Electrical argument

the fact is that you can connect capacitor in parallel
( spelling ??) or in series , when you connect2 or more capacitors in parallel they are forced to the same voltage, hence the coloumb charge is the same beetween all of them
qt=q1=q2..... when you connect 2 or more capacitor in series
the positive side forces the opposive side of the capacitor to be opposit , hence the charge is the sum
so for parallel capacitors ''c=c1+c2+c3
for series is the reciprocal of the sum of reciprocal
c=c1*c2/(c1+c2)

so you note that capacitors are ''dual'' of resistors
inductave bheaves just like resistors when not mutually coupled in ac ( alternate current )

quote:

---

Original post by v71
when you connect2 or more capacitors in parallel they are forced to the same voltage, hence the coloumb charge is the same beetween all of them

---

not if they have different capacitances, which they probably do

********


A Problem Worthy of Attack
Proves It''s Worth by Fighting Back

What was the exact proof? I don''t see how it can mathematically make sense, either. If in series, then the voltage across any of the capacitors is going to be less than that of the entire circuit (voltage of battery), by definition. Whether or not all the capacitors have the same capacitance, because voltage varies also, then it''s impossible for them each to have the same charge as the circuit. From a purely physical point of view, a certain coulomb charge also implies a certain quanity of electrons. That''s like saying the number of electrons in one of the capacitors is the same as the number of electrons in the entire circuit, which doesn''t make sense.

I have a feeling that we''re misinterpreting the proof.

That''s exactly the point I made to the teacher. He said he didn''t understand either but just "accept it for now"

********


A Problem Worthy of Attack
Proves It''s Worth by Fighting Back

What''s even more frusterating is that you know we''re not seeing something, or looking at something wrong, because there''s no way a textbook proof is going to be wrong Or at least a very, very small chance!

If the capacitors are in series then the section of circuit between capacitors is isolated from the rest of the circuit. Therefore you can''t get charge on to or out of that section. You can only redistribute the charge on that section. One side will be +q the other must be -q. Equaly the sections on the other side of the capacitors have the same charge. If not then a current must flow and the charge will be redistibuted untill it is the case. Therefore, the charge on each and every capacitor must be equal. At least that is how I remember it

I understand the charge sharing between capacitors bit, but the proof said the SUM of these charges equals any one of these charges.

BTW Zipster, my electronics textbook was riddled with mistakes, arrangements of logic gates which could never change states and things like that. I don''t have much faith in textbooks. The physics ones often made claims I knew to be false for the sake of simplicity, hours have been wasted arguing. I spent a whole lesson pointing out Newton''s law of gravity couldn''t possibly be right, the teacher eventually conceeded it was an approximation- but we would never have been told!

********


A Problem Worthy of Attack
Proves It''s Worth by Fighting Back

quote:

---

Original post by walkingcarcass
but we would never have been told!

---

Pedagogically, ignorance is bliss.

And, yes, what was your argument?

Cédric

my electronics textbook was riddled with mistakes, arrangements of logic gates which could never change states and things like that.

Get a copy of Linear and Non linear circuits , by L.Chua and others , and can you explain please why do you think that newton''s laws are ''incorrect'' ????

My argument was that the F=GMm/r^2 doesn't consider the density of the objects, which I KNOW makes a difference because of black holes.

[edit] Turns out I was right, Einstein developed a more complete theory but it's horribly complicated [/edit]

Anyway, about those capacitors....

********


A Problem Worthy of Attack
Proves It's Worth by Fighting Back

[edited by - walkingcarcass on June 2, 2003 12:57:02 PM]

No, F=Gmm/r^2 does not take density into account. It is true only for point particles and spherically symmetrical mass distributions, and hopefully your physics book doesn''t say otherwise.

I kept telling myself that I wasnt going to respond to this but the scientist in me wont let it go. In reference to the newtons law of gravity being wrong because it doesnt take into account the density consider this:
Density is mass/volume
Newtons formula is F=G*(m1m2)/r^2

If you rearrange the density formula you have
mass=density*volume

plug this into newtons formula and you have
F=G*(d1v1*d2v2)/r^2

Tatda, it now takes into account the density of the objects although it doesnt matter since it will give you the same answer.

*EDIT* Forgot to square the radius

"I may not agree with what you say but I will defend to the death your right to say it."
--Voltaire

[edited by - cmptrgear on June 2, 2003 1:06:08 PM]

But even though Newton''s law of gravitation is valid for objects that have volume (e.g., they are not point masses), it does not account for things like tidal forces, which are the results of a variation in the gravitational forces throughout the volume of the objects. It also does not really account for self-gravitation, which prevents objects from being torn apart by the gravitational forces of different neighboring objects. It purely represents the forces acting through the center-of-mass.

Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

It doesn't account for the movement of rigid bodies, but it works for particles. And a "rigid" body is made of particles, so Newton's formula isn't an approximation if you stick to what it was designed for (at least, in this respect).

A much more valid concern is that every force is modified by relativity (sadly, I don't remember how exactly, but I could revise my notes...), so as soon as you start moving, Newton's equation becomes inexact, even for point masses.

(well, the gravity equation can be kept IIRC, but F=ma is changed, so it amounts to the same)

Every problem you will ever solve in physics will have some approximation in it. As my teachers say, physics is the exact science of approximations.

Cédric

[edited by - cedricl on June 2, 2003 5:50:51 PM]

As Cédric implied,

The linear acceleration of true rigid bodies (which don''t exist in nature) is represented as accurately as point masses using Newton''s law of gravitation.

quote:

---

Original post by cedricl
EveryAs my teachers say, physics is the exact science of approximations.

---

I like that!


Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

Imagine a vast uniform low density sphere. Consider the gravitational field at a point on the surface, it is the sum of the fields of all the particles making up the body.

Some particles will be far from the centre, not only are they further away, but only a small component of the force field acts *in the direction of the centre of mass*.

If you squashed the body inwards horizontally, the particles at the edges would be closer and apply a larger force. Likewise if the body was squashed vertically. If it was squshed in the near/far axis, the nearest particles would be further and weaker but also the furthest particles would be nearer (although they wouldn''t quite compensate). If you think about it, the net gravitational field at the same point (now some distance from the surface) would be stronger.

Newton''s law isn''t subtle enough to model this effect because G isn''t known to a great deal of accuracy.

********


A Problem Worthy of Attack
Proves It''s Worth by Fighting Back

just adding on to previous replys

I have to brush up on my physics but here''s my 2cents:

You have to remember that phyics applies mathmatical "MODELS" to real world phenomena. i.e., there is no controlling mechanism in the universe that says you are an object on earth, you fall towards the center 9.8m/s/s. It is the sum affect of the total mass of the earth and the object applying gravitational forces on each other. We(humans) have just figured out that it that we can predict the motion of the object using our mathmatical model. Since the model gives good results and it can be reproduced over and over again, we call it a scientific law. Yet it is still just a MODEL of the way the universe works.

Now comes one of the greatest scientific quams of this century. The models that work in the macro world conflict with the models that work in the micro world. And the scientific community has been working toward a model that works in both realms.



Just a side note:
Gravity is an interesting phenomenon. It appears to the observer that it cause a force on other objects. Yet it appears to act instintaniously. So, if the universe is contrained by the speed of light, how can vastly distant objects instintaniously affect each other. I know Einstien and others have provided some explainations for this, but its been a while so Im a little rusty

quote:

---

In Einsteins theory, gravity doesn''t act instantaneously, but progress at the speed of light.

---

Righto. I remember now. It sort of ripples out, affecting the fabric of space/time.