Problem 4: A typical 12 V automobile battery has a resistance of 0.012 Ω. What is the terminal voltage of this battery when the starter draws a current of 100 A. Also calculate R, P, PR and Pr.
Given Electromotive force, emf = 12 V Internal resistance, r = 0.012 Ω
Current, I = 100 A
Required Starter resistance, R =? Power, P =? PR =? Pr =?
Automobile battery provides current to the starter. The internal resistance of the battery (acting as emf), is 0.012 Ω. We know that
Put values in (A) to get terminal voltage.
Terminal voltage is the voltage applied across the resistance.
In order to calculate resistance (of the starter), we apply Ohm’s law.
V = IR. Put the values,
10.8 = 100R OR R = 10.8/100 = 0.108 Ω
Power is the rate of work done (energy conversion) in the circuit. It is given by P=IV.
Put the values, power = 100 * 12 = 1200 watt
To calculate power of the starter, PR = 100 * 10.8 = 1080 watt
To calculate power of the battery, Pr = 100*1.2 = 120 watt