# Parallel Circuit Question

#### DJS0206

Joined Dec 4, 2020
3
How do you figure out the total resistance in a parallel circuit given the following information:

voltage: 150VDC
IR3: 2.5 amps
R1: 20 ohms
R2: 30 ohms
R3: ? (Not provided)

#### Tonyr1084

Joined Sep 24, 2015
6,467
That's easy. Volts divided by amps equals resistance. Then apply the known required resistance then calculate for the missing resistance.

#### Tonyr1084

Joined Sep 24, 2015
6,467
After having run the numbers - I don't believe your resistors are in parallel. I bet you mean to say "In Series".

Parallel resistors will ALWAYS have a lower resistance than the smallest resistor. For instance, two 100Ω resistors in parallel will have a resistance of 50Ω. Even if one of those two resistors are in the meg ohm range, the resistance will be less than the smallest, 100Ω resistor. So the problem as you present it is impossible. Unless I got something horribly wrong.

#### djsfantasi

Joined Apr 11, 2010
7,936
How are R1, R2, and R3 wired! Are all three resistors in parallel with each other?

#### WBahn

Joined Mar 31, 2012
26,398
After having run the numbers - I don't believe your resistors are in parallel. I bet you mean to say "In Series".

Parallel resistors will ALWAYS have a lower resistance than the smallest resistor. For instance, two 100Ω resistors in parallel will have a resistance of 50Ω. Even if one of those two resistors are in the meg ohm range, the resistance will be less than the smallest, 100Ω resistor. So the problem as you present it is impossible. Unless I got something horribly wrong.
Why is the problem impossible. He is given the current in the unknown resistance, not the total current. The key is in recognizing what the voltage is across that resistance if they are all in parallel (as indicated by the thread title).

#### djsfantasi

Joined Apr 11, 2010
7,936
• LymanT

#### WBahn

Joined Mar 31, 2012
26,398

#### djsfantasi

Joined Apr 11, 2010
7,936
Why is the problem impossible. He is given the current in the unknown resistance, not the total current. The key is in recognizing what the voltage is across that resistance if they are all in parallel (as indicated by the thread title).
Which implies that the values of R1 and R2 are irrelevant... so why give them?

#### WBahn

Joined Mar 31, 2012
26,398
Which implies that the values of R1 and R2 are irrelevant... so why give them?
To see if the student can recognize that they are irrelevant to the question asked.

#### djsfantasi

Joined Apr 11, 2010
7,936
And what if the TS is confused as to what parallel (or serial) means? Your previous comment (and my reply) only makes sense if the resistors are wired in serial.

#### djsfantasi

Joined Apr 11, 2010
7,936
To see if the student can recognize that they are irrelevant to the question asked.
I considered that as well...

#### WBahn

Joined Mar 31, 2012
26,398
And what if the TS is confused as to what parallel (or serial) means? Your previous comment (and my reply) only makes sense if the resistors are wired in serial.
The problem makes perfect sense if they are wired in parallel (or in series). You can solve it either way, you just get different values for the resistance since the voltage across it is different in each case.

• djsfantasi

#### DJS0206

Joined Dec 4, 2020
3
After having run the numbers - I don't believe your resistors are in parallel. I bet you mean to say "In Series".

Parallel resistors will ALWAYS have a lower resistance than the smallest resistor. For instance, two 100Ω resistors in parallel will have a resistance of 50Ω. Even if one of those two resistors are in the meg ohm range, the resistance will be less than the smallest, 100Ω resistor. So the problem as you present it is impossible. Unless I got something horribly wrong.
How do you figure out the total resistance in a parallel circuit given the following information:

voltage: 150VDC
IR3: 2.5 amps
R1: 20 ohms
R2: 30 ohms
R3: ? (Not provided)

#### DJS0206

Joined Dec 4, 2020
3
I attached the picture of the question. The possible answer choices are
2250 watts
1500 watts
375 watts

#### Tonyr1084

Joined Sep 24, 2015
6,467
OK, that's a different set of circumstances and a different question. I will get back to this shortly, I'm busy with window treatment right now. But knowing the current and voltage through a given resistor you should be able to calculate the resistance. Give me a while. Or someone else may help you. It's not difficult.

#### Tonyr1084

Joined Sep 24, 2015
6,467
OK, back.

First, we know the voltage and we know the current through R3. Divide voltage by current to get resistance. Then apply the rules for calculating the total resistance of R1, R2 and R3 to ascertain the overall current. Once you know the total current you multiply that by the voltage to ascertain the wattage.

I can do the math for you but then what do you learn?

Show me your work and I'll let you know if you've gotten it right or not.

#### Tonyr1084

Joined Sep 24, 2015
6,467
voltage: 150VDC
IR3: 2.5 amps
Why is the problem impossible. He is given the current in the unknown resistance, not the total current.
I missed IR3 being 2.5A. After realizing that from the full diagram and full question I was able to figure it out. I was assuming 2.5A through the entire circuit. My mistake. If the total current were 2.5A then there would be no resistor value large enough to account for the numbers. That was why I thought initially that the resistors could not be parallel. Sorry for adding to the confusion.

• djsfantasi

#### Tonyr1084

Joined Sep 24, 2015
6,467
Questions for DJS0206
1) What did you get for the value of R3?
2) What is the total current through the entire circuit?
3) What wattage did you calculate?

#### WBahn

Joined Mar 31, 2012
26,398
I attached the picture of the question. The possible answer choices are
2250 watts
1500 watts
375 watts
You actually don't need to find what R3 is at all.

Go back to my Post #5. What is the voltage across R3?

If you have a voltage drop and you know the current that is flowing through that voltage drop, can you find the power associated with that current flowing through that voltage drop.

Given the available answers, you can figure out the correct one without even doing that. What is the power being dissipated in R1 and in R2. What does that tell you about which of those three answers is even possible?

• Kjeldgaard