The starting motor of a car is turning too slowly, and the mechanic has to decid

Question

The starting motor of a car is turning too slowly, and the mechanic has to decide whether to replace the motor, the cable, or the battery. The car's manual says that the 12 V battery should have no more than 0.020 internal resistance, the motor no more that 0.200 resistance, and the cable no more than 0.040 resistance. The mechanic turns on the motor and measures 11.4 V across the battery, 1.5 V across the cable, and a current of 45 A. Which part is defective? (the "motor", the "cable", or the "battery")

Explanation / Answer

Checking the battery first..it was given that 11.4 Vwas measured..so 12 -11.4 = 0.6V drop was observed across battery internal resistance..

since circuit current = 50A given

internal resistance of the battery = 0.6/ 50 =0.0133 which is below the limit 0.02

Coming to cable 31.5 was observed,,so internal resistance = 1.5/45 =0.0333 which is below specification 0.04

Coming to motor, Vm = 11.4-1.5 = 9.9 V

Rm = 9.9/45 = 0.22 which is above specification 0.20

Hence motor is defective

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