Physics - electricity

As current(towards left or right) cut through the magnetic field lines,
the magnetic field should be into or out of paper.

By Feleming's Left Hand rule(FBI), if the particle carries positive
charge, current should flow with the SAME direction of the particle.
As the magnetic force exerting downwards on particle, the magnetic
field should be OUT OF paper.
The answer should be [Magnetic field : Out of paper ; Charge of
particle : Positive] in this case.

Thus, the answer goes to D.

* Note that if the particle carries negative charge, current should flow with the OPPOSITE direction of the particle.
As the magnetic force exerting downwards on particle, the magnetic
field should be INTO paper.
The answer should be [Magnetic field : Into paper ; Charge of
particle : Negative] in this case.
As the others do not satisify with the conditions, they are false.

Thus, the most satisfactory answer is D.

The direction of motion of the charge, the magnetic field and the direction of force should be mutually perpendicular.

2009-04-19 17:14:58 補充：
As A and B, the magnetic field and the motion of the particle are not perpendicular to each other. They are actually parallel to each other, so the path cannot be like the one as shown in the figure.

2009-04-19 17:15:04 補充：
For C, if the charge is neutral, then it should not be affected by the magnetic field. That is, the magnetic field cannot exert a force on the particle. By Newton's 1st law of motion, it should keep on moving in a straight line. So, C is wrong.

2009-04-19 17:15:11 補充：
So, the answer is D.

Consider the particle is positively charged.

By Fleming's left hand rule, if the magnetic field is out of the paper, then the particle will experience a downward force.

2009-04-19 17:15:18 補充：
The particle is actually moving to the right with a certain speed initially. And under the influence of the downward force, it will undergo a circular path as shown.