ENGR 12
Assigment 3 Hints
Chapter 2: 4,6,11,13,17
4) a) first simplify…determine Rparallel as the effective R for the mostly parallel group of resistors (5||4||(15+5))
[that's all resistors except the two R2 resistors]
and Rtotal for whole resistor network = 2 + 2 + Rparallel
Use OL to determine I coming from source (=120/Rtotal) and thus V2 = voltage across 2 ohm resistors
Now use KVL around source, R2, R2 and Rparallel loop to determine voltage drop Vparallel across
Rparallel section (5||4||(15+5))….this is, Vparallel = 120 – V2 – V2
Now that Vparallel is known, we can compute i0 = Vparallel/4 and v0 is a voltage divider of Vparallel
across R5 in series with R15 (Vparallel * 5/(5+15))
b) once we know Vparallel we can determine I thru the 5/15 pair using OL and thus P15 = I*I*15
c) P=IV with I as determined above
6) a) Rab = 2 + (12||24) + 6
First compute 12||24 then add the series resistors
b) Rab = 15||30||(7+(24||30||20)) First compute 24||30||20 and build from there
11) a) Since both R1k and R15k share the same node (and reference potential), we can calculate
vx = V15k – V1k i.e. voltage across 15k – voltage across 1k resistor.
both V1k and V15k are set up as voltage dividers with the 30 V source. So V1k = 30*(1/(1+5))
and similarly for V15k (Ans: 1V)
b) Re-solve using Vs instead of 30 (Ans: Vx = Vs/30)
13) Calculate Rtotal = 2 + [ (6 + 5||20) || (15 + 12 + 13) ]
it may help to redraw diagonal R20 as parallel to R5
Then Ig = 125/Rtotal
You can simplify the 6 resistors on the right as effectively two parallel resistor paths splitting up Ig
and use the current divider formula on those effective resistances for the two parallel paths.
And again, the current going down the left parellel branch is split into two parallel paths when it
reaches R5 and R20, so the current divider formula applies again to finally determine i0
(Ans: io = 2A, ig=12.5A)
17) a) use the voltage divider formula for vo with no load (Ans: 20V)
b) replace R20k with the parallel equivalent of R20k || R12k and recalculate vo (Ans: 15V)
c and d) divide answers to a and b by 25 (Ans: 0.8V, 0.6V)