The magnetic field produced by a moving charge is always perpendicular to the charge's velocity.

The magnetic field produced by a moving charge is always perpendicular to the charge's velocity.

• A. Sometimes
• B. False
• C. True
• D. Never

Answer: (C)

The magnetic field from a moving point charge comes from the charge’s motion: the field at a point in space is proportional to the cross product of the velocity with the direction from the charge to that point, B ∝ v × r̂. A cross product is always perpendicular to both inputs, so the resulting magnetic field is perpendicular to the charge’s velocity. An intuitive picture is that, for steady motion, the field lines wrap around the path of motion like circles around a wire, with the field direction tangent to those circles—again perpendicular to the direction the charge is moving. If the charge isn’t moving, there’s no magnetic field. So the statement is true.

The magnetic field from a moving point charge comes from the charge’s motion: the field at a point in space is proportional to the cross product of the velocity with the direction from the charge to that point, B ∝ v × r̂. A cross product is always perpendicular to both inputs, so the resulting magnetic field is perpendicular to the charge’s velocity. An intuitive picture is that, for steady motion, the field lines wrap around the path of motion like circles around a wire, with the field direction tangent to those circles—again perpendicular to the direction the charge is moving. If the charge isn’t moving, there’s no magnetic field. So the statement is true.