# Short Circuit Current, EMF, Internal Resistance

1. Jun 15, 2012

### geoff.bray

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 cant 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?

Last edited: Jun 15, 2012
2. Jun 15, 2012

### Staff: Mentor

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?

3. Jun 16, 2012

### sophiecentaur

"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.

4. Jun 16, 2012

### truesearch

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

5. Jun 16, 2012

### sophiecentaur

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.