iGCSE Physics (0625) 4.5.5 d.c. motor -Exam Style Questions Paper 1 - New Syllabus
▶️ Answer/Explanation
Detailed solution:
The total turning effect (or torque) experienced by a direct current motor coil inside a magnetic field is directly proportional to several key factors. Specifically, increasing the number of turns in the wire coil multiplies the force, and increasing the magnitude of the electrical current flowing through it also proportionally increases the force. Since Coil P has more physical turns (3) compared to Coil Q (2), it inherently has an advantage. To absolutely maximize the turning effect overall, we must pair the coil with the most turns (Coil P) with the highest possible current option available, which is 4 A.
▶️ Answer/Explanation
Detailed solution:
To increase the turning effect (or torque) of a d.c. motor, we need to increase the magnetic force acting on the sides of the coil.
The force on a wire is proportional to the magnetic field strength, the current, and the number of turns in the coil.
Increasing the current provides more charge flow, which directly boosts the motor’s strength, while a stronger magnet provides a more intense magnetic field for the current to interact with.
Additionally, having more turns on the coil means the magnetic force acts on more segments of wire simultaneously, effectively multiplying the total torque.
Since all three suggestions—increasing current, using a stronger magnet, and adding more turns—contribute to a higher turning effect, option A is the correct choice.
▶️ Answer/Explanation
Detailed solution:
The turning effect (torque) on a coil in a magnetic field is directly proportional to the current I and the number of turns N.
Mathematically, the turning effect can be expressed by the relationship τ∝B⋅I⋅A⋅N, where B is the magnetic field strength and A is the area.
To ensure an increase in the turning effect, both variables I and N should be changed in a way that maximizes their product.
Increasing the current I provides more force per unit length of the wire according to F=BIl.
Increasing the number of turns N adds more lengths of wire, thereby multiplying the total force acting on the sides of the coil.
Therefore, only option D, which suggests increasing both the current and the number of turns, guaranteed an increase in the turning effect.