Final answer:
To determine the resistance of a motor, subtract the back emf from the supply voltage and divide by the current, yielding a resistance value not given in the options. The starting current is calculated by dividing the full supply voltage by the resistance, which is 48 A in this case.
Step-by-step explanation:
The subject of this question is Physics, specifically focusing on electromagnetism in electrical motors. The question asks about determining the resistance of an electric motor and the current it draws at the start.
To find the resistance of a motor using the given voltage, back electromotive force (emf), and current, we use Ohm's law (V = IR) and the fact that the voltage drop across the motor at operating speed is the supply voltage minus the back emf.
Given a motor with a supply voltage of 240 V and a back emf of 180 V at operating speed, the voltage across the motor is 240 V - 180 V = 60 V.
The motor draws a current of 12.0 A, so using Ohm's law (V = IR), the resistance (R) is R = V/I = 60 V / 12.0 A = 5 ohms, which is not one of the options provided.
Therefore, there might be a mistake in the question options or the values given.
When the motor is first started, there is no back emf, and the entire supply voltage is across the motor.
Using the previously calculated resistance of 5 ohms, the starting current is I = V/R = 240 V / 5 ohms = 48 A.