The drawing shows four situations in which a positively charged particle is moving with a velocity v through a magnetic field B. In each case, the magnetic field is directed out of the screen toward you, and the velocity is directed to the right. In only one of these drawings is the magnetic force F physically reasonable. Which one is it? 3 1 2 4
Note:
physics diagram

Explanation:

A magentic force is defined as the cross product between the magnetic field and the speed of the particle. Mathematically, it would be

Where represents the charged particle.

So, according to the right hand rule, the resulting vector of a cross product is perpendicular to those vectors forming the product. If you simulate the product with your right hand, the thumb represents the magnetic force vector. So, if you point your index finger downside, you'll have the thumb pointing you (the magnetic field vector) and the middle finger will be pointing right side, as the image 1 shows.

Therefore, by the right hand rule, the correct figure is the first one.