# Homework Help: Help with circuit

1. Feb 26, 2014

### britt

1. You are part of a team designing a go-cart. A member of the team has given you a picture of the lighting for go-cart. Your mission is to analyze the circuit. The team has not yet decided on what lights or batteries to use yet, so you need to do the analysis using variables (into which numbers will be plugged once they are decided). In the picture below five light bulbs are connected to two batteries. Treat the batteries as sources of potential difference (∆V#) and the light bulbs as resistors (R#).

A) How many different currents are in this circuit? Draw where they are.

B) How does the potential difference across R4 compare to that across R3?

C) Determine the magnitude and direction of each current (in terms of R# and ε#). Suggestion: use Kirchhoff’s rules and Ohm’s law.

D) What is the total power dissipated in the circuit? E) What answers change if light bulbs R2 and R5 are swapped?

2. Relevant equations
Kirchhoffs Rules

3. The attempt at a solution

B) 5 currents

C) They are equal but have opposite signs

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2. Feb 26, 2014

### haruspex

You mean A). How do you get 5? Are you proposing a different current through each resistor? Can you see that some must be the same?
You mean B). Whether they have the same or opposite signs depends which way round you measure them. Since you are not told which way to measure them, the potential differences should be considered unsigned. But out of interest, let's say we measure both from bottom to top in the diagram; what is your answer now?

3. Feb 26, 2014

### britt

A) yes I assigned an I to each 5 resistors. Would the series ones be the same? R1 R2 and R5

B) I3=I4 so I34/2

4. Feb 26, 2014

### Staff: Mentor

The question asks for a comparison of potential differences.

5. Feb 26, 2014

### britt

R3i3=R4i4

6. Feb 26, 2014

### britt

1/R3 + 1/R4 = 1/R34 -- does this have anything to do with it?

7. Feb 26, 2014

### Staff: Mentor

Your first answer was "They are equal but have opposite signs" and haruspex questioned this. Now that you've had a chance to review your answer, would you express it any differently?

The answer will be in words.

8. Feb 26, 2014

### britt

The potential difference across both are equal?

9. Feb 26, 2014

### haruspex

Yes.
Yes. What about the current in the wire connecting the batteries?

10. Feb 26, 2014

### Staff: Mentor

Correct. Since the two are connected between the same two points, the voltage across each is equal.

11. Feb 26, 2014

### britt

C) don't all currents go like
_____>
| | |
^ ^ |
| | |
<___V

12. Feb 26, 2014

### britt

help!

13. Feb 26, 2014

### haruspex

Yes, those are the current directions.
Scratch that question - I see you answered it already. So yes, the currents through r1, r2, and r5 are all the same. Call it I. If the currents through R3 and R4 are I3 and I4, what two equations can you write down relating them to each other and to I?

14. Feb 27, 2014

### britt

I34=I3I4R3R4 / R34

I3=I4

Not sure about how to relate them

I = I3 + I4 ???

Last edited: Feb 27, 2014
15. Feb 27, 2014

### britt

C) ε2-(i4 / r4 + i3 / r3) +ε1-ir=0

d) p= iv = (i+(i3 + i4))(ε12)

e) none would change assuming r2 and r5 have the same resistance

16. Feb 27, 2014

### Staff: Mentor

wrong

wrong

p=iv is a good start.

What if R2 and R5 had unequal resistance?

Give some more thought to these.