Solving Series & Parallel Circuits: Calculating Voltage & Resistance

[Shakerhood]

I will try to describe this the best I can the circuit is in a rectangle, the left side has a battery at 18 volts, the top has a resistor in series with R1 = 9.25 Ohms, then it goes to the right side of the rectangle and the are 2 resistors in parallel R2 is on the outside and =3.45 Ohms and R3 is on the inside and = 5.90 Ohms and there is nothing along the bottom side of the rectangle. I hope this makes sense! I need to figure the voltage across all 3 resistors but am confused because it contains Series and parallel and I also need to find Equivalent resistance and the current in this Circuit. Any help is greatly appreciated!

 

[OlderDan]

Start by finding the equivalent resistance of the parallel combination. That combination is in series with the first resistor. That is all you need to find the current coming from the source voltage and through the first resistor. That will enable you to calculate the voltage drop across the first resistor, and the parallel combination. From there you can calculate the current through each of the resistors in parallel.

 

[WhirlwindMonk]

voltage is easy, using loop rule you know that it is equal to the voltage of the battery. Then for the current and equ. resistance, just do what OlderDan said.

 

[Shakerhood]

Start by finding the equivalent resistance of the parallel combination. That combination is in series with the first resistor. That is all you need to find the current coming from the source voltage and through the first resistor. That will enable you to calculate the voltage drop across the first resistor, and the parallel combination. From there you can calculate the current through each of the resistors in parallel.

I would do the parallel first, but I don't know what the voltage will be when it gets to them because it crosses over the series resistor first.

 

[Shakerhood]

voltage is easy, using loop rule you know that it is equal to the voltage of the battery. Then for the current and equ. resistance, just do what OlderDan said.

Not quite sure what you mean, I am running a little slow as I haven't had Physics in 15 years!

 

[OlderDan]

Not quite sure what you mean, I am running a little slow as I haven't had Physics in 15 years!

There are more sophisticated ways of treating circuits than simple series and parallel. You need to use concepts like loop voltages and loop currents for more complicated problems. This problem can be done by just considering series and parallel combinations.

When you calculate the equivalent resistance of two resistors in parallel, you are finding the value of a single resistor that will behave exactly like the parallel combination as far as the rest of the circuit is concerned.

Let's call your series resistor R1, and the two resistors in parallel R2 and R3. If you find the equivalent resistance for R2 and R3, and call it R4, as far as the voltage source and R1 are concerned they are in series with R4. As far as the voltage source is concerned there is some resistor R5 connected to it that is the series combination of R1 and R4. The source has no knowledge of how that resistance is created. By Ohm's law the current flowing from the source is completely determined by the voltage of the source and the total resistance R5 (assuming an ideal source with no internal resistance). All of that current must flow through R1. The voltage drop across R1 can be found from Ohm's law. The voltage drop across R4 is also found by applying Ohm's law using the total current and R4. The sum of the voltage drops across R1 and R4 must and will add up to the source voltage.

Each resistor R2 and R3 experiences the same volage difference as their equivalent resitance R4. Knowing that voltage, you can calculate the current through each of them. The sum of those two currents must and will be the current flowing from the source through R1.