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Need Help With Series-Parallel Ohms Law Circuit Math Problem

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ChainedBoxer

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I'm having a really tough time figuring out this Ohms Law problem. I am working on an electronics home study course and I don't want to submit the wrong answer; so I thought that I would post the problem here for help. I would like, but don't need, the formula listed that you used to get your answers. I know that I should figure this out on my own, and I will, but for now I need to submit this lesson to my school. I have figured out all of the other math questions, but this one problem has had me stuck for weeks now.

Can anyone help me answer these questions?


https://imageshack.us/photo/my-images/266/circuitn.png/


((NOTE: Where the question "What Is the Current Through R1 In Amps?", it should read "R2" instead.))

Known Circuit Values:

Total Voltage: 10 Volts
Resistor 1 - R1 - In Series - Value: 1K
Resistor 2 - R2 - In Parallel With R3 - Value: 4K
Resistor 3 - R3 - In Parallel With R2 - Value: 4K

Questions:

1. What Is The Total Current Equal to In Amps?

2. What Is The Voltage Across R1?

3. What Is The Current Through R3 In Amps?

4. What Is The Current Through R2 In Amps? (I First Listed It As R1 In The Pic, But It Should Be R2).

5. What Is The Current Value Through R1 In Watts?

6. What Is The Total Power Consumed In Watts?

7. What Is The Power Consumed By R3 In Watts
 

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Hint: First find out the combined resistance of R2 and R3 in parallel.
 
Hint: First find out the combined resistance of R2 and R3 in parallel.

Thanks for the hint! So once I figure out R2 and R3, then that part of the circuit will become a Series circuit right? So I will have two Resistors in Series. Then I can follow the Series rules correct? How do I find Current through a Resistor? Like with questions 3 and 5? I'm not 100% sure, but is the Total Current in the circuit 3.33? My workbook doesn't have a choice for 3.33, but they have a 0.0033 though. Am I right?
 
Your answer would be correct if the resistor values were in ohms but they are in k ohms.
 
So I will have two Resistors in Series. Then I can follow the Series rules correct?
Yes.
How do I find Current through a Resistor?
You use Ohm's Law. Let's call the combined resistance of R2 and R3 in parallel 'Rpar'. You now have R1 in series with Rpar, giving a total resistance Rtotal= R1+Rpar. So you divide the supply voltage V by Rtotal to find the current through both R1 and Rpar. Bear in mind that if voltage is in Volts and resistance is in kOhm then current is in mA (your figure of 3.3 above was in mA. 3.3mA=0.0033A).
You can then calculate the voltage across R1, the voltage (call it Vp) across Rpar, and hence knowing Vp you can calculate the individual currents through R2 and R3.
Multiplying the voltage across a resistor by the current through it gives you the power used by the resistor.
 
Last edited:
Yes.
You use Ohm's Law. Let's call the combined resistance of R2 and R3 in parallel 'Rpar'. You now have R1 in series with Rpar, giving a total resistance Rtotal= R1+Rpar. So you divide the supply voltage V by Rtotal to find the current through both R1 and Rpar. Bear in mind that if voltage is in Volts and resistance is in kOhm then current is in mA (your figure of 3.3 above was in mA. 3.3mA=0.00033A).
You can then calculate the voltage across R1, the voltage (call it Vp) across Rpar, and hence knowing Vp you can calculate the individual currents through R2 and R3.
Multiplying the voltage across a resistor by the current through it gives you the power used by the resistor.

When I enter the values for the resistors, do I enter into the calculator 4 or 4000 for the value of the KOhms resistors?
 
It's up to you. If you use 4 the answer will be in mA for current calculation. If you use 4000 it will be in Amps.
 
I think I finally have these figured out. The answers don't match up perfectly each time with the answer choices in my work book, but they are close enough. I'm not sure why they would list choices that don't fit perfectly with the real answers; that is something I need to ask them about. Hopefully these are the final answers.

Here are the choices as listed in the book. I rewrote the actually questions, because the questions in the book were poorly written and hard to understand.

The Questions Rewritten:

1. What Is The Total Current Equal To In Amps?
A - 0.5
B - 2
C - 0.0033
D - 3

2. What Is The Voltage Across R1?
A - 2
B - 3.3
C - 6
D - 1.5

3. What Is The Current Through R3 In Amps?
A - 3
B - 0.5
C - 1.65
D - 0.167

4. Total Current Through R2 In Amps?
A - .00165
B - 0.5
C - 0.333
D - 0.167

5. What Is The Current Value Through R1 In Amps?
A - 2
B - 3.3
C - 0.333
D - 0.167

6. What Is The Total Power Consumed In Watts?
A - 1.5
B - 3
C - .033
D - 0.01

7. What Is The Power Consumed By R3 In Watts?
A - 0.5
B - 1.5
C - .01089
D - 5


The Answers:


1. C - 3.33m
2. B - 3.33
3. C - 1.67m - The Book Has 1.65m, Which is Close Enough.
4. A - 1.67m - The Book Has .00165, Which Is Close Enough.
5. B - 3.33
6. C - 33.3m
7. C - 11.1m - The Book Has .01089, Which Is Close Enough.


Thanks to those who helped me out.
 
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There were no multiple choice answers when I was a student.

