Final answer:
Counterelectromotive force (CEMF) or back-EMF is the EMF generated by a motor's coil when turning within a magnetic field, opposing the input voltage, and altering the current and voltage in the motor circuit.
Step-by-step explanation:
Counterelectromotive force (CEMF), also known as back-EMF, is the electromotive force generated by a running motor because it consists of a coil turning in a magnetic field. This force opposes the voltage powering the motor. When a motor is first turned on and not yet moving, the back-EMF is zero, meaning the coil receives the full driving voltage and the motor draws maximum current.
As the motor's angular velocity increases, so does the back-EMF, opposing the driving EMF and consequently reducing the voltage across the coil and the amount of current it draws. This process can be observed when large appliances that contain motors—such as vacuum cleaners, refrigerators, or washing machines—are switched on, causing lights in the same circuit to dim momentarily. Moreover, back-EMF plays a critical role in the motor's operation; it helps prevent the motor from drawing excessive current, thereby protecting it from overheating and potential damage while also enabling efficient function under varying mechanical loads.