discharging a capaitor

pradeepbhat92

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12 years ago

hey guys first time here um the problem states that During a physics demonstration, a fully charged 70-µF capacitor is discharged through a 40-? resistor. How long will it take for the capacitor to lose 62% of its initial energy?

the issue is i used the formula and everything and i always get the wrong answer on webassign until i divided my answer by two i dont get what im doing wrong this is my calculations ln(0.38) * 40 * 70e-6 * -1.

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chemb

wrote...

12 years ago

Given

Resistance , R = 40 \({\Omega}\)
C = 70 x 10-6 F

Time constant , T = RC = (40 \({\Omega}\)) x (70 x 10-6 F)
   T= ANSWER in seconds (s)

Charge on the capacitor when discharged is

Q = Qo e-t/RC

The charge of the capacitor dropped to 62% of its initial charge

so Q = 0.62 Qo

Now, 0.62Qo = Qo e-t/(ANSWER s)

   ln(0.62) = -t /(ANSWER s)

-0.478 = - t / ANSWER s
   t = YOUR FINAL ANSWER

The time taken for the capacitor to drop to 65% of its initial charge is YOUR FINAL ANSWER in seconds (s).

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chemb

wrote...

12 years ago

Let me know if that's the correct answer in the back!

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pradeepbhat92 Author

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12 years ago

sadly no :/ if you look at my work and divide it by two thats the right answer. btw where did u get 65 and 77?

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chemb

wrote...

12 years ago

The 65 should be 40, and it should be 70 for 77 SORRY I misread it, crap/

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chemb

wrote...

12 years ago

I'll correct it: Check it out now, see if it works.

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pradeepbhat92 Author

wrote...

12 years ago

i figured and already tried that ur off by 0.00001 but if i do it on a different webassign practice question ur completely off

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chemb

wrote...

12 years ago

Quote from: pradeepbhat92 (12 years ago)

if i do it on a different webassign practice question ur completely off

Are you sure ur plugging the numbers into your calculator correctly Confounded Face

What answers do you get>

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pradeepbhat92 Author

wrote...

12 years ago

ive tried again and again the only way i get the right answer every time is the way i do it. for example During a physics demonstration, a fully charged 85-µF capacitor is discharged through a 50-? resistor. How long will it take for the capacitor to lose 55% of its initial energy?

1-0.55=0.45
ln(0.45) *85e-6 * -50 = my answer

final answer webassign says it is = my answer / 2

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