Can Two Batteries Simultaneously Act as Both the Source and Load in a Circuit?

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Discussion Overview

The discussion revolves around the behavior of two batteries in different circuit configurations, specifically whether they can act as both sources and loads simultaneously. Participants explore the implications of connecting batteries in parallel versus series and the resulting current flow in each scenario.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • In scenario 1, where both positive and negative terminals of the batteries are connected together, some participants suggest that no current flows, indicating an incomplete circuit.
  • In scenario 2, where the positive terminal of one battery connects to the negative terminal of the other, participants assert that a circuit is completed, allowing current to flow.
  • One participant questions the definition of a "circuit," noting that scenario 1 forms a loop without current, raising the idea of an "open circuit."
  • Another participant states that the total voltage in scenario 2 would be 3 V, with current calculated as 3/R A, where R is the total internal resistance.
  • There is a mention of Kirchhoff's laws, with one participant explaining that in scenario 1, the current is zero due to the equal and opposite electromotive forces (emfs) of the batteries.
  • Some participants discuss the concept of a battery acting as its own load when terminals are shorted, suggesting a low resistance path.

Areas of Agreement / Disagreement

Participants express differing views on whether scenario 1 constitutes a circuit and the implications of the batteries' configurations. There is no consensus on the definitions or the behavior of the batteries in these scenarios.

Contextual Notes

The discussion includes assumptions about internal resistance and the definitions of circuit completeness, which remain unresolved. The calculations presented depend on specific conditions and interpretations of circuit theory.

Evil Bunny
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Probably a very simple question, but I'm not sure of the answer...

We have two batteries.

Scenario 1: The positive terminals of both batteries are connected together and the negative terminals are connected together. Little to no current flows, correct?

Scenario 2:
The positive terminal of battery A is connected to the negative terminal of battery B.
The positive terminal of battery B is connected to the negative terminal of battery A.
Now we have lots of current flowing, correct?

In scenario 1, apparently no circuit was completed, yet in scenario 2 we must have completed a circuit... Why? Did both batteries (of scenario 2) simultaneously become the sources and the loads? How would you calculate the current flow?
 
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Hi Evil Bunny! :smile:
Evil Bunny said:
In scenario 1, apparently no circuit was completed, yet in scenario 2 we must have completed a circuit... Why? Did both batteries (of scenario 2) simultaneously become the sources and the loads? How would you calculate the current flow?

This is exactly what happens if you connect two 1.5 V batteries together the "correct" way, and then complete the circuit with only a wire …

the total voltage will be 3 V, and if the total internal resistance (plus the negligible resistance of the wire) is R Ω, then the current will be 3/R A, and the power drain (although you can't see it) will be 9/R W.
 
Evil Bunny said:
In scenario 1, apparently no circuit was completed, yet in scenario 2 we must have completed a circuit... Why? Did both batteries (of scenario 2) simultaneously become the sources and the loads? How would you calculate the current flow?


This makes me wonder exactly what we mean by the word "circuit". We certainly have a loop of some kind in scenario 1, but no current. (A simple metal ring with no battery is a loop, but without a current, unless there's a time-varying magnetic flux through the loop, of course.)

Scenario 1 has two batteries in parallel, so that "loop" is in itself a voltage source, having the same voltage as a single battery, like in this picture. But, should we call that a "circuit"? An open circuit perhaps? (Image from http://itp.nyu.edu/physcomp/Notes/Batteries")

[PLAIN]http://itp.nyu.edu/physcomp/uploads/battery_parallel.jpg
 
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I guess not... it's only a circuit if the negative side makes it's way back to the positive side. And apparently the source becomes it's own (very low resistance) load when we short the two terminals. Interesting...
 
It is a circuit (loop) no matter which way you connect them. The resistance of the circuit is 2r where r is the internal resistance of the battery. The sum of the emfs is E-E in the first case and E+E in the second case. (see Kirchoff's second law, for example).
So in the first case
I*2r= 0 and I=0
and in the second case
I*2r= 2E so I= E/r.
These cases bot refer to a circuit with just batteries. No external load.
 

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