What is intern. resist. of a 12.0 V car battery whose terminal voltage drops to 4.1 V when starter draws76 A?

Question

I believe I already know the resistance of the battery : 0.104 ohms, but the second part of the question asks" What is resistance of the starter?" I just needed help w/ the second part if possible...

Answer 1

Use ohm's law. Consider the current passing from the - terminal of the battery to the + terminal.

The total change in voltage is 4.1 V. That is also equal to the sum of the voltage changes as you cross the EMF of the battery and the internal resistance so the equation describing that would be:

4.1 = +12 - 76(r) Solve for r and you get

r = 0.104 ohms. (You are right about that!)

Now use Kirchoff's loop Rule: The sum of all the voltage changes across all the elements in a closed loop in any circuit is zero.

0 = +12 - 76(.104) - 76R

R = 0.054 ohms

is the resistance of the starter.

Answer 2

0.1039 ohms internal sounds correct. You can determine starter resistace by R = E / I ; Rstarter = 4.1v / 76amps = 0.0539ohms.
A quick check is: Rtotal = 0.0539 + 0.1039 = 0.1578ohms. Total current is then I = E / R = 12 / 0.1578 = 76amps. Checks out!