Power Supplied by the Leftmost element

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Discussion Overview

The discussion revolves around determining the power supplied by the leftmost element in a circuit, focusing on the application of the power equation (Power = Voltage × Current). Participants explore the implications of current direction and voltage polarity in relation to power output, with a specific emphasis on the context of circuit theory.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant calculates power as P = (-2A)(2V) = -4W and seeks confirmation of this calculation.
  • Another participant argues that the power supplied to the rest of the circuit should be +4W, suggesting a different interpretation of the signs involved.
  • Some participants express confusion about why the power would be +4W instead of -4W, referencing classroom discussions about negative power.
  • There is a discussion about the significance of current entering through the negative node and the implications for power output.
  • One participant explains that if current flows out of the positive terminal, it indicates that the element is doing work and supplying power to the circuit.
  • Another participant clarifies that if the current were to enter the positive terminal, it would represent work being done on the element, leading to negative power output.
  • Participants discuss the concept of voltage being present at a point rather than moving, and how both positive voltage and current contribute to positive power output.
  • One participant expresses their struggle with circuit theory, indicating a learning process.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the sign of the power calculation, with differing interpretations of the current and voltage relationships. There is ongoing confusion and debate regarding the definitions and implications of positive and negative power.

Contextual Notes

Participants reference classroom teachings about negative power, indicating that assumptions about power directionality may vary based on context. The discussion highlights the importance of understanding current flow and voltage reference points in circuit analysis.

Who May Find This Useful

This discussion may be useful for students learning circuit theory, particularly those grappling with concepts of power, voltage, and current relationships in electrical circuits.

Mosaness
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1. Determine the power supplied by the leftmost element in the circuit



Homework Equations



Power = Voltage ( Current)



The Attempt at a Solution



As power = voltage (current), that is the equation that will be used. The leftmost element shows a current of 2 AMPS entering through the negative node and a voltage of 2V.

Therefore the power ought to be :

P = (-2A)(2V) = -4W

I just wanted to know if I did this correctly as we do not have a solutions manual for the book we are using.
 

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The question asked how much power it was supplying (eg to the rest of the circuit). That would be +4W.
 
CWatters said:
The question asked how much power it was supplying (eg to the rest of the circuit). That would be +4W.

I am still slightly confused as to why that would be so? Why 4 W. Why not -4W? We were told in class that it could be a negative power as well.
 
Is it because both the current and the voltage are entering trough the negative node?
 
Mosaness said:
I am still slightly confused as to why that would be so? Why 4 W. Why not -4W? We were told in class that it could be a negative power as well.
The element on the left has a terminal marked +. Its current is shown leaving that terminal, flowing through a circuit external to that element, to return to its negative terminal. That represents power output from that element, the element is doing work, delivering energy to the external circuit. It's precisely how a battery-powered circuit is drawn, showing current emerging from the battery's + terminal.

Were the current to be drawn reversed, so that it enters the element at its + terminal, that represents work being done on that element, the external circuit is forcing current into the element's + terminal and doing work on it. That's a typical arrangement when a secondary battery is being recharged. So power coming from the element would be negative (because power is going to it, not from it).
 
Last edited:
NascentOxygen said:
The element on the left has a terminal marked +. Its current is shown leaving that terminal, flowing through a circuit external to that element, to return to its negative terminal. That represents power output from that element, the element is doing work, delivering energy to the external circuit. It's precisely how a battery-powered circuit is drawn, showing current emerging from the battery's + terminal.

Were to current to be drawn reversed, so that it enters the element at its + terminal, that represents work being done on that element, the external circuit is forcing current into the element's + terminal and doing work on it. That's would be a typical arrangement when a secondary battery is being recharged. So power coming from the element would be negative (because power is going to it, not from it).

And the Voltage is also shown leaving the positive terminal, which gives us a positive power correct?
 
Mosaness said:
And the Voltage is also shown leaving the positive terminal, which gives us a positive power correct?
We don't usually speak of a voltage moving or leaving. Current being comprised of a flow of charge carriers can be pictured as moving or leaving. Voltage is present at a point, it exists at a point (and is measured relative to the voltage at another point).

But yes, the voltage at the + terminal is positive (with reference to the other terminal), and current at the + terminal is in the positive direction for doing positive work. With voltage and current both being positive, so is power! :smile:
 
NascentOxygen said:
We don't usually speak of a voltage moving or leaving. Current being comprised of a flow of charge carriers can be pictured as moving or leaving. Voltage is present at a point, it exists at a point (and is measured relative to the voltage at another point).

But yes, the voltage at the + terminal is positive (with reference to the other terminal), and current at the + terminal is in the positive direction for doing positive work. With voltage and current both being positive, so is power! :smile:


Thanks! I am still fairly new at Circuit Theory, so I am kind of struggling! But hey, practice makes perfect!
 

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