Questions about current source and voltage

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Discussion Overview

The discussion revolves around the characteristics of ideal current sources, specifically their polarity and the implications of measuring voltage across them. Participants explore the application of Kirchhoff's Voltage Law (KVL) in circuits containing current sources, as well as the behavior of current sources when connected to other components like batteries and capacitors.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants propose that ideal current sources have a form of polarity since they produce a unidirectional current of fixed magnitude, but they can generate any voltage necessary to maintain that current.
  • One participant notes that measuring a negative voltage across a current source may indicate the direction of current flow relative to connected components, such as a battery or capacitor.
  • Another participant suggests that connecting a current source to a battery can either charge or discharge the battery depending on the connection, which affects the measured voltage.
  • There is a discussion about the use of KVL in circuits with current sources, with some indicating that a new variable must be introduced to represent the voltage across the current source, along with additional constraints for mesh currents.

Areas of Agreement / Disagreement

Participants express differing views on the concept of polarity in ideal current sources and the implications of measuring voltage across them. The application of KVL in circuits with current sources also appears to be a point of contention, with no consensus reached on the best approach.

Contextual Notes

Participants mention the need for additional equations when introducing new variables for KVL, indicating potential limitations in the analysis of circuits with current sources. The discussion also highlights the dependency on the configuration of connected components, which may influence the behavior of the current source.

BeefBowl
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hey guys, do ideal current sources have polarity? When I measure the voltage of a current source, it gave me a negative voltage. What does it mean? Also, can we use KVL in a loop containing a current source without converting it to equivalent voltage source?
 
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BeefBowl said:
hey guys, do ideal current sources have polarity? When I measure the voltage of a current source, it gave me a negative voltage. What does it mean? Also, can we use KVL in a loop containing a current source without converting it to equivalent voltage source?

Ideal current sources have a polarity of sorts -- they produce a unidirectional current of fixed magnitude. They will also, however, produce any voltage whatsoever that is required to enforce that current magnitude and direction, regardless of the voltage polarity. It's not a good idea to have an ideal current source unconnected to a load of some sort :smile:

You can write KVL equations for circuits with a current source, but you'll have to introduce a new variable to represent the voltage across the current source, and the current source is going to cause you to add another constraint equation for the mesh currents (of course, since you're introducing a new variable, you'll need another simultaneous equation anyways...).
 
BeefBowl said:
hey guys, do ideal current sources have polarity? When I measure the voltage of a current source, it gave me a negative voltage. What does it mean?

Imagine connecting a current source to a rechargeable battery. It will either charge the battery, or discharge it, depending on the connection. Reverse the connecting leads, and it will do the opposite. The voltage you measure across the terminals of the current source will be the voltage of the battery. Or if reversed, the reversed voltage.

The same will happen if you connect a current source to a charged capacitor. The initial polarity (of the voltage across the terminals of the current source) will be determined by the initial polarity of the capacitor. Eventually the polarity will reverse if the current flow is in a direction so as to discharge the capacitor (as it then charges it up with the opposite polarity).
 
Thanks everyone!
 

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