# Capacitors, why even bother?

by cpatel23
Tags: bother, capacitors
P: 15
 Quote by 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/ind...g1ctjuteom0ma0
wow. Is there a reason why all the capacitor tanks have low voltage (sub 9V)?

 Quote by yungman 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?

 Quote by jim hardy 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?

 Quote by nsaspook
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.
P: 581
 Quote by cpatel23 ... Sorry for all the questions, had to many, what seemed to be, close calls yesterday.
Before you hurt yourself, start here = Introduction to Electronics
P: 386
 Quote by cpatel23 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.
 P: 15 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?
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P: 10,391
 Quote by cpatel23 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.
P: 15
 Quote by Drakkith 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?
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P: 10,391
 Quote by cpatel23 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.
 P: 482 Someone has recently been developing a battery/capacator hybrid called an ultracapacitor http://en.wikipedia.org/wiki/Electri...ayer_capacitor http://www.technologyreview.com/news...acitor-hybrid/
P: 15
 Quote by Drakkith 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
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P: 10,391
 Quote by cpatel23 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.

 Quote by cpatel23 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.
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 Quote by cpatel23 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 text book, 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.
 PF Patron P: 10,391 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.
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 Quote by 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.
Sophie is a grumpy old git but he is right!
P: 315
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, lets 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
 PF Patron Sci Advisor P: 2,917 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
 PF Patron Sci Advisor P: 2,917 Here's an illustrated explanation of what Ratch was saying about capacitors, from Sophie's suggested site: http://hyperphysics.phy-astr.gsu.edu...ic/capchg.html a picture is worth a thousand words.
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 Quote by Ratch 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.
P: 315
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

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