Electronics and etc blog

[Ian Macmillan]

Have a look at my blog - there might be something of interest there:

merlins-mutterings.blogspot.com

All the best

Ian Macmillan

 

[Phil Allison]

"Ian Macmillan"

Have a look at my blog - there might be something of interest there:

merlins-mutterings.blogspot.com

** Oh dear - your very first item has a serious math error.

The formula for charge Q on a capacitor is simply:

Q = CV

The formula for the energy E stored in a capacitor is:

E = 0.5CV*2

In the case where a charged cap is connected in parallel with an uncharged
one of the same rating - the resulting voltage of the combination is found
by considering the law of " conservation of charge " - that is the CV of
the combination is the SAME as the CV of the first cap.

So, the combination will have exactly half the V across it - seeing as C
has now doubled.

The mystery in the puzzle is explaining what happened to the missing ENERGY
!!


...... Phil

 

[Ian Macmillan]

"Ian Macmillan"


** Oh dear - your very first item has a serious math error.

The formula for charge Q on a capacitor is simply:

Q = CV

The formula for the energy E stored in a capacitor is:

E = 0.5CV*2

In the case where a charged cap is connected in parallel with an uncharged
one of the same rating - the resulting voltage of the combination is found
by considering the law of " conservation of charge " - that is the CV of
the combination is the SAME as the CV of the first cap.

So, the combination will have exactly half the V across it - seeing as C
has now doubled.

The mystery in the puzzle is explaining what happened to the missing ENERGY
!!


..... Phil

Silly me, confusing symbols mainly. But I did point out that the mystery
energy loss was in the discharge path, which was the point of the article.
Thanks for your response.

Regards

Ian Macmillan

 

[Phil Allison]

"Ian Macmillan" >

"Phil Allison"
ENERGY !!


Silly me, confusing symbols mainly. But I did point out that the mystery
energy loss was in the discharge path,

** Where is that pointed out ?

What sort of losses are you alleging go on ?

which was the point of the article.

** Fraid it does not have much of one.

Be instructive to connect a scope across the uncharged cap and observe what
really happens when the charged one is first connected. Try a couple of 1uF
film caps.


..... Phil

 

[Phil Allison]

"Phil Allison"

"Ian Macmillan"

** Dear me - I must have upset the man.

He's pulled the item ...



... Phil

 

[Ian Macmillan]

"Phil Allison"


** Dear me - I must have upset the man.

He's pulled the item ...



.. Phil

Well, it obviously needs revision.

Ian

 

[Ian Macmillan]

"Ian Macmillan" >

** Where is that pointed out ?

What sort of losses are you alleging go on ?


** Fraid it does not have much of one.

Be instructive to connect a scope across the uncharged cap and observe what
really happens when the charged one is first connected. Try a couple of 1uF
film caps.


.... Phil

What really happens depends on the resistance of the path between the
charged and uncharged capacitors. Initially when connected the uncharged
capacitor is at 0 volts, so the current that initially flows is E/R,
asymtoting to zero when the capacitor voltages match. With your 1uF film
caps, there could be an oscillation, particularly if you just shorted them
together. Its easier to calculate using a current source, but obviously the
energy lost is dissipated in the resistor etc. If you just short the
capacitors together, the resistance is in the connection or internal in the
caps - its low and the current is high but it works out the same. I am
unhappy with my piece which was badly done, and I'll re-do it in due course.
As to the point of it, obviously none for you, but could be relevant for
others. By the way, your response also has a math error.

Regards Ian

 

[Phil Allison]

"Ian Macmillan"

"Phil Allison"

What really happens depends on the resistance of the path between the
charged and uncharged capacitors. Initially when connected the uncharged
capacitor is at 0 volts, so the current that initially flows is E/R,
asymtoting to zero when the capacitor voltages match. With your 1uF film
caps, there could be an oscillation, particularly if you just shorted them
together.

** That is the scenario posed in text book examples of the question.

Often the poor student is also asked to assume the capacitors are ideal.

This really throws 'em.

If you just short the
capacitors together, the resistance is in the connection or internal in
the
caps - its low and the current is high but it works out the same.

** In practical caps, the high frequency ESR and Q values are crucial to the
outcome.

Plus the series inductance of the parallel assembly.

I am
unhappy with my piece which was badly done, and I'll re-do it in due
course.

** Please don't lose any sleep over it .

Be assured, others have.....

As to the point of it, obviously none for you, but could be relevant for
others.

By the way, your response also has a math error.

** Did I use a " * " where I should have used a " ^ " ?

Where is the usenet etiquette on this convention published ?

I normally avoid any chance of mis-interpretations takes and just write the
word "squared".



..... Phil

 

[Phil Allison]

"Dennis the Pedant "

** Oh dear Phil, your 2nd formula has a serious math error;

E = 0.5CV^2 NOT E = 0.5CV*2

** That's a typo.

The math is just fine.



..... Phil

 

[fritz]

Good on you Ian. Without guys like yourself willing to stick some interesting stuff up on the net, the net would not be what it is
today.

A lot if 'interesting stuff' is incorrect, and that needs to
be pointed out, too.