An automobile starter motor has an equivalent resistance of

An automobile starter motor has an equivalent resistance of 0.0630 ohm and is supplied by a 12.0-V battery with a 0.0350-ohm internal resistance. What is the current to the motor? What voltage is applied to it? What power is supplied to the motor? Repeat these calculations for when the battery connections are corroded and add 0.0900 H to the circuit. (Significant problems are caused by even small amounts of unwanted resistance in low-voltage, high-current applications.)

Solution

Total resistance = 0.0630 + 0.0350

R = 0.098

(a)

I = V/R

I = 12.0/0.098 A

I = 122.44 A

(b)

V = I * 0.0630 V

V = 122.44 * 0.0630 V

V = 7.713 V

(c)

P = I^2 * R

P = 122.44^2 * 0.0630 W

P = 944.467 W

(d)

Total resistance = 0.0630 + 0.0350 + 0.090

R = 0.188

I = V/R

I = 12.0/0.188 A

I = 63.829 A

V = I * 0.0630 V

V = 63.829 * 0.0630 V

V = 4.02 V

P = I^2 * R

P = 78.95^2 * 0.0530 W

P = 330.35 W