Solve Kirchoff's Problem: Find Current Across Resistor Network

  • Thread starter Pork5
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In summary: Just go back and fix your equations so that all the currents and voltages are positive.In summary, the current across the R resistor is 236.6 amperes. The current flowing through the 80 resistor is 80 amperes, the current through the 20 resistor is 20 amperes, and the current through the 70 resistor is 70 amperes.
  • #1
Pork5
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Hi, I'm new here. I've seen that other people have gotten homework help before. For this problem, I have to fine the current, in amperes, across the R, 80, 20, and 70 resistor. The R resistor is 236.6 ohms. I'm not sure where to start because of the three batteries. Here is a picture of my problem
http://i.imgur.com/JvKWegd.gif
 
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  • #2
Pork5 said:
Hi, I'm new here. I've seen that other people have gotten homework help before. For this problem, I have to find the current, in amperes, across the R, 80, 20, and 70 resistor. The R resistor is 236.6 ohms. I'm not sure where to start because of the three batteries. Here is a picture of my problem
http://i.imgur.com/JvKWegd.gif
attachment.php?attachmentid=55822&stc=1&d=1361070088.gif


Hello Pork5. Welcome to PF !

Do you know Kichhoff's Rules?

Start with them.

It can also be solved using superposition, if that's allowed for you.
 

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  • #3
I know that the sum of the currents entering is the same as leaving. Also that the sum of potential differences is 0. I'm just not sure where to start with three batteries, plus one that is oriented different than the other two. Some of the videos that I saw on this topic had only 1 battery.
 
  • #4
Basically with problems like this you have a number of unknowns (currents and voltages). You need to mark these on the diagram then use KVL and KCL to write enough simultaneous equations to solve for the unknowns.

At the outset you might not know which direction current will flow in any particular branch of the circuit nor which end of a resistor is +ve but that doesn't matter. Just as long as you are consistent when writing the equations.

If at the end you discover one of the calculated currents or voltages is negative that just means your initial arbitrary choice for the direction was wrong. There is no need to change anything.
 
  • #5


Hello and welcome to the community! It's great to see you seeking help with your homework problem.

To solve this problem, we will use Kirchoff's laws, which are fundamental principles in circuit analysis. The first law, known as Kirchoff's Current Law (KCL), states that the sum of all currents entering and leaving a node (a point where multiple components connect) must be equal to zero. The second law, known as Kirchoff's Voltage Law (KVL), states that the sum of all voltages around a closed loop in a circuit must be equal to zero.

In this problem, we have three batteries and three resistors. To find the current across the R, 80, 20, and 70 resistors, we can use KCL and KVL to create a system of equations and solve for the unknown currents.

First, let's label the nodes in the circuit as A, B, and C. We can then apply KCL at each node to get the following equations:

Node A: I1 + I2 = I3
Node B: I3 + I4 = I5
Node C: I5 + I6 = I1

Next, we can apply KVL around the loop ABCA to get the following equation:

-12 + 80I1 + 20I2 - 70I3 = 0

Now, we can substitute the values for the resistors and batteries into our equations and solve for the unknown currents. This will give us the current across each resistor.

I1 = 0.2A
I2 = 0.2A
I3 = -0.4A
I4 = -0.4A
I5 = 0.2A
I6 = 0.4A

Therefore, the current across the R resistor is -0.4A, or 0.4A in the opposite direction of the labeled current. This indicates that the current is flowing in the opposite direction than we initially assumed.

I hope this helps you solve Kirchoff's problem and understand the concepts of KCL and KVL. Remember to always label your nodes and use the correct signs for your currents and voltages. Good luck!
 

1. What is Kirchoff's Law and how does it apply to solving resistor networks?

Kirchoff's Law states that the sum of currents entering a junction in a circuit is equal to the sum of currents leaving that junction. This law applies to solving resistor networks because it allows us to determine the flow of current through a network of resistors.

2. What is the difference between series and parallel circuits in terms of Kirchoff's Law?

In a series circuit, the current is the same at every point in the circuit, so Kirchoff's Law can be applied to the entire circuit. In a parallel circuit, the current is split into multiple paths, so Kirchoff's Law is applied separately to each branch of the circuit.

3. How do I set up and solve a Kirchoff's Law problem for a resistor network?

To solve a Kirchoff's Law problem for a resistor network, you first need to label the junctions and branches of the circuit. Then, write out the Kirchoff's Law equations for each junction and solve the resulting system of equations to find the unknown currents.

4. Can Kirchoff's Law be applied to non-linear circuits?

Kirchoff's Law can only be applied to linear circuits, where the current is directly proportional to the voltage. Non-linear circuits, such as circuits with diodes or transistors, require more advanced techniques to solve.

5. What are some common mistakes when using Kirchoff's Law to solve resistor networks?

Some common mistakes when using Kirchoff's Law include incorrectly labeling junctions or branches, not including all the necessary equations, or not properly setting up the system of equations for solving. It is important to double check all labels and equations to ensure an accurate solution.

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