# Kirchhoff's Law Problem -- Resistors in Series and Parallel and a Switch

1. Apr 22, 2016

### DevonZA

1. The problem statement, all variables and given/known data

2. Relevant equations
Rparellel=1/R1+1/R2...
IR1=R2/R1+R2 x I

3. The attempt at a solution

I am unsure of how to answer d) and e) using KVL because I count 4 junctions?
Where should I start?

2. Apr 22, 2016

### BvU

Check your first equation ! It's not for Rpalrallel but for 1/=Rpalrallel !

And the 20 and 25 Ohm resistors are NOT parallel to e.g. the 12 Ohm !

3. Apr 22, 2016

### DevonZA

Yes sorry that was a typo.
The total resistance for the parallel resistors still equals 15.18ohms but yes I should have done them separately:
(1/12+1/10+1/16)^-1 = 4.07ohms
(1/20+1/25)^-1= 11.11ohms

Do my values look correct?

4. Apr 22, 2016

### BvU

I only know $${1\over 12} + {1\over 10}+ {1\over 16}+ {1\over 20}+ {1\over 25} \ne 15.82$$

5. Apr 22, 2016

### DevonZA

(1/12+1/10+1/16+1/20+1/25)^-1=15.18 ohms

Yes values are answers in the book

6. Apr 22, 2016

### BvU

Then why ask if the values are correct ?

7. Apr 22, 2016

### BvU

No. $${1\over 12} + {1\over 10}+ {1\over 16}+ {1\over 20}+ {1\over 25} = {1\over 2.98}$$However,$${1\over {1\over 12} + {1\over 10}+ {1\over 16} } + {1 \over {1\over 20}+ {1\over 25} } = 4.07 + 11.11 = 15.18$$

Last edited: Apr 22, 2016
8. Apr 22, 2016

### DevonZA

9. Apr 22, 2016

### SammyS

Staff Emeritus
What sort of device is R7 that its resistance changes depending on whether the switch is opened or closed

10. Apr 22, 2016

### BvU

The parts d and e are a bit weird, because basically you use KVL to calculate R7 in a and b.
And it wouldn't be proof, just showing.
I should think they want you to show that 1.2 A * (4.07 + 60 + 11.11 + 24.82) Ohm = 120 V

11. Apr 22, 2016

### CWatters

+1

Although I prefer to make the voltages around the loop sum to zero.

12. Apr 22, 2016

### SammyS

Staff Emeritus
@DevonZA ,

Are you saying that this problem is from a textbook?

What book? Who is the author ?

Last edited: Apr 22, 2016
13. Apr 23, 2016

### DevonZA

Can you show this?

14. Apr 23, 2016

### DevonZA

Hi Sammy this is from our varsity study guide. The author is most likely the lecturer who doesn't respond to me hence why I must ask these questions on here.

15. Apr 23, 2016

### CWatters

With reference to the equivalent circuit below... KVL says that going around a loop the voltages sum to zero.

If I arbitrarily choose to start at the -ve terminal of the battery and go around clockwise we have to prove that..

+120 + (-V1) + (-V2) + (-V3) + (-V4) = 0

V1 = 1.2 * 4.07 = 4.884V
V2 = 1.2 * 60 = 72V
V3 = 1.2 * 11.11 = 13.332V
V4 = 1.2 * 24.82 = 29.784V

Substitute..

+120 - 4.884 - 72 -13.332 - 29.784 = 0

16. Apr 23, 2016

### DevonZA

Thank you for the nice clear explanation CWatters