Homework Help: Jumping a dead battery in a car

1. Dec 13, 2011

Eastonc2

1. The problem statement, all variables and given/known data
so the problem, along with the figure, can be found here

2. Relevant equations
I=V/R,

3. The attempt at a solution

I am able to calculate parts a and b, however, once the batteries are in parallel, I am completely confounded. The concept of the current that would be running through the dead battery completely eludes me.

2. Dec 13, 2011

SammyS

Staff Emeritus
Have you learned to use Kirchhoff's Laws?

That should do it.

3. Dec 13, 2011

Eastonc2

Ok, so, how do I take the second, 'dead', battery into account? I am actually working on a slightly different problem in terms of voltage and resistance, but the arrangement is essentially identical. Do I calculate the current that it would pass through the resistors as well? it's just that when we went over kirchoff's laws we didn't really cover any instances involving batteries in parallel like this, and it's really throwing me for a loop lol, get it? sorry bout that last part....

4. Dec 13, 2011

Eastonc2

What I'm trying to find is a break down of the work required. I know how to do the calculations, I am just unsure of how the numbers would work out because of this second battery. I might be approaching this thinking it's more complicated than it actually is, but I see that second battery, the dead one, and don't know what to do with it, and the effect it has on the rest of the circuit.

5. Dec 13, 2011

SammyS

Staff Emeritus
You will need voltage equations for two of the three possible loops.

You may need a junction equation, depending upon how you define the currents.

6. Dec 13, 2011

Eastonc2

so, I guess what I'm looking for is, can the dead battery still pass current through the right loop given that the battery on the left is charging it?

7. Dec 13, 2011

netgypsy

Set up your Kirchoff equations for the loops and your currents in and out of a junction. Just make a guess as to the direction of the current through the second battery if you have no intuition about it. Solve and if you chose the wrong direction you'll get the opposite sign. Once you solve for the currents it should make sense to you.

8. Dec 13, 2011

SammyS

Staff Emeritus
Yes, it can ... but will current pass through it with this model ??? The answer should be interesting.

9. Dec 13, 2011

netgypsy

Set up the equations correctly and you don't have to guess.

10. Dec 13, 2011

SammyS

Staff Emeritus
[STRIKE]Well, the bird flies a finite distance, in a finite amount of time, but does that in an infinite number of intervals !!![/STRIKE]

Thanks gneill . My computer killed my reply & I jumped to the wrong thread to re-post.

Last edited: Dec 13, 2011
11. Dec 13, 2011

Staff: Mentor

Well, THAT post was a bit of a train-wreck.

12. Dec 13, 2011

netgypsy

help on the bird problem just up the list???