• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Calculating total resistance of a combination circuit

combination circuit.png
1. Homework Statement
Find the total resistance of the circuit
2. Homework Equations
Parallel resistance: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ..., Series resistance: R1 + R2 + R3...
3. The Attempt at a Solution
The 1470 ohm resistor is parallel to the 50 ohm resistor and the 1000 ohm resistor, but there is a junction between them, which confuses me on how to add the 1470 ohm resistor to each of them... that probably didn't make sense but it's the best way I could word it. Once that is solved I know how to do the rest.
 
Last edited:
627
112
1. Homework Statement
Find the total resistance of the circuit
2. Homework Equations
Parallel resistance: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ..., Series resistance: R1 + R2 + R3...
3. The Attempt at a Solution
The 1470 ohm resistor is parallel to the 50 ohm resistor and the 1000 ohm resistor, but there is a junction between them, which confuses me on how to add the 1470 ohm resistor to each of them... that probably didn't make sense but it's the best way I could word it. Once that is solved I know how to do the rest.
Try adding a picture.
 

vela

Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,426
1,109
None of the resistors are in series or in parallel with any other.
 
None of the resistors are in series or in parallel with any other.
How would i go about adding up the resistances then?
 

vela

Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,426
1,109
Connect the combination to a voltage source V and calculate the current I it draws. The equivalent resistance is V/I.
 
Connect the combination to a voltage source V and calculate the current I it draws. The equivalent resistance is V/I.
But to find the current that is drawn from the battery (6V) i need to know the total resistance of the circuit...
 

vela

Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,426
1,109
You're just going to have to write down equations using Kirchoff's laws and solve the resulting system of equations.
 
627
112
Connect the combination to a voltage source V and calculate the current I it draws. The equivalent resistance is V/I.
I would think that there is no point to this. You need the total resistance to calculate the current, then use I=V/R, and then go back to use R=V/I. That's just a lot of unnecessary extra steps that you would take after you find the total resistance.

To solve this you would need to either use kirchoff's laws or kirchoff's laws with the delta y transform.
 

scottdave

Science Advisor
Homework Helper
Insights Author
1,632
619
I would think that there is no point to this. You need the total resistance to calculate the current, then use I=V/R, and then go back to use R=V/I. That's just a lot of unnecessary extra steps that you would take after you find the total resistance.

To solve this you would need to either use kirchoff's laws or kirchoff's laws with the delta y transform.
At first, I thought @vela was joking (like actually get a battery and some resistors.. Ha). But I think what was meant is to create a circuit on paper, then go through the loop analysis. Once you find the current delivered by the test voltage, then R = Vsource/Isource.
 

vela

Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,426
1,109
I would think that there is no point to this. You need the total resistance to calculate the current, then use I=V/R, and then go back to use R=V/I. That's just a lot of unnecessary extra steps that you would take after you find the total resistance.
Really? There's no other way to find the current I? Like perhaps the way you mention below...

To solve this you would need to either use kirchoff's laws or kirchoff's laws with the delta y transform.
 
627
112
Really? There's no other way to find the current I? Like perhaps the way you mention below...
Right, but to find the current that way wouldn't you have to find the total resistance first?
 

vela

Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,426
1,109
I can't help but feel you're trolling here.
 

gneill

Mentor
20,489
2,615
Right, but to find the current that way wouldn't you have to find the total resistance first?
No. Solve the circuit, and you'll find all the currents even if you only really require one of them.
 

Janus

Staff Emeritus
Science Advisor
Insights Author
Gold Member
3,393
1,066
View attachment 203597 1. Homework Statement
Find the total resistance of the circuit
2. Homework Equations
Parallel resistance: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ..., Series resistance: R1 + R2 + R3...
3. The Attempt at a Solution
The 1470 ohm resistor is parallel to the 50 ohm resistor and the 1000 ohm resistor, but there is a junction between them, which confuses me on how to add the 1470 ohm resistor to each of them... that probably didn't make sense but it's the best way I could word it. Once that is solved I know how to do the rest.
One method is the loop-equation method. Draw out the possible current loops for the circuit, write an equation for each loop, and then solve the simultaneous equations to find each loop current. The sum of these give you the total current for any given source voltage, and from that you can determine the total resistance of the circuit. Here's the circuit redrawn with the loop currents marked:
bridge.gif
 

davenn

Science Advisor
Gold Member
8,818
6,088
Here's the circuit redrawn with the loop currents marked:
just out of personal curiosity for my learning

can you do that with just the three loops you have done or should it really be for all loops ?
as there is a 4th loop
 

vela

Staff Emeritus
Science Advisor
Homework Helper
Education Advisor
14,426
1,109
just out of personal curiosity for my learning

can you do that with just the three loops you have done or should it really be for all loops ?
as there is a 4th loop
You'd find that only three of the loop equations are independent. The equations from the other loops would be linear combinations of the three.
 

ehild

Homework Helper
15,360
1,766
just out of personal curiosity for my learning

can you do that with just the three loops you have done or should it really be for all loops ?
as there is a 4th loop
Independent loops are needed, and there are three in this case. Assign current to each loops, so that there is one or two loop currents flowing through each resistor. If it is possible, the loops are separate. @davenn's construction is too complicated. See the next one:
upload_2017-5-18_6-25-8.png


The currents through the resistors are: I1, through 100Ω, I2, through 330Ω, I2-I1, through 220Ω, I2-I3, through 50Ω, and I3-I1, through 1000Ω. The generator current is I1. Anti-clockwise direction is positive. Write Kirchhoff's Loop Rule for each loops, that makes three equations for the three currents, eliminate I2 and I3, solve for I1. The total resistance is R=V/I1.
 

Want to reply to this thread?

"Calculating total resistance of a combination circuit" You must log in or register to reply here.

Related Threads for: Calculating total resistance of a combination circuit

Replies
7
Views
446
Replies
4
Views
2K
Replies
4
Views
416
Replies
5
Views
1K
Replies
12
Views
2K
Replies
10
Views
643
Replies
4
Views
393

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top