I have a homework assignment with confusing combination circuit of parallel and series. I would appreciate any help solving. My voltage is 55V and resistors are R1=190 R2=140 R3=290. Thanks.

 

1) What is it you are trying to solve for?
2) What type of course is this for?
3) What methods have you been introduced to?
4) What is it that you find confusing?
5) What attempts have you made to solve it and how far have you gotten?

 

1) Voltage of 1, 2, and 3. Total resistance. As well as total current and current of 1, 2, and 3.
2) A Electrostatics and Circuits course.
3) Methods such as Ohm's law but nothing else.
4) How to merge the different methods of finding these answers for parallel and series.
5)I have made multiple attempts at figuring it out and have only figured out the total resistance so far.

 

Well, if you know the total resistance, than you can use that to find total current in the circuit.

From here, try to remember that resistors in parallel share the same voltage.

 

What value did you get for the total resistance?

What did you get for the total current?

What is the voltage across R3?

What do you know about the current in R1 compared to the current in R2?

What do you know about the voltage across R1 and the voltage across R2, in relation to the battery voltage?

 

1) 154.3548387
2) .3563218391A
3) 55
4) I don't really understand I know that the current is R1 is larger than the current in R2
5) I know they should add up to equal the total voltage

 

1) 154.3548387
2) .3563218391A
3) 55
4) I don't really understand I know that the current is R1 is larger than the current in R2
5) I know they should add up to equal the total voltage

1) Correct (but don't leave off the units, which are ohms).
2) Correct
3) Correct (but don't leave off the units, which are volts).

4) Why do you think the current in R1 is greater than the current in R2? Remember, current is the flow of electrons past a point in a circuit. So imagine that you take a really big magnifying glass and can see the electrons flowing along the wire that connects R1 and R2. Since I am talking about electrons, we know that the electrons flow from negative to positive and you are saying that the number of electrons, per second, flowing up out of R2 is smaller than the number of electrons, per second, flowing on into R1. Where are the rest of the electrons flowing into R1 coming from?

5) Correct.

New questions:

1) What is the current flowing in R3?
2) Given your total current of 0.356A and your answer to the previous question, how much current is coming out of the battery that is NOT going into R3?
3) Where must the missing current from the previous question be going?

 

1) .1896551724A
2) .1666666666
3) Through resistors 1 and 2

 

1) .1896551724A
2) .1666666666
3) Through resistors 1 and 2

Don't forget the follow-up questions about #4 that I asked in the last post. Those are the key to where you are having problems.

You answer to #1 and #2 above are correct. But, again, get in the habit of carrying your units throughout your work.

You do not need to carry every digit that your calculator spits out. Generally, it is considered sufficient to give answers to three significant digits, so the answers to the first two questions should be give as:

1) 0.1897A
2) 0.1667A

Note that a leading 1 is typically not counted as a "significant digit".

Now, even though you still need to say why you think the current in R1 will be greater than the current in R2, let's assume that the missing 0.1167A is going through both R1 and R2.

4) What will the voltage across R1 be?
5) What will the voltage across R2 be?
6) How do these voltages compare to your answer to question #5 in Post #6?

NOTE: I should have kept numbering my questions consecutively instead of starting over with #1 in each new set. Sorry for the confusion it might be causing. My bad and we just have to deal with it.

 

Follow up to #4: Actually the current is equal in R1 and R2 one is not larger than the either.

4) 31.667V

5) 23.334V

6) They add up to equal 55

 

Congratulations, you have solved the entire problem! Perhaps not in the easiest or most elegant way, but at the end of the day any method to a valid solution is a valid method.

Now consider any insight you might have gained and how you might have tackled the problem from scratch a little bit more easily.

 

I would have responded much sooner but I did not see there was a second page so I did not know you responded. I would like to thank you so much for the help through your unique teaching process. I really appreciate all the help. This is a worksheet for my 8th grade Science class.

 

It was, truly, a pleasure working with you.

The method I used is known as the Socratic Method and involves answering questions with questions designed to focus the learner's own thinking on specific points in order for them to find their own answers. You might have noticed that I (don't think I) answered a single one of your questions or really told you specifically what to do. I merely asked questions that, in my view, you already had all the information you needed in order to solve the next piece of the puzzle. Once you came up with the next piece, you then had all the pieces to solve another piece. You did all the work.

The Socratic Method is often claimed to be the most powerful method for teaching, particularly for teaching critical thinking. But it requires not only a willing teacher, but a willing learner. You were. So many aren't.

Hope to see you around the board some more!