What Happens When You Connect Different Voltage Batteries in Parallel?

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Discussion Overview

The discussion revolves around the hypothetical scenario of connecting a 6V battery and a 4V battery in parallel, both assumed to have negligible internal resistance, along with resistors. Participants explore the implications of applying Kirchhoff's laws in this context and the contradictions that arise from such a configuration.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about determining the potential difference across the resistor in the circuit due to contradictory answers when applying Kirchhoff's voltage rule.
  • Another participant suggests that the assumptions of circuit theory break down in this scenario, indicating that the presence of impedance in the cells or wires must be considered.
  • A third participant agrees that the scenario leads to contradictions and emphasizes that valid circuit analysis should not yield infinite voltages or currents.
  • Further, it is noted that the circuit definition inherently contains a contradiction related to Kirchhoff's voltage law, necessitating the specification of internal resistances before analysis.
  • One participant elaborates on the limitations of Kirchhoff's laws, stating that they are applicable only to ideal components and that real-world components may require more complex modeling to avoid nonsensical scenarios.
  • Additionally, the discussion touches on the existence of impossible scenarios in network theory, particularly those involving only voltage sources or current sources, highlighting the complexity of circuit analysis.

Areas of Agreement / Disagreement

Participants generally agree that the scenario presents contradictions and that Kirchhoff's laws may not apply straightforwardly. However, there is no consensus on how to resolve these contradictions or the implications for circuit analysis.

Contextual Notes

Participants note that the discussion is limited by the assumptions made about ideal components and the necessity of including internal resistances in the analysis. The implications of using idealized models versus real-world components remain unresolved.

Amaterasu21
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Hi all,
I've been thinking about a hypothetical circuit with (say) a 6V cell of negligible internal resistance, a 4V cell of negligible internal resistance, and a resistor in parallel with each other, and I can't figure out what the potential difference across the resistor would be. I've tried to apply Kirchoff's voltage rule about the emfs and p.d.s around a closed loop, but I can't see how to apply it without contradictory answers. Any help?

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The circuit theory assumptions have to break down for this circuit, for if the wires are truly resistance-free then KVL applied around the upper loop implies ##6-4 = 0##. There must be some impedance lumped with the cells (be it in the wires or the cells themselves).
 
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Amaterasu21 said:
I can't see how to apply it without contradictory answers.
That is correct. It is a self contradictory scenario and therefore all you can get is contradictory answers.
 
You can conceive of several other contradictions leading to seeming paradoxes.

For example, a short circuit across an ideal voltage source. Or an open circuited ideal current source.

They can all be resolved by remembering that valid circuit analysis should never result in infinite voltage or infinite currents. If your scenario leads to such an infinity, then circuit analysis rules do not apply.
 
Amaterasu21 said:
... a 6V cell of negligible internal resistance, a 4V cell of negligible internal resistance, and a resistor in parallel with each other, ...
That circuit definition contains a KVL contradiction.
You must specify the internal resistance of both cells before solving the circuit.
 
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An important point to remember here is that Kirchoff's Voltage and Current laws are only applicable for circuit analysis. Circuit analysis is a highly abstract method of modelling the real world using ideal components. These are generally restricted to resistors, capacitors, inductors, voltage sources, and current sources (there are a few others that you rarely see). These aren't the same as real voltage sources, inductors, etc. Think of it as a graphical version of simple mathematical equations. One consequence of this is that you can construct circuits that don't make sense. Like a 4V battery in parallel with a 6V battery. This is equivalent to the algebraic problem x=4 and x=6, find the value of x.

So, for real world electrical components we will use these simple ideal circuit elements to model the real component as a small network. For example, batteries may be shown as a voltage source with one (or more) resistors. How you choose to model your components allows you to decide how accurate (and difficult) your solutions are.

There are a lot of really well educated people that don't understand this. Including a rather famous MIT physics professor. It's not that they don't know physics, it's that they don't know how EEs talk about network analysis with "lumped element" models (also, they may not know about the problems with voltage and/or current probes in the lab).

If you study network theory, you'll learn of two subtle, but impossible scenarios*. The first is any circuit loop that includes only voltage sources and/or capacitors. The second is any circuit node that only has current sources and/or inductors connected to it. Your network has the former.

* There is a trivial solution where all of the initial conditions match perfectly.
 
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