DC Circuit Theory Question - Any help?

[rdbanks]

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 drained  or 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 'I_s' = 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!!

[rdbanks]

Simulating it in Proteus give me the same answers, therefore if i'm wrong it must be due to having calculated R3 incorrectly...

[Ian.M]

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.

[rdbanks]

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!! 

[Andy Watson]

It must be a typo. I get 36V and therefore 36\$\Omega\$

[IanB]

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.

[Cerebus]

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.

Ditto, but in doing so I got 10R for R₃ - which makes me suspect that R₃ was supposed to be the reference in Q9.

Oh, and rdbanks - welcome.

[IanB]

Ditto, but in doing so I got 10R for R₃ - which makes me suspect that R₃ was supposed to be the reference in Q9.

Yes! 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.

[rdbanks]

Cerebus, IanB - That would make sense - As i've arrived to V₁ before R₁ therefore as they are both unknowns, Question 9 was likely asking for R₃ which 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!! 

[rdbanks]

Lecture replied, the answer is correct, a mistake in the exam paper...

Thanks again!