# Homework Help: Ohm's Law

1. Sep 30, 2004

### Euclid

If a 12V car battery has an internal resistance of .4 ohms, what is the current if the battery is shorted momentarily?

Maybe this is stupid, but if the battery is shorted, it shouldn't be delivering any current, right?

2. Sep 30, 2004

### cepheid

Staff Emeritus
I hear that one should not try to short a car battery (connect the + and - terminals directly with a wire of negligible resistance) because they have a tendency to explode. This is because in circuit loop of such paltry resistance, the current drawn will be HUGE. Think about it...the battery supplies an EMF of 12V. There has to be a corresponding (and equal) potential drop around the circuit. The only resistance available to provide such a voltage drop is the internal resistance of the battery, because the potential drop is given by V = RI. So 12V = RI ==> I = 12V/.4Ohm = 30 A. Did I get this right?

3. Sep 30, 2004

### Euclid

That makes sense... I guess I didn't know what it meant to "short" a battery. I thought it meant it went dead.

4. Sep 30, 2004

### cepheid

Staff Emeritus
Yeah...same thing it means as in a "short circuit". The current takes the path of least resistance, and if the that path is not the one you intended (i.e., there is a 'short' or unintended conducting pathway somewhere), then the increased current means bad news.