Final answer:
Magnetic force on a charged particle is only present if the particle is moving, and the force is perpendicular to both the velocity of the particle and the magnetic field's direction. The force is zero if the particle moves parallel to the magnetic field and does not cause the particle to gain kinetic energy. Straight-line motion of a charged particle does not imply a zero magnetic field.
Step-by-step explanation:
Regarding the statements about the force on a charged particle placed in a magnetic field, we can assess their validity based on the principles of electromagnetism:
- (a) A magnetic force is exerted only if the particle is moving. This statement is true, as a stationary charged particle will not experience a magnetic force.
- (b) The force is a maximum if the particle is moving in the direction of the field. This statement is false; the force is actually zero when the particle moves parallel to the magnetic field because the magnetic force is always perpendicular to the velocity and the magnetic field's direction.
- (c) The force causes the particle to gain kinetic energy. This statement is false; while the magnetic force changes the moving charged particle's direction, it does not do work on the particle and hence does not change its kinetic energy.
- (d) The direction of the force is along the magnetic field. This is also false; the force direction is perpendicular to both the velocity and the magnetic field.
- (e) A magnetic field always exerts a force on a charged particle. This is false; a magnetic field exerts a force only on moving charged particles and the force depends on the angle between the particle's velocity and the magnetic field.
When a charged particle moves through a uniform magnetic field at an angle perpendicular to the field, it follows a circular path due to the force applied, which changes only its direction and not its speed. The magnetic force is given by the cross product of the velocity and the magnetic field, indicating that it will always act perpendicular to both. Moreover, the force on a moving charge in a magnetic field does not depend on simply the field's magnitude but also on the velocity of the particle and the angle between the particle's velocity and the magnetic field. In other words, changing the direction of the magnetic field changes the direction of the force, but not necessarily its magnitude.
If a charged particle moves in a straight line, we cannot conclude that the magnetic field is necessarily zero; it may be that the velocity of the particle is parallel to the field, resulting in no force acting on the particle.