Simple Combination Circuit, Find Current

In summary, you need to subtract the voltage lost across the resistors when calculating the resistance of a component.
  • #1
blue_lilly
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Homework Statement


Combinations of Resistors

Im not sure if the image will show up. If the image doesn't show up; it is similar to this picture http://www.ceb.cam.ac.uk/data/images/groups/CREST/Teaching/impedence/sparal6.gif
In the link's picture R1 is in the correct spot, R2 is R6-5-4 and R3 is R3-2. Sorry if there is any confusion.

Given the circuit shown below, the power supply is set to a voltage V=18.3 V, R2 = 33.2 Ω, and R3 = 46.5 Ω. Suppose you place an ammeter into the circuit and find that the current through R2 is 0.263 A. Answer the following questions:

A)What is the voltage across resistor R2? 8.74 V This is correct
B)What is the current through resistor R3? 0.188 A This is correct
C)What is the current flowing through R1? 0.451 A This is correct

D)What is the resistance of resistor R1? I NEED HELP ON THIS ONE

Homework Equations


V2 = R2 * I2
I3 = V3 / R3
I1(series)= I2 + I3 (Parallel)
R1 = V1 / I1

The Attempt at a Solution


A) V2 = R2 * I2 = 33.2 *.263 = 8.74
Voltage going through resistor 2 is 8.74 V

B) I3 = V3 / R3 = 8.74/46.5 = .188 A
Current going through resistor 3 is .188 A

C) I1(series)=I2 + I3 (parallel) = I1 = .188 + .263= .451 A
Current going though resistor 1 is .451 A

D) THIS IS THE PART I NEED HELP WITH!
R1 = V1 / I1 = 18.3/.451 = 40.57 ohms

This answer is not correct and I am not sure why. I thought that the way to find resistance was R= V/I. I solved for the correct current in the previous problem and we are given that V= 18.3 V in the wording of the problem.

Any help would be greatly appreciated!
 
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  • #2
Your image is a bit hard to see, given that it's blue lines on a black background (for my viewer at least). So here I've rendered it as I understand it (note that you can upload gif files if you use the Advanced editing panel):

attachment.php?attachmentid=66634&stc=1&d=1392426211.gif


Note that your voltage V1 = 18.3 V is not across R1 alone. Try writing KVL around a loop with R1 in it.
 

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  • #3
gneill said:
Your image is a bit hard to see, given that it's blue lines on a black background (for my viewer at least). So here I've rendered it as I understand it (note that you can upload gif files if you use the Advanced editing panel):

attachment.php?attachmentid=66634&stc=1&d=1392426211.gif


Note that your voltage V1 = 18.3 V is not across R1 alone. Try writing KVL around a loop with R1 in it.

So, the voltage that they gave is for the total circuit then?
If that's the case then I can subtract the V1(8.74) and V2(8.74) from it.
Vtot = V1 + V2 + V3
18.3 = V1 +8.74 + 8.76
V = .82 volts going though R1

Then would you take R1 = V1 / I1 = .82/.451 = 1.818 ohms ?
 
  • #4
blue_lilly said:
So, the voltage that they gave is for the total circuit then?
If that's the case then I can subtract the V1(8.74) and V2(8.74) from it.
Vtot = V1 + V2 + V3
18.3 = V1 +8.74 + 8.76
V = .82 volts going though R1

Then would you take R1 = V1 / I1 = .82/.451 = 1.818 ohms ?

No, that's not correct.

Okay, first, voltage does not go "through" a component. Voltage is across components. It's the potential difference from one end to the other. Second, the voltages across parallel components are the same voltage. You don't sum them! They are not in series.

So when I suggested that you write KVL for a loop including R1, you cannot have both R2 and R3 in that loop, you can only have one or the other. Try tracing a loop through the voltage source and R1...
 
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  • #5
gneill said:
No, that's not correct.

Okay, first, voltage does not go "through" a component. Voltage is across components. It's the potential difference from one end to the other. Second, the voltages across parallel components are the same voltage. You don't sum them! They are not in series.

So when I suggested that you write KVL for a loop including R1, you cannot have both R2 and R3 in that loop, you can only have one or the other. Try tracing a loop through the voltage source and R1...

I think I get it. The current would only go down one of the parallel resistors so I shouldn't have subtracted the voltage lost across the resistors for both R2 and R3.
Vtot= V1 + V2
18.3 = V1 + 8.74
V1 = 9.56 Volts lost across R1

R1 = V1 / I1 = 9.56 / .451 =21.2 ohms
 
  • #6
blue_lilly said:
I think I get it. The current would only go down one of the parallel resistors so I shouldn't have subtracted the voltage lost across the resistors for both R2 and R3.
Vtot= V1 + V2
18.3 = V1 + 8.74
V1 = 9.56 Volts lost across R1

R1 = V1 / I1 = 9.56 / .451 =21.2 ohms

Huzzah! Yes, that looks good! :smile:
 
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  • #7
gneill said:
Huzzah! Yes, that looks good! :smile:

All right, thanks so much for your help! I really appreciate it!
 

FAQ: Simple Combination Circuit, Find Current

What is a simple combination circuit?

A simple combination circuit is a circuit that combines multiple elements, such as resistors and capacitors, in a series or parallel configuration to create a single circuit with a single power source.

How do you calculate the current in a simple combination circuit?

The current in a simple combination circuit can be calculated using Ohm's Law, which states that current (I) is equal to the voltage (V) divided by the resistance (R). This can be represented by the equation I=V/R. The total resistance in a series circuit is equal to the sum of all individual resistances, while in a parallel circuit, the total resistance is equal to the reciprocal of the sum of the reciprocals of each individual resistance.

Can the current change in a simple combination circuit?

Yes, the current in a simple combination circuit can change depending on the elements and their configurations within the circuit. In a series circuit, the current remains the same throughout, while in a parallel circuit, the current can split into multiple paths and vary at different points in the circuit.

What is the difference between a series and parallel combination circuit?

In a series combination circuit, the elements are connected in a single path, with the same current flowing through each element. In a parallel combination circuit, the elements are connected in multiple paths, with the total current divided between the paths. Additionally, the total resistance in a series circuit is the sum of all individual resistances, while in a parallel circuit, the total resistance is less than the smallest individual resistance.

How does the voltage affect the current in a simple combination circuit?

The voltage in a simple combination circuit affects the current through Ohm's Law. As the voltage increases, the current also increases, assuming the resistance remains constant. This is because a higher voltage provides more energy for the current to flow through the circuit.

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