WAEC Physics 2015 Paper

Concept:The deflecting force on a current-carrying coil in a magnetic field is given by $F = B I L \sin\theta$.Here, $I$ is the current, $B$ is the magnetic field strength, and $L$ is the length of the conductor.Explanation:In a moving-coil galvanometer, current flows through a coil placed in a radial magnetic field.The magnetic force experienced by the coil depends directly on the magnitude of the current flowing through it.This force also depends on the strength of the magnetic field produced by the permanent magnets.In addition, the force depends on the length of the wire wound in the coil, because a longer conductor experiences a larger force.Since the field is radial, the angle $\theta$ is $90^\circ$, so $\sin\theta = 1$ and the equation simplifies to $F = B I L$.Therefore, the force on the pointer depends on the current strength, the magnetic field strength, and the length of the coil.Thus, options i, ii, and iii are all correct.Answer:D. i, ii and iii