was better. So far, no one has shown it to be wrong. I only see vague statements from detractors like "everyone says it the old <wrong> way", or "that is what they said <wrongly> in the beginning".