Basic circuit problem -- Resistor network and a voltage source

In summary: It is a scalar.In summary, the conversation discusses a problem with a variable resistor circuit and questions the validity of an equation for a loop. It is noted that the direction of the current affects the sign of the equation, but it is also pointed out that current is a scalar quantity, not a vector. The conversation concludes with a clarification that the direction of current only affects the sign, not the result of the equation.
  • #1
edgarpokemon
93
1

Homework Statement


http://www.chegg.com/homework-help/questions-and-answers/problem-2-variableh-resistor-r-circuit-fig-p224-adjusted-va-equals-60-v-find-value-r-q8083343

Homework Equations

The Attempt at a Solution


Hi, I attached a pdf file with the problem. My question is, for the lower right loop, why is the equation that they have valid? I worked the problem, but the equation that I got for that loop is...

voltage of 180 ohm + voltage of 30 ohm + voltage of 12 ohm + voltage of 18 ohm =0. so the voltage for the 180ohm, is -180V, but they have a positive 180V. How did they got that?
 

Attachments

  • Week1_SampleProblem4_Theory (6).pdf
    47.9 KB · Views: 2,098
Physics news on Phys.org
  • #2
edgarpokemon said:

Homework Statement


http://www.chegg.com/homework-help/questions-and-answers/problem-2-variableh-resistor-r-circuit-fig-p224-adjusted-va-equals-60-v-find-value-r-q8083343

Homework Equations

The Attempt at a Solution


Hi, I attached a pdf file with the problem. My question is, for the lower right loop, why is the equation that they have valid? I worked the problem, but the equation that I got for that loop is...

voltage of 180 ohm + voltage of 30 ohm + voltage of 12 ohm + voltage of 18 ohm =0. so the voltage for the 180ohm, is -180V, but they have a positive 180V. How did they got that?

I just noted something. Does the direction of the current affects the signs (+ or -)? For example, if i have a KVL with a loop going clockwise, and when it reaches a resistor, that has a current going in the opposite direction (to the left), does that matter? If a kvl with a loop going clockwise hits the + sign of a resistor, will it always have a + sign regardless of the direction of the current on that resistor?
 
  • #3
Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.
 
  • #4
analogdesign said:
Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

but if a keep the negative answer, I get a different result?
 
  • #5
No, the negative answer just means the current goes in the opposite direction from what you initially assumed. Remember current is a vector quantity with a magnitude and a direction.
 
  • #6
analogdesign said:
Yes the direction of the current affects the sign. Typically you assume a direction and if you're wrong you'll get a negative answer. It's as simple as that.

look at this picture from my book. In this example, what you said is correct, the assumed direction of current affects the sign. But on the other picture that i posted, they are not doing this. I think they are using a counterclockwise loop for the lower left loop in the other picture, but that should not matter. I used a clockwise direction and i got a negative answer, they used a counterclockwise direction, and they got a positive.
 

Attachments

  • 20180213_194110 (1).jpg
    20180213_194110 (1).jpg
    21.6 KB · Views: 396
  • #7
analogdesign said:
Remember current is a vector quantity with a magnitude and a direction.
I agree with your explanation of the negative current in OP's answer, but current is NOT a vector.
 

1. What is a resistor network?

A resistor network is a collection of resistors connected together in a circuit. The resistors can be connected in series, where they are connected end to end, or in parallel, where they are connected side by side.

2. What is the purpose of a voltage source in a circuit?

A voltage source is a component in a circuit that provides a constant voltage to the rest of the circuit. This voltage is necessary for the flow of current through the circuit and powering any connected devices or components.

3. How do I calculate the total resistance in a series circuit?

In a series circuit, the total resistance is equal to the sum of all the individual resistances. This can be calculated using the formula Rtotal = R1 + R2 + ... + Rn, where R is the resistance value of each individual resistor.

4. How do I calculate the total resistance in a parallel circuit?

In a parallel circuit, the total resistance is less than the smallest resistance of any individual resistor. This can be calculated using the formula 1/Rtotal = 1/R1 + 1/R2 + ... + 1/Rn, where R is the resistance value of each individual resistor.

5. How does the presence of a voltage source affect the current in a circuit?

The voltage source determines the amount of current that flows through the circuit. According to Ohm's Law, the current (I) is equal to the voltage (V) divided by the total resistance (R), or I = V/R. So, the higher the voltage, the higher the current will be, and vice versa.

Similar threads

  • Engineering and Comp Sci Homework Help
Replies
10
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
2
Views
700
  • Engineering and Comp Sci Homework Help
Replies
2
Views
788
  • Engineering and Comp Sci Homework Help
Replies
0
Views
383
  • Engineering and Comp Sci Homework Help
Replies
9
Views
3K
  • Engineering and Comp Sci Homework Help
2
Replies
35
Views
4K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
976
  • Introductory Physics Homework Help
Replies
3
Views
475
  • Engineering and Comp Sci Homework Help
Replies
7
Views
817
  • Engineering and Comp Sci Homework Help
Replies
6
Views
623
Back
Top