An electron enters a magnetic field parallel to B. The electron's

Question

BlackTom AI · Accepted Answer

Consider the situation: an electron enters a magnetic field that is parallel to the velocity vector (parallel to B). In a magnetic field, the force on a charged particle is given by F = q(v × B). When v is parallel to B, the cross product v × B is zero, so there is no magnetic force acting on the electron.

Option 1: «motion is unaffected.» This is correct because without any magnetic force, there is no change in the trajectory; the electron continues in a straight line at the same speed along the direction of its initial velocity.

Option 2: «direction is changed.» This would require a magnetic force to bend the path. Since v × B = 0 when v is parallel to B, there is no change in direction.

Option 3: «speed is changed.» The magnetic force, even if present, does no work on a charged particle because it is always perpendicular to velocity. When there is no force (as here), the speed remains constant; when a force is perpendicular, speed stays the same while direction changes. In this case, there is no force, so speed is not altered.

Option 4: «energy is changed.» Similar to speed, the magnetic force does no work, so the kinetic energy remains constant. Since there is no force acting, the energy does not change.

In summary, with velocity parallel to B, the magnetic field exerts no force, so the electron’s motion remains a straight line with unchanged speed and energy. The only statement that aligns with this is that the motion is unaffected.

An electron enters a magnetic field parallel to B. The electron's单项选择题

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