Final answer:
To answer the student's question: (a) The torque of a DC motor will decrease by 5% if its permanent magnets lose 5% of their strength. (b) The current must be increased by 5% to restore the original torque.
Step-by-step explanation:
The subject of the question is related to electric motors, specifically the torque-speed characteristics of a permanent magnet DC motor. In part (a), the question asks by what percentage the torque of the motor decreases if the strength of its permanent magnets is reduced by 5%. The torque of a DC motor is directly proportional to the magnetic field strength. Therefore, if the magnetic field strength decreases by 5%, the torque will also decrease by 5%.
For part (b), to compensate for the reduced magnetic field strength and return the torque to its original value, the current flowing through the motor must be increased. Since torque is also proportional to the current, a 5% increase in the current would restore the original torque.
The student's question also includes information about the electrical characteristics of a DC motor, such as resistance, emf, and current. When examining these characteristics, it's essential to understand the function of the back emf, which opposes the applied voltage and is proportional to the motor's speed.