Why Does a Battery's EMF Seem Constant Over Time?

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Discussion Overview

The discussion centers around the behavior of a battery's electromotive force (EMF) in circuits, particularly why it is often treated as constant despite its actual decrease over time during operation. Participants explore theoretical simplifications and practical implications in circuit design, as well as comparisons with alternating current (AC) systems.

Discussion Character

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

Main Points Raised

  • One participant questions why the EMF of a battery is considered constant in circuits, noting that it actually decreases over time while in operation.
  • Another participant suggests that treating EMF as constant is a simplification that is useful for many purposes, but not for circuit design.
  • A participant introduces the concept of residential AC voltage fluctuations, highlighting that it can vary significantly depending on demand, while noting the consistency of frequency.
  • One participant expresses a desire for accurate results in building and testing a battery in a circuit, acknowledging the theoretical nature of the constant EMF assumption.
  • A participant discusses the nature of alternating current, explaining that it involves an electric field moving back and forth, and contrasts this with the unidirectional flow of electrons in direct current (DC) circuits.
  • Another participant elaborates on AC, describing it as a result of circular motion between magnets and coils, which allows for efficient voltage transformation and reduced transmission losses.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the treatment of battery EMF in circuit design, with some acknowledging the simplification while others emphasize the need for accuracy in practical applications. The discussion also reveals differing perspectives on the nature of AC versus DC.

Contextual Notes

Limitations include the assumption that EMF can be treated as constant for theoretical purposes, which may not hold in practical scenarios. The discussion also touches on the complexities of AC and its generation, which may not be fully resolved.

Who May Find This Useful

This discussion may be useful for individuals interested in circuit design, battery technology, and the differences between AC and DC systems.

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just a simple question, why is it that in almost all circuits where a battery is the source of DC , the EMF is considered to be constant over time while in reality is it not ? The EMF is a decreasing function w.r.t time considering the battery is operating.
 
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It's just a simplification that works for many purposes. It definitely does not work if you are doing the circuit design, though...

Welcome to the PF, BTW.
 
Also, residential AC voltage commonly varies some 20 volts (~6%) in most of the US depending on demand (not to mention fluctuations due to surges). What one can count on with the former is the accuracy of frequency.
 
Thanks to both of you.

berkeman, so I understand that this is for purely theoretical purposes. I ask because my purpose is building a battery and then testing it in a simple circuit. So the more accurate results, the better.

Loren Booda, now that you've mentioned AC , I am curious about its multi-direction flow of electrons. I have read that somewhere, and I think it means electrons move forward or backward unlike in DC ciruits where electrons flow in one direction , correct ? If this is so , why does it occur ?

Thanks,
 
Alternating current may be understood more accurately as an electric field alternating, i. e., moving back and forth, through a wire - although the alternating electron (particle) picture is mostly correct. The main advantage with AC is that it may be transformed down from high voltage, having fewer losses in transmission. The generator which creates AC inherently converts relatively circular motion between magnets and coils into alternating current.
 

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