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How to resolve a DC Theory problem or a Newbies Quandary

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Inquisitive

Super Moderator
This is a problem from a book that I'm working with. So this would be considered property of the rightful owner with respect to copyright. The book is All New Electronics, by Harry Kybett, Earl Boysen, published by Wiley Publishing. Page 32, question 8.

Attached is a copy of the circuit schematic

The question I'm having a bit of trouble with states.

VS=12V, I=300mA, R1= 50ohms

Find R2= ____, P1= ____

The correct answer according to the text is R2= 200ohms, P1=2.88 W

I have Ohms Law pie chart. But this is just not obvious to me.

How did they extract R2 and P1?

Could somebody step me through the logical approach to this scenario?

Thank you, for any and all help in explaining this to me.
 

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  • Figure 1 - 21.GIF
    Figure 1 - 21.GIF
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It is stated that the total current draw is 300mA. First thing you must do it find how much current R1 pulls.

Take a look at your Ohms law chart.
You know the Voltage to be 12V, and R1 is 50Ω.

Current would then be calculated as I = V/R = 12/50 = 0.24A or 240mA.
This would mean that R2 is pulling the last 60mA.

To find the value of R2: R = V/I = 12/0.06 = 200Ω

As for P1 which is apparently Power through R1: P = V*I = 12 * 0.24 = 2.88W


All you need is two values to find the third.
 

Inquisitive

Super Moderator
Thanks for the explaination.

I see where I went wrong. My answer was .24 and I did not convert it to mA.

It not always clear to me whether the answer is in A or mA. Guess that comes with repetative drill
 

carbonzit

Active Member
It not always clear to me whether the answer is in A or mA. Guess that comes with repetative drill

If it's not clear to you, it probably won't be to anyone else, either. So you've learned that you need to be explicit with your quantities, specifying either 0.24A or 240mA. And yes, it comes with practice.
 
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