The strength of a magnetic field produced by a charged particle is greatest when that particle is:

The strength of a magnetic field produced by a charged particle is directly related to the velocity of that particle. When a charged particle is moving rapidly, it generates a stronger magnetic field due to the increased motion of the charge. This is based on the principle that magnetic fields are produced through the motion of electric charges.

In practical terms, as the speed of the charged particle increases, the amount of magnetic flux generated around it also increases, resulting in a stronger magnetic field. This is why charged particles such as electrons moving rapidly in a conductor can produce significant magnetic fields, which are essential in applications like electromagnets and electric motors.

In contrast, if a charged particle is stationary, it does not create a magnetic field at all. When it is moving slowly, the resulting magnetic field is weaker compared to when it is moving rapidly. While being in an existing magnetic field may allow the charged particle to experience forces, it does not specifically enhance the strength of the magnetic field that the particle itself produces. Therefore, the scenario where the particle is moving rapidly is the one that results in the greatest strength of the magnetic field produced.