The correct answer is option-3, which states that the force experienced by a unit positive test charge placed at a point is called the electrical field at that point.
To understand this, let`s break down the options:
Option-1: Magnetic field at that point. The magnetic field is a separate force that is generated by moving charges or currents. It does not directly determine the force experienced by a unit positive test charge, so this option is incorrect.
Option-2: Gravitational field at that point. The gravitational field is the force exerted by a mass on other objects. While it is an important force in physics, it does not apply to electric charges. Therefore, this option is also incorrect.
Option-3: Electrical field at that point. The electrical field is the force exerted by an electric charge on other charges. When a positive test charge is placed in an electric field, it experiences a force due to the interaction with the other charges in that field. Therefore, this option is correct.
Option-4: Nuclear field at that point. The nuclear field is not a concept in physics. It is not relevant to the force experienced by a positive test charge, so this option is incorrect.
In summary, the force experienced by a unit positive test