Calculating the voltage at a certain Resistor

[arhzz]

Homework Statement

Calculate the voltage at the Resistor R3 when S1 closses

Relevant Equations

U=RI

Hello!

Consider this circuit;

Now the value are given as follows; U = 10V; R1 = 150 Ohm R2 = 470 Ohm R3 = 330 Ohm;

I am susposed to calculate the voltage at R1 when S1 clossed;so when there is a current flowing through it.

I did that using the loaded voltage divider,since I can see that R2 and R3 are parallel and than in series with R1 so I used the formula;

$$ U2 = U * \frac{R2||R3}{R1+R2||R3} $$ and that should give me that U2 = 5,65 V and since U2 is parallel to U3 the voltages are the same so U3 = 5,65V.

Pretty sure that is correct,at least according to my solutions.

But here is the question I had.Whenever I googled or really though about solving a relatively simple circuit where I need to calculate the voltage at a certain resistor there would always come up the following method.

1. You simplify the circuit as much as possible (parallel,series circuits etc.)
2.Then you find the total Resistance of the circuit.
3.Find the total current flowing through it;
4. Than use Ohm's Law to find the voltage

I tried using that here but it doesn't seem to work,and I don't know why. Here is how I tried.

So first I calculated the total resitance; $R_total = 343,875 Ohm$ R2 and R3 are parallel in series with R1;

Than since I know the voltage (10V) I calculated the current flowing through the circuit; $I = \frac{U}{R_total}$ Should be I = 0,029A

And than I used Ohm's Law to get U3;

U3 = I * R3 = 9,57V

That is not right.So my question is why cannt I use this "method" to calculate the voltage (or voltage drop) across R3,even though this is,if you google it or try looking it up the way that these problems are suggested to be solved.Many thanks!

 

[berkeman]

Homework Statement:: Calculate the voltage at the Resistor R3 when S1 closses
Relevant Equations:: U=RI

And than I used Ohm's Law to get U3;

U3 = I * R3 = 9,57V

You forgot to use the parallel combination here...

 

[FactChecker]

How do you know how much current goes through $R_3$ and how much through $R_2$? You are not using the correct current for $R_3$.
Alternatively, now that you know the current through $R_1$, you can calculate the voltage drop across it and subtract that from 10V to get the remaining voltage drop across the $R_2, R_3$ combination.

 

[arhzz]

Yes of course I need to multiply with the parallel resistance of R2 and R3.It worked obviously.

That with the voltage drop is also a way,thanks for the insight and help!

Cheers!