The direction of the magnetic field in a certain region of space is determined by firing a test charge into the region with its velocity in various directions in different trials.The field direction is:
A)one of the directions of the velocity when the magnetic force is zero
B)the direction of the velocity when the magnetic force is a maximum
C)the direction of the magnetic force
D)perpendicular to the velocity when the magnetic force is zero
E)none of the above

Question

Quizplus · Accepted Answer

The direction of the magnetic field is determined by the direction of the velocity of a test charge when the magnetic force on it is zero because the magnetic force is given by $F = q(\mathbf{v} \times \mathbf{B})$, where $q$ is the charge, $\mathbf{v}$ is the velocity of the charge, and $\mathbf{B}$ is the magnetic field. The cross product $\mathbf{v} \times \mathbf{B}$ is zero when $\mathbf{v}$ and $\mathbf{B}$ are parallel (or antiparallel), meaning no magnetic force is exerted on the charge, indicating the direction of the magnetic field.

The Direction of the Magnetic Field in a Certain Region