DC Circuit Theory Question - Any help? - Page 1
Forum section: electronics / beginners

author: rdbanks
author_rank: Contributor
timestamp: January 19, 2018, 11:33:13 pm
content: Hi all!My name is Ryan, I'm a EE student in the UK and have been a follower of the EEV channel for many years - I'm new to the forum and thought I'd ask the question as this is driving me mad! Its a simple past exam question (have an exam on Wednesday) and its pulled from the multiple-choice section - I've emailed the lecture however its 23:30 here and I doubt he'll reply. I'm pretty confident with DC theory however this has left me stumped - maybe its late and i'm drainedor maybe, just maybe I've got it correct.. Please see the picture for reference.I've calculated that (Question 9) - R1 = 36(ohms) however that doesn't appear to be an answer... and Question 10 - V1 = 36V.My assumption is R3 = 20V, 10(ohms), 2A (this is in parallel with the resistor to the right) therefore 'i2' = 4A. Considering 'Is' = 5A, 'I1' = 1A.Therefore, V1 = ('i2'*( ((10*R3)/(10+R3)) + (2+2)) ) = ('4'*( ((10*10)/(10+10)) + (2+2)) ) = ('4'*( 5 + 4 )) = 36V (R1 and the rest to the right are in parallel thus same voltage).Knowing V1 = 36V, R1 =36V/1A = 36(ohms)Then for Question 10, V1 = 36V (as calculated above).I must be wrong as the questions are from a previous examination paper...I know there are a lot of highly experiences guys & girls here, to be honest, I've never signed up (although i've been on this forum as a guest for year) due to being a little afraid of comments such as"if you can't work that out god help you!!"haha - please go easy on me...Thanks ALL!!

author: rdbanks
author_rank: Contributor
timestamp: January 19, 2018, 11:39:57 pm
content: Simulating it in Proteus give me the same answers, therefore if i'm wrong it must be due to having calculated R3 incorrectly...

author: Ian.M
author_rank: Super Contributor
timestamp: January 20, 2018, 12:02:23 am
content: Although exam papers are carefully checked, they are not miraculously free of misprints.Your reasoning appears to be sound and your SPICE simulation in Proteus gives the same results, so I would suspect a defective question or answer set.

author: rdbanks
author_rank: Contributor
timestamp: January 20, 2018, 12:10:55 am
content: I was thinking this however, considering it was an exam question I'm finding it hard to trust my understanding and workings - We haven't been given any solution for this paper so can't check our answers... I just dont want to think I have the correct method and understanding only to go on to make mistakes with this misinformation - especially as I'm first year and to me this is the foundation of what I'm going on to learn over the next few years...Thank you for your comment though!!

author: Andy Watson
author_rank: Super Contributor
timestamp: January 20, 2018, 12:18:16 am
content: It must be a typo. I get 36V and therefore 36\$\Omega\$

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2018, 12:26:48 am
content: I also just calculated the answer without reference to any previous workings and I get 36 V for V1, 1 A through R1, and thus 36 ohms for R1. My guess is that there is a typo in the paper.

author: Cerebus
author_rank: Super Contributor
timestamp: January 20, 2018, 12:48:25 am
content: Quote from: IanB on January 20, 2018, 12:26:48 amDitto, but in doing so I got 10R for R3- which makes me suspect that R3was supposed to be the reference in Q9.Oh, and rdbanks - welcome.

author: IanB
author_rank: Super Contributor
timestamp: January 20, 2018, 01:01:37 am
content: Quote from: Cerebus on January 20, 2018, 12:48:25 amYes! That makes sense.The 10\(\Omega\) applies to the far right resistor, and R3 is unknown. Then V3 is 2 x 10 = 20 V, whereupon R3 = 20^2 / 40 = 10\(\Omega\), which is what Q9 was presumably asking for.After that the solution follows to the left arriving in logical steps at V1.

author: rdbanks
author_rank: Contributor
timestamp: January 20, 2018, 01:21:00 am
content: Cerebus, IanB - That would make sense - As i've arrived to V1before R1therefore as they are both unknowns, Question 9 was likely asking for R3which I calculated as 10(ohms) - Ideal!!Ian.M, Andy Watson, IanB, Cerebus - Thank you!You guys have been excellent! I am genuinely grateful that you've all taken time to reply!I'll post back with the lectures answer!Thanks again!!

author: rdbanks
author_rank: Contributor
timestamp: January 20, 2018, 10:10:17 am
content: Lecture replied, the answer is correct, a mistake in the exam paper...Thanks again!