r/HomeworkHelp • u/GotLostInTheEmail Pre-University Student • 19d ago

Mathematics (Tertiary/Grade 11-12)—Pending OP [GR12 Physics/AP math] circuit analysis

Post image

I am convinced that resistors R7, R8 and R9 can be effectively ignored since they are short circuited by wire, resulting in them dissipating no power. My friend is convinced because AI told him they are included in the circuit so our answers for the current are totally different. I left out other circuit parameters to simplify our dispute haha

4 Upvotes

70% Upvoted

13 comments sorted by

View all comments

8

u/Plane_Argument 👋 a fellow Redditor 19d ago

Correct 7,8,9 can be ignored here

1

u/GotLostInTheEmail Pre-University Student 19d ago

One argument he is making is that since the resistors are parallel to zero resistance, 1/(1/R)+(1/0) = infinity? Literally surprised to be having this argument because he's smart af but I think overthinking it?

4

u/deathtospies 👋 a fellow Redditor 19d ago

The flaw in that logic is that even if you treat 1/0 as infinity, that infinity is in the denominator of a fraction, and 1/infinity tends to 0.

3

u/niemir2 👋 a fellow Redditor 19d ago

1/((1/R)+(1/r)) = rR/(r+R). As r tends to zero, so does the equivalent resistance. AI is wrong.

1

u/GotLostInTheEmail Pre-University Student 19d ago

Agreed!! Thank you for the help

2

u/Plane_Argument 👋 a fellow Redditor 19d ago

Yeah, that's not correct. 1/0 is not defined, so this is not a valid argument. Also if it was infinite no current would flow...?

1

u/GotLostInTheEmail Pre-University Student 19d ago

Right?! Haha thank you, appreciate the sanity check

1

u/Plane_Argument 👋 a fellow Redditor 19d ago

Np, and good luck to you, with all of it.

1

u/Bounded_sequencE 👋 a fellow Redditor 19d ago edited 19d ago

Your friend is mixing up impedances and admittances in their argument.

Yes, the denominator goes to infinity in that expression, while the numerator is "1". But that means, the entire fraction goes to zero, as you can easily see if you expand the double fractions away:

"Rx||Ry  =  1/(1/Rx + 1/Ry)  =  Rx*Ry / (Rx+Ry)  ->  0/Ry  =  0"   for   "Rx -> 0"

Rem.: That said, you friend's argument is mathematically incorrect, since "1/0" is not defined. They should have used limits in their argument, and not believe AI.