For a question like this one, we had to check our answers by techniques such as checking whether Kirchoff's laws were satisfied.

Nor did we have the internet to ask for advice. We had to advise ourselves or seek it from our fellow students.

Judging by the home work questions in this forum & other info, I have a distinct feeling that the standard of education has gone down hill.

what do others think?
 
That problem can be generally analyzed by loop equations or simple applications of series and parallel circuits.

I = V/R, so if I is in mA and R is K ohms, the problem works out because of the implicit conversions. Ohms law assumes that R is in ohms; I is in Amps and V is in Volts.
 
I figured out the right answers. It took me a bunch of attempts but I found the right answers. The problem is the right answers don't match the choices given. So I had to use Google to find the numbers that match the most closely, which is not how a test should be given. I'm still waiting to hear back from the school on why the test and its answers were listed this way. I got a 95, but should have gotten a 100. The question that I failed had numbers that didn't match the answer and the closest value was the choice that I picked. I don't know who wrote the test, but they did it all wrong. We shouldn't have to pick the answer that most closely matches the right answer, because that type of conversion isn't even taught in the book. So the choices and answers should match up perfectly. I just feel bad for all those students that failed parts of the test,because of this problem.
 
Len -- this is what happens when:

A) Kids replace seasoned workers because they're willing to work cheap,
B) You buy a poorly-designed online class instead of being in a classroom where you can raise your hand,
C) Training manuals are written by people who shouldn't be writing training manuals, and
D) Education is treated as a bottom-budget afterthought rather than seen as critically important to ALL our futures.

Yes, this is (really) simple Ohm's law and middle-school math. It's not complicated. But if you don't have some decent foundation in what it's about, it's all Greek, and it sounds like that's what this student is up against (also possible he just didn't do the reading assignment).

I'll bet he gets a great score in Call To Action III, though.
 
No question about it. When you get freshmen entering college who are incapable of even doing long division, what do you expect. We have to spend most of our time teaching remedial subjects rather than what we should be doing. 50 years ago, we didn't have this problem.

It's a vicious circle. Students leave high school with extremely limited math knowledge. Some of those want to be teachers, but they aren't prepared to study complex technical subjects. Therefore, they spend much of their college time in remedial courses, and they aren't prepared to teach math in high school. So their students leave high school with extremely limited math knowledge...
 
ljcox,

The New, New Math

I purchased a burger at Burger King for $1.58. The counter girl took my $2 and I was digging for my change when I pulled 8 cents from my pocket and gave it to her. She stood there, holding the nickel and 3 pennies, while looking at the screen on her register. I sensed her discomfort and tried to tell her to just give me two quarters, but she hailed the manager for help. While he tried to explain the transaction to her, she stood there and cried... Why do I tell you this? Because of the evolution in teaching math since the 1960s:


1. Teaching Math In 1960s (when I was in school)

A logger sells a truckload of lumber for $100. His cost of production is 4/5 of the price. What is his profit ?


2. Teaching Math In 1970s

A logger sells a truckload of lumber for $100. His cost of production is 4/5 of the price, or $80. What is his profit?


3. Teaching Math In 1980s
A logger sells a truckload of lumber for $100. His cost of production is $80. Did he make a profit ? Yes or No


4. Teaching Math In 1990s

A logger sells a truckload of lumber for $100. His cost of production is $80 and his profit is $20. Your assignment: Underline the number 20.


5. Teaching Math In 2000s

A logger cuts down a beautiful forest because he is selfish and inconsiderate and cares nothing for the habitat of animals or the preservation of our woodlands. He does this so he can make a profit of $20. What do you think of this way of making a living? Topic for class participation after answering the question: How did the birds and squirrels feel as the logger cut down their homes? (There are no wrong answers, and if you feel like crying, it's ok).


6. Teaching Math In 2010

Un hachero vende una carretada de madera para $100. El costo de la producciones es $80. Cuanto dinero ha hecho?

ANSWER: His profit was $375,000 because his logging business is just a front for his pot farm.

Ratch
 
5. Teaching Math In 2000s

A logger cuts down a beautiful forest because he is selfish and inconsiderate and cares nothing for the habitat of animals or the preservation of our woodlands. He does this so he can make a profit of $20. What do you think of this way of making a living? Topic for class participation after answering the question: How did the birds and squirrels feel as the logger cut down their homes? (There are no wrong answers, and if you feel like crying, it's ok).
BaAaaawwww !!! Because logger is Unemployed Engineer...Computer has replaced him ;_;
 
Judging by the home work questions in this forum & other info, I have a distinct feeling that the standard of education has gone down hill.

what do others think?
I think the current generation of students and professors are being ruined by the pervasive world of computer simulations and computers in general doing all the thinking for them. It disconnects their minds from the reality of the circuit and means they never acquire the ability to do simple problems in their heads as we had to do when working in the lab. They also never develop the "common sense baselines" that every engineer needs to know when a computer has given them an answer that just can't be correct.
 
There were no multiple choice answers when I was a student.
For a question like this one, we had to check our answers by techniques such as checking whether Kirchoff's laws were satisfied.
Nor did we have the internet to ask for advice. We had to advise ourselves or seek it from our fellow students.
There is so much information to learn right now in 4 year engineering course , that one cannot concentrate on 1 subject .. we have to do lot of Cram up !
 
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