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  #1  
Old 04-19-2007, 04:36 PM
optimus_prime optimus_prime is offline
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Default Circuit Analysis Question (Electrical Engineering majors holla)

Alright so finished up my exam in electricity and magnetism and was kind of thrown off by a question.

I'm 90% certain it was a trick question of sorts but was hoping some of the 2p2 brains could confirm/deny.

Here's the question as I remember it:



The question asked for the potential difference across R1 and R4, the current magnitude and direction across R3, and the source current.

My buddy found the equivalent resistance:

(1/R1+1/R2)^-1+R3+(1/R4+1/R5)

but I'm damn near positive the whole "path of least resitance" thing means that the current wouldn't even pass through R1, R2, R4 and R5 and would instead take this path:



and that I1 and I2 would be 32/R3 and V1 and V2 would both be 32V.

Any help appreciated.
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  #2  
Old 04-19-2007, 05:14 PM
The Legend The Legend is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

the five resistors are in parallel. It's just drawn a little funky. So the voltage across all of them is 32. Currents are 32/R.
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Old 04-19-2007, 05:18 PM
The Legend The Legend is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

Currents across each resistor is Ix=32/Rx I2 is the sum of all the currents across all the resistors.
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Old 04-19-2007, 05:21 PM
optimus_prime optimus_prime is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

N/M
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  #5  
Old 04-19-2007, 05:22 PM
Matt R. Matt R. is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

Your current can split at any junction. You can't just ignore a possible pathway because a resistance is lower through another path. Unless it's some theoretical question where your resistance is infinite at one point in the circuit, you need to apply Kirchoff's rules (like your friend did, I think).

Your buddy is on the right track, although I didn't check his derivation. You need to find the current passing through R1 and R4, then just use Ohm's Law to get the voltage drop. Same for R3. Find the equivalent resistance across the entire circuit, and use V=IR again to get the source current.

Everything follows from Kirchoff's rules and V=IR, so just do the analysis from there.
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Old 04-19-2007, 05:29 PM
Matt R. Matt R. is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

I type slow.

As Legend noted, an easy way to do this is to use your first "loop" as the outer-most loop of the circuit. Since sum(V) = 0 you know immediately that V2=32V and I2=32V/R1. This circuit is really simple once you break it down into the loops, but it's hard to see at first since it's drawn kind of weird.
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Old 04-19-2007, 06:00 PM
carlo carlo is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

Been awhile(with trepidation)-isn't node @R1 the same as R4? therefore isn't the voltage= 0 from R1 to R4?
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  #8  
Old 04-19-2007, 06:01 PM
BruceZ BruceZ is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

Each resistor is connected directly across the battery, so all 5 resistors are in parallel. V1 = 32, V2 = 32, I1 = 32/R3. The equivalent resistance is actually Req = R1 || R2 || R3 || R4 || R5 = 1/(1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5). I2 = 32/Req.

Electricity doesn't only take the "path of least resistance". It takes all possible paths to ground, with a current in each path in inverse proportion to the resistance of that path.

EDIT: If the voltage source is actually a 32 volt battery with some internal resistance Rint, then compute I2 as 32/(Req+Rint) and scale the voltage across each resistor as 32*Req/(Req + Rint). This would not apply to an idealized 32 volt source, or if the 32 volts refers to the actual voltage across the battery in the circuit.
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  #9  
Old 04-19-2007, 06:19 PM
carlo carlo is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

Ok, try again.Aren't R1 and R2 in parellel and in series with R4 and R5 which are also in parellel? Also, isnt R3 in parallel with the series circuit just referred to?

Thereforethe voltage across R1 to R4 is 32 volts and the voltage across R3 is 32 volts. My drawing doesn't hve them all in parallel but R1/R2 which series with R4/R5 and this resultant resistance in parallel with R3. The voltage is across the resulting circuit.
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  #10  
Old 04-19-2007, 08:58 PM
optimus_prime optimus_prime is offline
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Default Re: Circuit Analysis Question (Electrical Engineering majors holla)

In case I wasn't clear, my buddy was arguing that R1 and R2 were in parallel, R4 and R5 were in parallel, and that these were both in series with R3.

I started to look at R1, R2 and R3 being in parallel but couldn't put it all together. That, and my prof is a bit of a douche who loves tricky [censored].

Anyways I'm pretty sure I see how they're all in parallel now and am sobbing gently.

Edit to add: The resistors had values on the exam, but I couldn't remember them and they weren't important for my purposes here.
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