Short Circuit Current, EMF, Internal Resistance

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Discussion Overview

The discussion revolves around the determination of short circuit current in a battery pack, particularly focusing on the relationship between electromotive force (EMF), internal resistance, and the implications of measuring short circuit current. The context includes experimental methods and theoretical considerations related to battery behavior.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes their experiment to find internal resistance and EMF, proposing that short circuit current can be calculated as EMF divided by internal resistance.
  • Another participant raises safety concerns about shorting batteries and questions the context of the experiment, suggesting that it may not be safe to measure short circuit current directly.
  • A third participant critiques the assumption that internal resistance is constant across all currents, noting that batteries may not behave ohmically and that the resistance of the ammeter could also affect measurements.
  • Another participant agrees that if only internal resistance is considered, the formula for short circuit current holds, but emphasizes the need for caution in practical applications.
  • There is a reiteration that the variability of internal resistance in batteries complicates the assumption that it remains constant, which could lead to inaccuracies in determining short circuit current.

Areas of Agreement / Disagreement

Participants express differing views on the validity of using EMF/internal resistance to calculate short circuit current, with some supporting the approach under certain conditions while others caution against it due to the non-ohmic behavior of batteries and safety concerns. No consensus is reached regarding the best method for determining short circuit current.

Contextual Notes

Participants highlight limitations related to the assumptions of constant internal resistance and the potential influence of the ammeter's resistance on measurements. The discussion does not resolve these complexities.

Who May Find This Useful

This discussion may be of interest to students conducting experiments with batteries, educators teaching concepts of electromotive force and internal resistance, and hobbyists exploring battery behavior in practical applications.

geoff.bray
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Okay so I have conducted an experiment to find the internal resistance of a battery pack by gradient of a graph of voltage against current. All so far okay.

I went on to find the EMF also, again all so far okay.

Now I need the short circuit current, and I can't find any information anywhere unbelievably. I don't know if its known otherwise elsewhere.

But anyway, I thought it would simply be EMF/Internal Resistance.

The battery pack is in series with 4 batteries, it is not parallel.

Can anyone confirm/disprove this for me?
 
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geoff.bray said:
Okay so I have conducted an experiment to find the internal resistance of a battery pack by gradient of a graph of voltage against current. All so far okay.

I went on to find the EMF also, again all so far okay.

Now I need the short circuit current, and I can't find any information anywhere unbelievably. I don't know if its known otherwise elsewhere.

But anyway, I thought it would simply be EMF/Internal Resistance.

The battery pack is in series with 4 batteries, it is not parallel.

Can anyone confirm/disprove this for me?

You need to be careful shorting batteries in the real world. What is the context of the question? Is this for a school lab? If so, have they assured you that it is safe to short the battery pack to measure the short circuit current?
 
"Internal resistance" is a loose term to describe the way a battery (or any power source) functions. The assumption is made that it is the same for all currents - i.e. that the battery is 'ohmic'. If you plot the voltage drop over a range of practical currents then you do get a straight line, which justifies this to an extent. I wouldn't say that measuring short circuit current would necessarily reveal the same answer, particularly if the battery gets cooked in the process. (Also, the resistance of the Ammeter would be a factor if the battery happened to be a really beefy one.)
The proposed experiment is fine as a thought exercise but not the best way to find out a useful property of the battery. And you could always blow it up if you're unlucky.
 
If yo connect the terminals of a battery (generator) together the ' short circuit ' current equals emf/total resistance .
If the only resistance in the circuit is the internal resistance then you are correct
 
That assumes that the resistance is the same for all currents. It may well not be because batteries are not made of metals so they can't be relied upon to follow Ohm's law and may well not have a fixed value of resistance.
Also, your current meter has resistance that needs to be taken into account.
 

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