Question

a proton, moving north, enters a magnetic field. because of this field, the proton curves downward. we may conclude that the magnetic field must have a component

a) towards the east.
b) upward.
c) towards the west.
d) downward.
e) towards the north.

Explanation:

Step1: Recall the right - hand rule for magnetic force on a charged particle

The magnetic force on a charged particle is given by $\vec{F}=q\vec{v}\times\vec{B}$, where $q$ is the charge of the particle, $\vec{v}$ is the velocity vector of the particle, and $\vec{B}$ is the magnetic - field vector. For a positive charge (like a proton, $q>0$), we can use the right - hand rule. The velocity of the proton is northward.

Step2: Apply the right - hand rule

Point the fingers of your right hand in the direction of the velocity of the proton (north). The proton curves downward, so the magnetic force is downward. Curl your fingers towards the magnetic - field direction. Your thumb points in the direction of the magnetic force. When you point your fingers north and your thumb downward, your curled fingers point towards the west.

Answer:

C. towards the west