## Mashy 2 years ago i have an odd question.. in some videos regarding making your own batteries.. i saw some homemade batteries produce high voltage but do not light up LEDs.. now.. why is that.. i mean ohm's law says v= IR.. so if any other battery of same voltage works then this should give out same current for the same led.. right? :-/.. or is it the internal resistance that comes into play?

1. mathmate

Certain batteries have high voltage, but cannot sustain a current. They are great for measuring the voltage, but lights no bulb!

2. Mashy

i dunno why!?? i mean.. how?? what happens to ohms law??

3. Mashy

the terminal pd very less?? compared to emf?? so is it the internal resistance?

4. mathmate

Depending on the nature of the battery. With the high voltage, once there is a current, the energy of the battery could be depleted before the bulb is lit. Ohm's law applies, but there is probably not enough energy to light a bulb long enough for you to see. Ever tried to use a 9 volt battery and two AA batteries to start a car?

5. Mashy

well even then .. there should be a FLASH atleast.. like how a capacitor works.. it gets depleted instantly creating a flash so definitely there cannot be any depletion cause there is not current flowing and as for car.. they need higher voltage.. 200 to 250 volts.. !!

6. mathmate

A car battery is 12 volts, = 9+2*1.5. Would you expect the engine turn before the batteries are exhausted?

7. Mashy

can you do some math to explain it? :P..

8. Mashy

so you mean.. the moment you close the circuit.. the potential difference instantly decreases?? and becomes almost zero?

9. mathmate

The math part depends a lot on the nature of the battery, and the characteristics associated with it. @mashy yes, that's what happens when you draw a large current from a small energy source. I would not say it becomes zero, but decreases exponentially to zero, like: |dw:1358008592637:dw|

10. Mashy

oh wait is it like.. the battery doesn't get enough time to make its red ox reactions.. and maintain the required EMF??.. so that would mean.. the moment i open circuit.. the battery comes back to normal??

11. mathmate

The redox reaction is choked when you close the circuit (Energy=Vit). It doesn't supply fast enough. Yes, most batteries recuperate when you give it time, but not a 100%.

12. Mashy

aha.. that makes sense now!!>. thanks you should name yourself.. math and physics mate.. cheers :)

13. mathmate

Thank you for the honour! :)