I thought circuits had to be closed

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    Circuits Closed
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Discussion Overview

The discussion revolves around the concept of potential difference in electrical circuits, particularly addressing the question of whether circuits need to be closed for voltages to exist. Participants explore the relationship between current flow and potential difference, and how to calculate voltage in scenarios where current may not be present.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that circuits must be closed for current to flow, but potential differences can exist even without current.
  • One participant mentions that a "KVL walk" can be performed to calculate potential differences as long as there is a path between points, regardless of current flow.
  • There is a discussion about using the formula V = I*R, with questions raised about how to calculate potential difference when current (I) is zero.
  • Some participants propose that if there is no current, there would be no voltage drop across resistors, leading to questions about the equality of potential differences at various points.
  • One participant suggests that a battery maintains a fixed potential difference regardless of whether current is flowing.
  • Questions are raised about finding potential difference across terminals of a battery when they are not connected, and the potential difference in unconnected wires connected to a battery.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between current flow and potential difference, with no consensus reached on how to calculate potential difference in the absence of current.

Contextual Notes

Some assumptions about circuit behavior and definitions of potential difference may not be fully articulated, leading to unresolved questions about voltage calculations in specific scenarios.

joel amos
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Homework Statement


I am supposed to determine the potential differences between the points. However, I thought circuits had to be closed...
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Such a circuit would have to be closed in order for current to flow, but not for voltages to exist. If there is no current, then no voltage is dropped across each resistor.
 
Circuits have to be closed for current to flow continuously, but potential difference does not depend upon a closed path or on continuous current flow. It is enough that there is some path between the points so that can do a "KVL walk" from one to the other, adding up the potential changes along the way.
 
Stephen Tashi said:
If there is no current, then no voltage is dropped across each resistor.
So how then do I calculate potential difference without current?

I'd usually use the formula V = I*R.
 
joel amos said:
So how then do I calculate potential difference without current?

I'd usually use the formula V = I*R.
Yes, and if I=0?
 
gneill said:
Yes, and if I=0?
Well, then there would be no drop in voltage. Does this mean that the potential difference at all three points is the same?

EDIT: Well I guess there would be current through R2.

So then would the potential difference between A and B be 9 V, the difference between A and C be 9 V, and the difference C and B be 0 V?
 
Last edited:
joel amos said:
Well, then there would be no drop in voltage. Does this mean that the potential difference at all three points is the same?
Not quite. The battery will maintain a fixed potential difference regardless of current flow or not.
EDIT: Well I guess there would be current through R2.

So then would the potential difference between A and B be 9 V, the difference between A and C be 9 V, and the difference C and B be 0 V?
Yup.
 
joel amos said:
So how then do I calculate potential difference without current?

How do you find potential difference across the two terminals of a battery when they aren't connected ?
What would the potential difference be in the ends of two unconnected wires that had their other ends connected to opposite terminals of a battery?
 

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