Simple electric circuit confusion

[AlexCrafter]

Now, I'm not quite sure where this goes but its my homework that led me to the question I have now.

 

The original task was to do an experiment to show the relationship between amps and resistance, the resistance was made using varying lengths of wire. Now the original task went well, I showed the relationship wrote a conclusion etc. but, I measured the voltage as well and it seems to change with the different lengths of resistance wire.

Now, I used the same battery each time and tried each length multiple times but the voltage has different value for each length. I would assume that the voltage would stay the same.

The next day I did it again and the voltage did stay the same, I used a different battery that time. What is causing this? Does the first battery have a built in feature to change its voltage depending on the resistance or am I imagining things?

 

Any help is appreciated. This is not my homework only a question it brought up.

[YT2095]

if the voltage was dropping under a Reasonable load, but not on the other battery, it`s quite possible the 1`st Batt was nearly dead.

[AlexCrafter]

No, thats not it. I tried the different lengths in succession 3 times; 1, 2, 3, 1, 2, 3, 1, 2, 3. And the results for each try for each length matched. Furthermore, it didn't drop, at first it increased, as the resistance did, then it dropped.

[JohnFromAus]

Any battery has an internal resistance R which is in series with the resistor you were measuring. Low values of the external resistor cause more current to be drawn and therefore more voltage drop across the internal resistor resulting in a drop in the terminal voltage of the battery.

 

The second battery you used must have had a smaller internal R.

 

You can measure the internal R by connecting a variable resistor across the battery - when the terminal voltage drops by 50% the external R equals the internal R.

 

DONT try this with any large battery!!!! Say the internal resistance was 0.1 ohms then a 12v car battery would deliver 60 amps into a 0.1 ohms external resistor! This would dissipate 360 watts - smoke and heat !!