Solving for current in circuit with several light bulbs

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Homework Help Overview

The discussion revolves around a circuit problem involving several light bulbs, where the original poster attempts to solve for the current through different branches of the circuit using given voltage values and current measurements.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the application of series and parallel circuit rules, questioning how to correctly apply these laws to find unknown currents. There is confusion regarding the direction of current flow and the use of Kirchhoff's loop rule. Some participants suggest reconsidering the assumptions about current direction and the relationship between currents in different branches.

Discussion Status

Participants are actively engaging with the problem, raising questions about the relationships between currents and the application of circuit laws. There is no explicit consensus on the approach to take, but several lines of reasoning are being explored, particularly regarding the application of Kirchhoff's laws and the implications of current direction.

Contextual Notes

There is mention of potential confusion due to the original poster's use of electron current versus conventional current. Additionally, the problem involves multiple voltage values and a single voltage source, which may influence the analysis of the circuit.

Derpicus
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Homework Statement


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Homework Equations


Series[/B]
Vt= V1 + V2 + V3 etc
It = I1 = I2 = I3 etc
Parallel
Vt = V1 = V2 = V3 etc
It = I1 + I2 + I3

The Attempt at a Solution



Vt= 20V V2= 5.0 V V4= 8.0V
It= 150 mA I2= 40 mA
Find I3, and I4

I've solved for the values of V already
V1 = 7.0V
V3 = 5.0V
So Vt = V1+V2=V3+V4
= 7.0V + 5.0V + 8.0V = 20V
But I'm getting confused on solving for I which is current.

I1 is in series with I2 and I4, but I2 is also parallel to I3. So which set of laws would I apply for solving for I3?
I know I1=I4 since they're in series.
I'm assuming I1=I1=It - I2
= 150 mA - 40mA = 110mA
But still stuck on how to solve for I3, since its not stated the bulbs are identical so I'm unsure if I3 would also be 40mA. I'd appreciate any help I can receive.
 
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Shouldn't current be in the opposite direction?
Also, Have you tried applying Kirchoff's loop rule?
 
Titan97 said:
Shouldn't current be in the opposite direction?
Also, Have you tried applying Kirchoff's loop rule?
I copied everything from the diagram the current flows the way it does since it starts from the negative part of the cell and flows along to the positive part of the cell. I also don't believe I've learned about Kirchoff's loop rule yet. Once it reaches the box where I2 is it has two possible paths going down to V2 or proceeding forward and then down to I3/V3.
 
Derpicus said:
I copied everything from the diagram the current flows the way it does since it starts from the negative part of the cell and flows along to the positive part of the cell. I also don't believe I've learned about Kirchoff's loop rule yet. Once it reaches the box where I2 is it has two possible paths going down to V2 or proceeding forward and then down to I3/V3.
The direction of current is conventionally from +ve terminal to -ve terminal of the battery, as Titan97 said earlier. But using the given convention, it won't affect the calculations once you flip the voltage polarities accordingly. Also, there is only one source present(Vt), rest all are bulbs.
Where is It in your diagram?
 
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cnh1995 said:
The direction of current is conventionally from +ve terminal to -ve terminal of the battery, as Titan97 said earlier. But using the given convention, it won't affect the calculations once you flip the voltage polarities accordingly. Also, there is only one source present(Vt), rest all are bulbs.
Where is It in your diagram?
Itotal is just past V4
 
Derpicus said:
Itotal is just past V4
Then isn't It=I1? You have I1 and I2. What is the relation between i1, i2 and i3?
 
cnh1995 said:
Then isn't It=I1? You have I1 and I2. What is the relation between i1, i2 and i3?
I know I1 is 110mA
Since I2 is 40mA
As I1=I4=It-I2
Since the total incoming has to equal the total outgoing.

But the issue I'm having is I3 is parallel to I2

Since 110ma+40 mA = 150 mA which is the It.

But the relationship for parallel is It= I1+I2 etc, so I'm unsure how to go about solving for I3.
 
Derpicus said:
As I1=I4=It-I2
If It is just past V4, then I1=I4=It.
 
cnh1995 said:
If It is just past V4, then I1=I4=It.
KCL says incoming current at a point is equal to the outgoing current. If you apply that, you can see
I1=I2+I3.
 
  • #10
Where are the several sources? It looks like only one. Shouldn't all V's be the same if there are no resistors?
 
  • #11
justaman0000 said:
Where are the several sources? It looks like only one. Shouldn't all V's be the same if there are no resistors?
There appears to be only one source.

No. The V-s are not all the same. Several are given.

Apparently the problem statement is that:
Vt= 20V, V2 = 5.0 V, V4= 8.0V
It= 150 mA, I2= 40 mA

Find I3, and I4​

And it looks like OP uses electron current, rather than conventional current..
 
  • #12
SammyS said:
There appears to be only one source.

No. The V-s are not all the same. Several are given.

Apparently the problem statement is that:
Vt= 20V, V2 = 5.0 V, V4= 8.0V
It= 150 mA, I2= 40 mA

Find I3, and I4​

And it looks like OP uses electron current, rather than conventional current..
Thank you SammyS
 

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