Assigning plarities of circuit elements

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Discussion Overview

The discussion revolves around the assignment of polarities for electric circuit elements, particularly resistors, within the context of circuit analysis. Participants explore the rules and conventions for determining polarities in various scenarios, including the implications of current flow and voltage sources.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions how to assign polarities for resistors in a given circuit diagram.
  • Another participant notes that the polarities may already be assigned in the diagram and suggests that the current entering a resistor indicates the positive terminal.
  • A participant provides a general rule stating that the point where current enters a resistor is marked as positive, while the exit point is negative, with a special note on voltage sources.
  • It is proposed that polarities can be assigned arbitrarily, as long as the measurement of potential difference remains consistent, regardless of the assignment.
  • A later reply adds that there are cases where current can enter the positive terminal of a voltage source, particularly when connecting multiple sources in series, complicating the relationship between voltage polarity and current flow.

Areas of Agreement / Disagreement

Participants express varying views on the assignment of polarities, with some agreeing on the basic rules while others highlight exceptions and complexities. The discussion remains unresolved regarding the best approach to assigning polarities in all scenarios.

Contextual Notes

Participants acknowledge that the assignment of polarities can depend on specific circuit configurations and the presence of predefined markings on components. There is also an indication that assumptions about current flow may vary based on the arrangement of voltage sources.

skeem
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How can I assign polarities for electric elements , say resistors..

You can explain on this circuit

cir.jpg
 
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skeem said:
How can I assign polarities for electric elements , say resistors..

You can explain on this circuit

cir.jpg

I am in introductory circuit analysis so my response is not intended to be the definitive answer... in your diagram, aren't the resistor polarities already assigned?

jason
 
Yes they are already assigned. But here's the general rule: If a current is entering a resistor (or any circuit device for which the voltage drops across it in the direction of current flow), then the point where the current enters is denoted + and the other -. Special case for voltage sources (denoted as cells here): the current enters at the -ve terminal and exits at the positive terminal.
 
You can assign polarities (to resistor voltages) any way you wish... That just means, "Put your voltmeter / multimeter probes like this across the resistor" (where the +ve is the red probe and -ve is the black one). The polarity (which end of the resistor has the higher potential) should come out the same regardless of what you assign, assuming your analysis is correct.

If you were to take a voltmeter or DMM (Digital MultiMeter) and measure the terminals of a battery one way, and then measure the terminals the other way, the only difference you should see would be the sign of the potential. This is a favorite question in basic circuit analysis classes because it's something very easy to overlook when you're in a rush.

EDIT: Unless the + and - signs are already on the resistor (as they are in this question, and as I alluded to earlier). Then the question says, "Measure the potential this way"
 
One thing I should add to my post:

Sometimes it's possible that the current would enter the positive terminal of the voltage source and exit from the negative. This happens for example, when you connect two voltage sources (or cells) with their positive points pointing away from each other in series. In this case the voltage polarities on that cell isn't an indication of which way the current flows, but which way it would flow if the other cell isn't connected backwards in series with this one.
 

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