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#1
04-05-2006, 03:04 PM
 tjjam2003 Junior Member Join Date: Apr 2006 Posts: 22

In a Series-Parallel circuit, how do you find the resistance and the current if only the voltage is given? 5 resistors in series and 1 resistor in parallel.

R1= 3 ohm Voltage= 3V Current 1A
R2= 5 ohm Voltage =1.65 current 0.33A

R3= ??? voltage= 4 volts (parellel)

R4= 4 ohm Voltage 1.32 Current 0.33
R5= 1 ohm voltage 1 volt current 1A
R6=3ohm voltage 0.99 current 0.33
#2
04-05-2006, 10:28 PM
 windoze killa Senior Member Join Date: Feb 2006 Location: Melbourne Australia Posts: 605

Quote:
 Originally posted by tjjam2003@Apr 6 2006, 02:04 AM In a Series-Parallel circuit, how do you find the resistance and the current if only the voltage is given? 5 resistors in series and 1 resistor in parallel. R1= 3 ohm Voltage= 3V Current 1A R2= 5 ohm Voltage =1.65 current 0.33A R3= ??? voltage= 4 volts* (parellel) R4= 4 ohm Voltage 1.32 Current 0.33 R5= 1 ohm voltage 1 volt current 1A R6=3ohm voltage 0.99 current 0.33 Quoted post
Could you draw the circuit please. What you have here doesn't seem to make sense.

If all these resistors that are in series are really in series then the current through them all will be the same. If R1 and R2 are in series you can't have 1A through one and .33A through the other.
__________________
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#3
05-03-2006, 03:30 PM
 rodn.m Junior Member Join Date: May 2006 Posts: 12

Quote:
 Originally posted by windoze killa@Apr 6 2006, 08:28 AM Could you draw the circuit please. What you have here doesn't seem to make sense. If all these resistors that are in series are really in series then the current through them all will be the same. If R1 and R2 are in series you can't have 1A through one and .33A through the other. Quoted post
hi, if you are adding resistors in series
Rt=R1+R2+R3
For resistors in parallel
Rt=R1*R2/R1+R2
to add more than 2 resistors in parallel
do above calc for R1 and R2
Then do same calc for above result and R3
then simply add total series and parallel resistor values together.
#4
06-01-2006, 02:44 AM
 radiosmoke Junior Member Join Date: May 2006 Posts: 17

Quote:
 Originally posted by tjjam2003@Apr 5 2006, 08:04 AM In a Series-Parallel circuit, how do you find the resistance and the current if only the voltage is given? 5 resistors in series and 1 resistor in parallel. R1= 3 ohm Voltage= 3V Current 1A R2= 5 ohm Voltage =1.65 current 0.33A R3= ??? voltage= 4 volts* (parallel) R4= 4 ohm Voltage 1.32 Current 0.33 R5= 1 ohm voltage 1 volt current 1A R6=3ohm voltage 0.99 current 0.33 Quoted post
------------------------------
All current that goes into a circuit must leave the same circuit.

(since you have 3 resistors that have .33a they must be in series.)

R2 + R4 + R6 so 5ohms + 4ohms +3ohms = 12ohms

Rt1 = 12
It1 = .33amps
Et1 = Rt1 X It1 or 12 X .33 = 3.96v (I will round that to 4v)
---------------------------
and

two resistors have 1 amp of current flowing they must also be in series.

Rt2 = R1 + R5 or 3ohms + 1 ohm = 4ohms

so

E2t = It2 X Rt2 or 4ohms X 1 amp = 4volts
------------------------------------------
Since both have a total of 4 volts across the series branches
both of these series circuits are in parallel

Then add the R3 with a 4volt drop also to the circuit, and since its in parallel, it uses E = I X R

or 4V = 1amp X 1ohm, Since the 4volts is across the resistance of R3 it must be 4ohms and a current of 1 amp.
ohms law proves this R =E / I or 4v / 1 amp = 4 volts

----------------------
To continue:
you have a series parallel circuit (that is actually three series circuits, that are in parallel
-------------
Just for fun:
Adding the currents and using ohms law, we see that
Rt = Et / It or 4v / 2.33 amps = 1.73 ohms of Total resistance.

This is a close approximation of the total resistance of the circuit.
---------
There are other methods that may work also, but, I find this the most logical, without going into higher math, (which I abhor)
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