Hello everyone, having troulbes applying Kirchoff's Rule~DC circuits

In summary, the conversation is about a problem dealing with Kirchoff's Rule and a circuit with resistances and ideal batteries. The questions ask for the current through each battery and the potential difference between two points. The person solving the problem provides three equations but is unsure where to plug in a specific value. Another person suggests substituting the value into the equations and points out a mistake in the second circuit.
  • #1
mr_coffee
1,629
1
Hello everyone, this is my first problem dealing with Kirchoffs Rule and I thought I did it right...here is the circuit...
http://www.webassign.net/hrw/hrw7_27-40.gif
the info is the following:
In Figure 27-40, the resistances are R1 = 1 , R2 = 2.2 , and the ideal batteries have emfs E1 = 2.0 V, and E2 = 3 = 3 V.


Fig. 27-40

(a) What is the current through each battery? (Take upward to be positive.)
battery 1 A
battery 2 A
battery 3 A

(b) What is the potential difference Va - Vb?
V

I came out with the following 3 equations:
E1 + (-R1)(I_1) + R2*I_2 - E2 -R1*I_1;
-R2*I_2 + R1*I_3 - E3 + R1*I_3 + E2;
I_1 + I_2 + I_3 = 0;

When i solve for I_1 = -I_2 -I_3
Where do I plug I_1 into? i tried the first equation and i couldn't solve for any I, and i can't for the 2nd, because there is no I_1, did i do this right? thanks!
 
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  • #2
Substitute for I_2 = -I_1 - I_3 into both the first two eqns, since it is common to both.
Then you will have two eqns in two unknowns.
 
  • #3
Thanks! that worked out great, but I messed somewhere writing out the 2nd circuit on the right...This is the equation i got and the work:
http://img477.imageshack.us/img477/956/lastscan6lq.jpg
its wrong...
The first one, i did, on the left, was right though. Any help would be great.
 
Last edited by a moderator:
  • #4
On your last line - in the attachment - the -R1.I2 should be +R1.I3.

It looks like this mistake was copied through from the line before it.
 

1. What is Kirchoff's Rule?

Kirchoff's Rule, also known as Kirchoff's Circuit Laws, are two principles used to analyze and solve problems in direct current (DC) circuits. They are the Kirchoff's Current Law (KCL) and Kirchoff's Voltage Law (KVL).

2. How do I apply Kirchoff's Rule in DC circuits?

To apply Kirchoff's Rule in DC circuits, first identify all the nodes and branches in the circuit. Then, apply KCL at each node by stating that the sum of currents entering the node is equal to the sum of currents leaving the node. Next, apply KVL in each closed loop by stating that the sum of voltage drops is equal to the sum of voltage rises.

3. What are some common mistakes when applying Kirchoff's Rule?

One common mistake when applying Kirchoff's Rule is forgetting to consider the direction of current and voltage. Another mistake is not properly labeling the nodes and branches in the circuit. It is also important to make sure that all loops in the circuit are closed and that all components are included in the analysis.

4. Can Kirchoff's Rule be applied to AC circuits?

Yes, Kirchoff's Rule can also be applied to alternating current (AC) circuits. However, in AC circuits, the current and voltage values are constantly changing, so the analysis can become more complex.

5. How can I check if my application of Kirchoff's Rule is correct?

You can check the accuracy of your application of Kirchoff's Rule by verifying that KCL and KVL are satisfied at each node and loop in the circuit. This means that the sum of currents at each node should be equal to zero and the sum of voltage drops and rises in each loop should also be equal.

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