Quick Physics Question...

[Cobra]

Doing a bit of revision at the moment, and im struggling with this forumla as i never did maths for A-Level...

What do you get as the answer to this formula:

V = 9 e ^ (-1 / 220000 x (470 x 10^-3)

(above = this formula : V = Voe ^ -t/RC

The book answer is 6.3v but i can't seem to get that!

Help very much needed, exam on Tues & studying for this one alone so no one to ask!

[expat]

It keeps working out at 42 for me

Matt

[Ashar]

It keeps working out at 42 for me

Matt

Hahaha

[EGNX]

Ha!... I was doing this at school today!... But for charge rather than voltage.

I'm trying it and I don't get 6.3 Volts!

[Cobra]

Ha!... I was doing this at school today!... But for charge rather than voltage.

I'm trying it and I don't get 6.3 Volts!

Ah good man. Yeh, the formula is interchangable for V/Q/I. Im thinking the answer must be wrong! What do you get? I keep getting 1.03 if i remember correctly!

[Omag 2.0]

Man, that takes me back... I used to be able to solve that kind of equations. The e-function bothers me here... I know that it's a logarithmic function, but I can't recall if e has a value...

[Chris_F]

Check your units.  Double check your units.  Tripple check your units.

Man, that takes me back... I used to be able to solve that kind of equations. The e-function bothers me here... I know that it's a logarithmic function, but I can't recall if e has a value...

e is approximately 2.818 IIRC.

[FsNovice]

wikipedia'ed it

e = 2.71828 18284 59045 23536 to 20 decimal places


Still dont have the right answer though

EDIT :I followed that equation exactly as you put it, and i get 0.9999993169 V which i assume is wrong!!!

sure there isnt one too many 0's in any of the thousands ? as this would affect the answer

[Chris_F]

EDIT :I followed that equation exactly as you put it, and i get 0.9999993169 V which i assume is wrong!!!

I bet the error is in the units.  One of the inputs is probably in the wrong units and needs to be converted.

Cobra, please give us the entire problem, typed verbatum the way it appears.

[Saitek]

I'm familair with the e function but with the data you give me (and it is more difficult to read on a computer screen) I get 8.999 (got that twice). Are you sure your values are correct that you have provided?

Right in stages:

10^-3=0.001
470x0.001=0.47
22,000x0.47 =10,340
-1/10,340=0.00009671179884
9x (e^0.00009671179884)

= 9.0  :o

[Cobra]

EDIT :I followed that equation exactly as you put it, and i get 0.9999993169 V which i assume is wrong!!!

I bet the error is in the units.

[TacitBlue]

Thank God I go to a trade school instead of a regular college/university. I don't have to do anything that complex.

Here's a stupid question though. If e always equals 2.71828... etc. then why not just put 2.71828 rounded to whatever place is accurate enough? I mean, to me that's like saying "from now on when you see an A it's really a seven"...

[Chris_F]

Thank God I go to a trade school instead of a regular college/university. I don't have to do anything that complex.

Here's a stupid question though. If e always equals 2.71828... etc. then why not just put 2.71828 rounded to whatever place is accurate enough? I mean, to me that's like saying "from now on when you see an A it's really a seven"...

The same reason people use pi to mean 3.1415... "e" gets used a lot in math so if you used 2.718... first it would take longer to write, second people wouldn't be sure if you meant 2.718... or e.

e by the way is a very special number, like pi.  Pi relates different aspects of circles, which is neat, but e is the "Natural Logarithm" and has a number of really useful mathmatical properties that make some things like calculus easy.  That's it in a nutshell, or you can read:

http://en.wikipedia.org/wiki/Natural_logarithm

[TacitBlue]

Actually, that helps. I didn't know that it was "special" like Pi. I've used Pi a lot, and I know how useful it is in volume and area equations with cylinders and circles.  ;)

[Mobius]

Ben's doing it correctly, although you need to use 470 x 10^-6 instead of 470 x 10^-3 (for 470 micro-farads like you said).  You need to use the order of operations.  You would first do R*C, then -1 divided by your answer, then e raised to the power equal to -1/RC, the multiply that by your initial voltage.  When I do this I get 8.91 volts, using the correct equations for an RC circuit.  I would re-check that you have the right values, otherwise I don't know what the problem is either.