The starting motor of a car is turning too slowly, and the mechanic has to decide whether to replace the motor, the ca- ble, 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 than 0.200 resistance, and the cable no more than 0.040 resistance. The mechanic turns on the motor and mea- sures 11.4 V across the battery, 3.0 V across the cable, and a cur- rent of 50 A. Which part is defective?

a. the motor
b. the cable
c. the battery

The cable is defective

Explanation:

First of all, let's check if the battery is okay from the equation;

Ir = ΔV

Where;

I is the current

r is the internal resistance gotten

ΔV is the difference in potential difference

Thus;

50(r) = (12 - 11.4)

r = 0.6/50

r = 0.012 ohms

Now, thus value is less than the internal resistance of 0.02 ohms and thus it meets the requirement, so the battery is okay.

Now, let's check the cable from the equation;

V = IR

Where;

V is voltage

I is current

R is resistance

So,

3 = 50(R)

R = 3/50

R = 0.06 ohms

This is bigger than the maximum of 0.04 ohms required in the question. Thus, the cable is defective.

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