Ohm's law question

Thread Starter

Kid347

Joined Aug 14, 2015
70
I am trying to teach myself electronics. I am currently reading/studying a book Complete Electronics Self-Teaching Guide. I am getting stuck on a bunch of Ohm's law questions, one of which says Vs = 12 Volts, I = 300mA, R1 =50 Ohms. It is in a parallel circuit with Vs, I, R1, R2, and I1, I2. I thought I would have divided 12 Volts by 300 mA I get 40. The answer in the book says that R2 should be 200 Ohms and P1 2.88 Watts. I can't seem to get these answers. Can someone help me out. Thank You in advance.
 

shteii01

Joined Feb 19, 2010
4,667
I am trying to teach myself electronics. I am currently reading/studying a book Complete Electronics Self-Teaching Guide. I am getting stuck on a bunch of Ohm's law questions, one of which says Vs = 12 Volts, I = 300mA, R1 =50 Ohms. It is in a parallel circuit with Vs, I, R1, R2, and I1, I2. I thought I would have divided 12 Volts by 300 mA I get 40. The answer in the book says that R2 should be 200 Ohms and P1 2.88 Watts. I can't seem to get these answers. Can someone help me out. Thank You in advance.
Lots of words. Lots of numbers. Not at all organized.
 

Thread Starter

Kid347

Joined Aug 14, 2015
70
20150831_194212.jpg

here is the schematic. I had to take a picture on my phone, please let me know if you can see it. Thank You.
 

shteii01

Joined Feb 19, 2010
4,667
40 Ohm is the total resistance of the circuit.

They are not asking you for total resistance.

They are asking you for R2.
 

Thread Starter

Kid347

Joined Aug 14, 2015
70

MikeML

Joined Oct 2, 2009
5,444
So this is not an Ohm's Law question, rather a Kirchoff Current Law problem. Write an equation which describes the sum of current either entering or leaving the top center node (the one where the positive pole of Vs connects to R1 and R2.
 

shteii01

Joined Feb 19, 2010
4,667
Total resistance is not given, only R1 which is 50 ohms.
You just calculated the total resistance.

You have 300 mA leaving the voltage source. That is the total current of the whole circuit. Apply Ohm's Law: V=IR. You know V. You know I. Solve for R, you get total resistance of the circuit.
 

Thread Starter

Kid347

Joined Aug 14, 2015
70
You just calculated the total resistance.

You have 300 mA leaving the voltage source. That is the total current of the whole circuit. Apply Ohm's Law: V=IR. You know V. You know I. Solve for R, you get total resistance of the circuit.
This is where I get confused, Vs = 12 volts, I = 300 mA, so 12/.300 = 40. but the answer the book gives is R2 = 200 ohms ? I just don't get it.
 

shteii01

Joined Feb 19, 2010
4,667
This is where I get confused, Vs = 12 volts, I = 300 mA, so 12/.300 = 40. but the answer the book gives is R2 = 200 ohms ? I just don't get it.
40 Ohm is TOTAL RESISTANCE of the circuit.

R2 IS NOT total resistance of the circuit. Therefore R2 is not 40 Ohm.
 

Thread Starter

Kid347

Joined Aug 14, 2015
70
40 Ohm is TOTAL RESISTANCE of the circuit.

R2 IS NOT total resistance of the circuit. Therefore R2 is not 40 Ohm.
I agree, but is my calculation correct for total resistance? I don't think it is if the answer in the book is R2 = 200 ohms. do you know the correct calculation to get the answer?
 

shteii01

Joined Feb 19, 2010
4,667
I agree, but is my calculation correct for total resistance? I don't think it is if the answer in the book is R2 = 200 ohms. do you know the correct calculation to get the answer?
I got 40 Ohm for total resistance (Rtotal). What do you have?
 

WBahn

Joined Mar 31, 2012
24,674
R2 is NOT the total resistance. So if you do a calculation that yields the total resistance, is should not be surprising that this doesn't match R2.

Ohm's Law related the voltage across THAT resistance to the current through THAT resistance.

The 300 mA is the total current flowing in the total, effective resistance due to the 12 V source. The total effective resistance is the parallel combination of R1 and R2.

Because they are in parallel, you know that R1 and R2 each have 12 V across them. How much current is flowing in R1? If you know that the total current is 300 mA and you know the current flowing in R1, what is the current flowing in R2? If you know the voltage across R2 and the current flowing in R2, what is the value of R2?
 

Thread Starter

Kid347

Joined Aug 14, 2015
70
I got 40 Ohm for total resistance (Rtotal). What do you have?
I also came up with 40 ohm total, but I think that is wrong because the answer in the book is R2 = 200 ohms. I don't know how the total ohm's can be 40 if R2 is 200 ohms.
 

shteii01

Joined Feb 19, 2010
4,667
I also came up with 40 ohm total, but I think that is wrong because the answer in the book is R2 = 200 ohms. I don't know how the total ohm's can be 40 if R2 is 200 ohms.
R2 is not total resistance.

Do you know the formula for finding total resistance of resistors in parallel?
I posted it in reply #9: http://forum.allaboutcircuits.com/threads/ohms-law-question.115001/#post-896771

That formula has 3 pieces, Rtotal, R1, R2. To find any one piece you need to know other two pieces. In your case you know Rtotal=40 Ohm and R1=50 Ohm. Now plug them into formula and find R2.
 
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