# Have A Little Question

#### logmode

Joined Sep 5, 2011
48
Hello,
I was working in the worksheets, Kirchhoff's Laws, Question 5: Reveal Answer and found:

Notes:
Nothing here but series-parallel calculation practice, combined with KVL.

So where would I use KVL in question 5?

First of all let me say thank you for these files! They are great stuff!

So let me show you my work and thought.

find: $$\frac{R_3 R_4}{R_3 + R_4}$$ $$\Large= R_3_&_4$$

Then: $$R_T= R_1+R_2+R_3_4+R_5$$

Then: $$E_T/R_T=I$$

Then work out all the voltage drops with I R. I could use KVL to find these voltages as so.

$$26V - 1.9548V - 7.24V - E_3_&_4 - 7.24V = 0$$

Is that what is meant by combined with KVL. Or is there something I do not understand about KVL. If so please explain.

Thank you
logmode

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#### WBahn

Joined Mar 31, 2012
26,074
It looks like the battery is listed as having quite a bit of internal resistance. Don't forget to take that into account. Otherwise, you are on the right track and, yes, I would imagine that the "combined with KVL" means just what you surmised since setting the total of the voltage drops around a loop equal to zero IS what KVL is all about.

Joined Dec 26, 2010
2,148
Looks OK to me: once you have worked out the total resistance of the parallel pair, you have a simple series circuit with only one loop and one voltage source.

As the last poster pointed out though, do not forget to take the battery internal resistance into the total: with the values shown, you will get noticeably different values if you neglect it - due to its resistance, the battery terminal voltage will in fact be quite a bit lower than it's open-circuit value.

#### logmode

Joined Sep 5, 2011
48
Hello, thank you.

I never knew about resistance in the battery. Your files were very helpful. I understand what it is and why now, but how do you know what its going to be?

I know what RT is in the circuit. I also know what rT in the battery is if I know open circuit voltage and load current. How do you find the current if you dont know rT (battery) and you dont have the circuit made yet?

RT = VT / (VT / RT)

r= (E-IR)/I

Thank you
logmode

#### WBahn

Joined Mar 31, 2012
26,074
The internal resistance of a battery is not a fixed number. It varies depending on lots of different things. The age of the battery, the number of times it's been cycled (assuming it's rechargable), the temperature is a BIG factor for most batteries, and even the current draw.

People designing batteries can estimate what the internal resistance will be by relying on lots of data developed over lots of years, some of it theoretical/analytical and much of it empirical. Depending on the battery manufacturer, you can find data and curves that will let you estimate what a particular battery *might* have as its internal resistance under a particular set of circumstances, but the only way to really know is to measure it under conditions, including load, comparable to how you expect to use it.

In practice, if the behavior of your circuit turns out to be very sensitive to the internal resistance of the battery, then it is either a poorly designed circuit or you are using a poorly chosen battery (or both). Which you choose to deal with will depend on the specifics of the situation.