# Capacitance Q unit 4Watch

#1
Stuck on the question below;

I don't know what to do, method is shown in pic that I tried to use, in the end I had to guess the answer as I couldn't figure it out.

Help is appreciated!

Thanks, Jay

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4 years ago
#2
0.14 i think

Q=Qe^-t/RC
so 0.5Q=Qe^-t/5
Q cancels
ln0.5=-t/5
t=0.139
0
#3
(Original post by Cheesegrater4)
0.14 i think

Q=Qe^-t/RC
so 0.5Q=Qe^-t/5
Q cancels
ln0.5=-t/5
t=0.139
Answer is D by the way, sorry i forgot to mention the answer. I tried the method you did when doing the exam under timed conditions, but it didn't work.
0
4 years ago
#4
(Original post by Jaydude)
Answer is D by the way, sorry i forgot to mention the answer. I tried the method you did when doing the exam under timed conditions, but it didn't work.
oops entered it into caluclator wrong! ln0.5 x -5 = 3.47 (so it works!)
1
#5
(Original post by Cheesegrater4)
oops entered it into caluclator wrong! ln0.5 x -5 = 3.47
I want to cry. I mis-read time constant 5 seconds as 0.5 seconds hence where i went wrong...

Thanks!
0
4 years ago
#6
I can't really follow what you're trying to do there tbh...

so

Or there's a couple of rules of thumb for exponential decay

0.37 times the original value after 1 time constant

half life = 0.7 times the time constant

the rule of thumb way works better than longhand on this example because 5 times 0.7 is 3.5 exactly
0
#7
(Original post by Joinedup)
I can't really follow what you're trying to do there tbh...

so

Or there's a couple of rules of thumb for exponential decay

0.37 times the original value after 1 time constant

half life = 0.7 times the time constant

the rule of thumb way works better than longhand on this example because 5 times 0.7 is 3.5 exactly
I did the correct method, just Mis read 5 seconds for 0.5 seconds...(see Above posts!) Thanks for tips, I'll try and follow a clearer method in the future

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