Final answer:
When a motor starts up, it is protected by established effective resistance and the concept of back EMF, both of which are crucial for safe motor operation. The effective resistance affects the startup current, and back EMF provides opposition to the applied voltage, mitigating current after initial surge.
Step-by-step explanation:
When a motor starts up, it is protected by the establishment of an effective resistance and by understanding the concept of back electromotive force (back EMF). The effective resistance of a motor influences the current that flows through it upon startup. For example, if we have a car's starter motor where 150 A flows through as the battery applies 11.0 V, we can calculate this effective resistance using Ohm's law, V = IR, which leads to R = V/I. In this case, the effective resistance would be 11.0 V / 150 A = 0.0733 ohms.Back EMF is important in understanding motor behavior as well; it is an induced EMF that opposes the input voltage. In a specific scenario, if we have a 120 V motor that normally draws 8.00 A and draws 20.0 A when first starting, the back EMF can be calculated by considering the difference in current due to the opposition it creates. The presence of back EMF indicates that the motor's speed is such that it is generating an electrical force opposing the applied voltage, thus moderating the current after the initial surge at startup.Therefore, both the effective resistance and back EMF play critical roles in the initial operation and protection of motors. Not properly accounting for these can lead to issues such as the inability to start a car, as might be the case when dealing with an old car with battery issues.