Hey! Sign in to get help with your study questionsNew here? Join for free to post
 You are Here: Home >< Physics

# PLEASE HELP with Capacitor!!

Announcements Posted on
Four hours left to win £100 of Amazon vouchers!! Don't miss out! Take our short survey to enter 24-10-2016
1. About 4(f)(II), I used to V=V0e^(-t/RC) equation to find the value of R as 137ohm but it is wrong

Can somebody please explain to me how we use use proportion equation of T=RC and original R value from graph?
Attached Images

2. (Original post by mystreet091234)
About 4(f)(II), I used to V=V0e^(-t/RC) equation to find the value of R as 137ohm but it is wrong

Can somebody please explain to me how we use use proportion equation of T=RC and original R value from graph?
even so 137 is still closer to 150 than it is to 100. why didn't you go for 150 anyway?
3. (Original post by Joinedup)
even so 137 is still closer to 150 than it is to 100. why didn't you go for 150 anyway?
Cuz when R is larger, T is also larger. I was thinking if R is 150, time needed will be more than 12s.
4. (Original post by mystreet091234)
Cuz when R is larger, T is also larger. I was thinking if R is 150, time needed will be more than 12s.
Well you already worked out from the experimental data that the real circuit was about 10% quicker than the theoretical calculation... so for a new time interval making R 10% greater, i.e. 10% slower than the theoretical calculation should get you close.
137+13.7=150.7
----

anyway

V0 = 12 V
V = 5 V
C= 100x10-6F

new value for time elapsed t=12 s

ln V - ln V0 = -t/RC

get everything that's not changed onto the same side of the equals sign

C (ln V - ln V0) = -t/R

what this means is that if everything else is held constant then -t/R is constant ratio... so it'll be the same for the new value of R that makes t=12s as it was for the old value of R when t was measured at 17.5 s

told/Rold=tnew/Rnew

17.5/(220x103)=12/Rnew

12/(17.5/(220x103))=R
= 220x103 * (12/17.5)
5. (Original post by Joinedup)
Well you already worked out from the experimental data that the real circuit was about 10% quicker than the theoretical calculation... so for a new time interval making R 10% greater, i.e. 10% slower than the theoretical calculation should get you close.
137+13.7=150.7
----

anyway

V0 = 12 V
V = 5 V
C= 100x10-6F

new value for time elapsed t=12 s

ln V - ln V0 = -t/RC

get everything that's not changed onto the same side of the equals sign

C (ln V - ln V0) = -t/R

what this means is that if everything else is held constant then -t/R is constant ratio... so it'll be the same for the new value of R that makes t=12s as it was for the old value of R when t was measured at 17.5 s

told/Rold=tnew/Rnew

17.5/(220x103)=12/Rnew

12/(17.5/(220x103))=R
= 220x103 * (12/17.5)
Thanks!
So we should check whether the experimental value matches the theoretical one before we apply an equation to calculate the new value?
6. (Original post by mystreet091234)
Thanks!
So we should check whether the experimental value matches the theoretical one before we apply an equation to calculate the new value?
in this case yes because it seems to be asking us what the designer will choose to make t=5 based on information from the experiment rather than calculate a theoretical value.
calculating a %age difference between expected and actual was an earlier part of the question (I looked up the paper to see the the whole question) , if you've worked through the question in order it could have been a clue.
7. (Original post by Joinedup)
in this case yes because it seems to be asking us what the designer will choose to make t=5 based on information from the experiment rather than calculate a theoretical value.
calculating a %age difference between expected and actual was an earlier part of the question (I looked up the paper to see the the whole question) , if you've worked through the question in order it could have been a clue.
Understood! THANKS A LOT!!!

Write a reply…

Submit reply

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
that username has been taken, please choose another Forgotten your password?
2. this can't be left blank
this email is already registered. Forgotten your password?
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
your full birthday is required
1. Oops, you need to agree to our Ts&Cs to register
2. Slide to join now Processing…

Updated: April 12, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Who is getting a uni offer this half term?

Find out which unis are hot off the mark here

### How to get a 1st class degree

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.