Question

A commutator changes the direction of current in the coil of ___________
A) a DC motors
B) a DC motor and an AC generator
C) a DC motor and a DC generator
D) an AC generator

Answer 1

Here we must know the role of components used in motors and generators and the principle behind the working i.e. Fleming’s left-hand rule and Fleming’s right-hand rule.

Complete step by step solution:
Before starting with working, first we look into Fleming's left- and right-hand rule.
Fleming’s right-hand rule: Stretch the thumb, forefinger, and middle finger of right hand so that they are perpendicular to each other, If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current.
Fleming’s left-hand rule: Stretch the thumb, forefinger, and middle finger of right hand so that they are perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.
Electric motors and electric generators play an important role in our daily life. Most of the appliances which we use in our life work on the principle of electric motor and generator. Let us briefly understand the components(parts) of the motor and generator.

Electric generator: The electric generator works on the principle of electromagnetic induction. In this case mechanical energy is converted to electricity. It consists of a rotating rectangular coil placed between two poles of permanent magnets; it is connected to the two rings. These two rings are connected to an external circuit through brushes. This external circuit consists of a galvanometer to detect the presence of current. When the rectangular coil is rotated, a magnetic field is produced around the coil. By applying Fleming’s right-hand rule, the current is induced in 2 arms of the coil which are parallel to the electromagnets. The current in the outer circuit will flow from second brush to first brush. After half of the cycle, both arms will get interchanged then the direction of induced current also interchanged.

Electric motor: The construction of an electric motor is almost the same as an electric generator, only difference is it is used to convert electric energy into mechanical energy. This works on the principle of Fleming’s left-hand rule.
A device that reverses the direction of flow of current through a circuit is called a commutator. A commutator is a rotary electrical switch in certain types of electric motors and electrical generators that periodically reverses the current direction between the rotor and the external circuit. It consists of a cylinder composed of multiple metal contact segments on the rotating armature of the machine. Two or more electrical contacts called "brushes" made of a soft conductive material like carbon press against the commutator, making sliding contact with successive segments of the commutator as it rotates. The windings (coils of wire) on the armature are connected to the commutator segments.
Commutators are used in dynamos (DC generators) and many DC motors as well as universal motors. In a motor the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced. In a generator the commutator picks off the current generated in the windings, reversing the direction of the current with each half turn, serving as a mechanical rectifier to convert the alternating current from the windings to unidirectional direct current in the external load circuit.
In electric motors, the split ring acts as a commutator.

Hence, the correct answer is option C.

Note: Most of the time we may get confused about which rule we have to use for motors and generators. Fleming's left-hand rule is used for electric motors, while Fleming's right-hand rule is used for electric generators. Different hands need to be used for motors and generators because of the differences between cause and effect. In an electric motor, the electric current and magnetic field exist (which are the causes), and they lead to the force that creates the motion (which is the effect), and so the left-hand rule is used. In an electric generator, the motion and magnetic field exist (causes), and they lead to the creation of the electric current (effect), and so the right-hand rule is used.