Calculate current through resistors

[Andrei0408]

Homework Statement

Find the currents I1 and I2 through the resistors R1 and R2

Relevant Equations

U = I*R

I'm trying to grasp the basics here, we know Us = 70 V and I want to find out the currents I1 and I2 through the resistors R1, respectively R2. I just need the steps explained, not necessarily the results, please. Thanks in advance!

 

[berkeman]

Can you write the KCL equation for that middle node where all 3 resistors come together? There's an easier way to do this, but it's good to be sure you understand the basics of using KCL equations to solve for circuit node voltages...

 

[DaveE]

Have you studied the Kirchhoff voltage and current laws yet?
https://www.electronics-tutorials.ws/dccircuits/dcp_4.html

If you set V as a variable could you find the current through R1 given V and Us? What about R2?

 

[Andrei0408]

I think this is it, right?

[Mentor Note -- Added improved version of dark picture]

 

[willem2]

I think this is it, right?

It's a bit hard to read, but I Think it's correct.
Now, if you knew V, you could compute all those currents, so you can use that equation to find V.

 

[berkeman]

I think this is it, right?

[Mentor Note -- Added improved version of dark picture]View attachment 279526

I touched up your dark picture for you. To type your work into the PF, please see the "LaTeX Guide" link below the Edit window. That will help you to post your equations and work in a legible way, and make it easier for us to help you. Thanks.

 

[Andrei0408]

Sorry about the picture, could you tell me how to find V from this equation? I'm thinking about Ohm's law but I don't know the voltage through the 2 resistors

 

[berkeman]

could you tell me how to find V from this equation?

Have you used KCL before? As you indicated, you write the equations for each node, with the sum of the currents leaving the node equal to zero.

In the circuit you have above, label the unknown voltage of the center node $V_n$ and then for each branch, you write the branch current in the form:

$$\frac{V_n - V_x}{R_x}$$

where $V_x$ is the voltage of the node on the far side of the resistor $R_x$. There are 3 branches leaving that node in your circuit, so you add each of those currents up and set the sum equal to zero. Then just solve the equation for the voltage $V_n$ and that determines all of the currents.

Give that a try?

 

[DaveE]

Khan Academy has some really good examples of DC circuit analysis using KVL, KCL, and ohms law. Like this one, for example:
https://www.khanacademy.org/science.../ee-application-of-the-fundamental-laws-setup