   04-11-2015, 08:10 PM   #21
NN5I
Carl, nn5i

Join Date: Sep 2011
Location: Tallahassee, FL
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 Originally Posted by wa8yxm (*!^@#(*&#@([email protected] B (unprintable comment) Thanks for the correction... That was what I meant to type but I messed up.
I blew it too. My arithmetic was wrong when I was fixing your arithmetic. I've edited the correction, and now I think I've got it right.
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-- Carl   04-11-2015, 08:27 PM   #22
NN5I
Carl, nn5i

Join Date: Sep 2011
Location: Tallahassee, FL
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 Originally Posted by W7JZE Any homework tonight, Teach?
Yup. Memorize the following:

A watt is a joule per second.

A volt is a joule per coulomb.

A volt is a coulomb per farad. Thus a farad is a coulomb per volt.

An ampere is a coulomb per second.

A henry is a volt per (ampere per second), i.e. a volt-second per ampere.

An ohm is a volt per ampere. Thus a mho is an ampere per volt.

Manipulate these to show that:

An ampere per second is a volt per henry. Understand this to mean that, to change the current through an inductor by one ampere per second (increasing or decreasing), you must impose one volt across the inductor for each henry of its inductance.

A watt is a volt-ampere.

... and so forth.

Now then, how many joules are stored in a two-farad capacitor charged to three volts?
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-- Carl   04-11-2015, 10:41 PM   #23
W7JZE
RV in 2016

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 Originally Posted by NN5I ... ... A watt is a volt-ampere. A joule is a farad-volt-squared. ... and so forth. Now then, how many joules are stored in a two-farad capacitor charged to three volts?
Ouch! Ouch! Just thinking about that makes my...

(I will try to work it out, though)

HMmmm... "A joule is a farad-volt squared"

1 farad * 1 volt = 1 farad-volt and 1^2 = 1 Joule. OK. Then, maybe...

2 farads * 3 volts = 6 farad-volts and 6^2 = 36 Joules.

36 joules? Maybe?
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73,

Bill - W7JZE
"Dar" - XYL
(And our little Bichon Frisé "Sparkle")    04-11-2015, 11:49 PM #24 NN5I Carl, nn5i   Join Date: Sep 2011 Location: Tallahassee, FL Posts: 1,441 Actually I threw you a curve there, and also erred when I wrote, A joule is a farad-volt squared. It ain't. A farad (volt squared) is half a joule. Only the volts get squared, and I omitted the 1/2. To derive that from the simple equations requires the integral calculus. Now then, how many joules are stored in a two-farad capacitor charged to three volts? Most people would use the calculus to solve this. Let's see if we can do it another way. Let's charge those two farads [C] with a constant two-ampere current. The voltage e will go up at one v/s. At start, the capacitor is at zero volts, and the current is two amperes, so the power going in is zero. But the power starts rising immediately, because the voltage [e] starts rising immediately and the current is constant. After one second, it's charged to one volt. Our two amperes are now flowing at one volt, so the power going in at that instant is two watts. After two seconds, it's at two volts, and still two amperes, so the power going in is four watts. After three seconds, it's at three volts, and still two amperes, so the power going in is six watts. Since the voltage went up linearly and the current was constant, the power went up linearly too, from zero to six watts. So the average power was three watts over the three seconds. The energy, then, must be nine joules. Does this consist with the answer from calculus? Yes. The result from integral calculus would be that the energy = (1/2)Ce^2, using your excellent notation. (I wish I could use superscripts here.) C = 2. e = 3. e^2 = 9. Thus (1/2)Ce^2 is nine joules. We're good to go. We did it at constant current, but actually the energy in that capacitor doesn't depend on how it got there. It depends only on the capacitance and on the voltage across it. At two farads and three volts, it's nine joules, regardless. The capacitor doesn't "remember" the history, it only remembers the end result. __________________ -- Carl   Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules » Recent Threads