Calculating the voltage at a certain Resistor

In summary, when the switch is closed, current flows through the circuit and the voltage is calculated using Ohm's Law.
  • #1
arhzz
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Homework Statement
Calculate the voltage at the Resistor R3 when S1 closses
Relevant Equations
U=RI
Hello!

Consider this circuit;

ce se rasprsnem.png


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!
 
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  • #2
arhzz said:
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...
 
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  • #3
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.
 
Last edited:
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  • #4
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!
 
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Related to Calculating the voltage at a certain Resistor

1. How do you calculate the voltage at a certain resistor?

To calculate the voltage at a certain resistor, you can use Ohm's Law which states that voltage (V) is equal to the current (I) multiplied by the resistance (R), or V=IR.

2. What units are used to measure voltage?

Voltage is measured in units of volts (V) which represents the potential difference or electrical pressure between two points in a circuit.

3. How do you determine the resistance of a resistor?

The resistance of a resistor can be determined by using a multimeter, which measures the flow of current through the resistor and displays the resistance in units of ohms (Ω).

4. Can the voltage at a resistor change?

Yes, the voltage at a resistor can change depending on the current flowing through it and the resistance of the resistor. If the current or resistance changes, the voltage will also change accordingly.

5. What is the relationship between voltage and resistance?

There is a direct relationship between voltage and resistance, meaning that as the resistance increases, the voltage also increases. This is because a higher resistance means there is more opposition to the flow of current, resulting in a higher voltage needed to maintain the same current.

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