Motion is required for charged particles to experience magnetic forces.

Motion is required for charged particles to experience magnetic forces.

• A. Sometimes
• B. Not at all
• C. False
• D. True

Answer: (D)

Magnetic forces arise on a charged particle only when there is motion relative to the magnetic field. The force is given by F = q v × B, which means its magnitude is q v B sinθ and it points perpendicular to both the velocity and the magnetic field. If the particle isn’t moving (v = 0) or if its velocity is parallel to the field (θ = 0 or 180 degrees), the cross product is zero and no magnetic force acts. Only when there’s a velocity component perpendicular to the field does a magnetic force appear, pulling the charge into a curved path. For a positive charge, the direction follows the right-hand rule; for a negative charge, the direction is opposite. A stationary charge in a changing magnetic field experiences no magnetic force, though a changing field can induce an electric field that can push it—this is an electric, not magnetic, effect. Therefore, motion is required for magnetic forces to act on charged particles.

Magnetic forces arise on a charged particle only when there is motion relative to the magnetic field. The force is given by F = q v × B, which means its magnitude is q v B sinθ and it points perpendicular to both the velocity and the magnetic field. If the particle isn’t moving (v = 0) or if its velocity is parallel to the field (θ = 0 or 180 degrees), the cross product is zero and no magnetic force acts. Only when there’s a velocity component perpendicular to the field does a magnetic force appear, pulling the charge into a curved path. For a positive charge, the direction follows the right-hand rule; for a negative charge, the direction is opposite. A stationary charge in a changing magnetic field experiences no magnetic force, though a changing field can induce an electric field that can push it—this is an electric, not magnetic, effect. Therefore, motion is required for magnetic forces to act on charged particles.