Question

26) An electron, moving toward the west, enters a uniform magnetic field. Because of this field the electron curves upward. The direction of the magnetic field is A) towards the north. B) towards the south. C) towards the west. D) upward. E) downward.

Answer 1

Answer: A

Step-by-step explanation:

There is one important fact about magnetic fields. Magnetic fields are always perpendicular to the velocity of the particle, and the force acting on the particle. This is evident in the Lorentz force equation for magnetic fields, which states:

`F=q(v x B)`

The cross product between between the velocity and the magentic field forms a force vector that is perpendicular to both the velocity and the field. In our example, the electron is traveling to the left, and the force it feels is upwards. One thing to keep in mind now, is that the Lorentz force equation is for positively charged particles. Therefore, whatever result we obtain for the field, it is actually the opposite since our particle is negatively charged.

Point your thumb in the direction of the velocity, and point your palm in the direction of the force. Your fingers should be pointing towards yourself (the south). This would be correct for positive charges, but it is the opposite for negative charges. The magnetic field points away from your body, which is the north.