## Fundamentals of Physics Extended (10th Edition)

$-z$
We know that $q$ is negative and we want $\vec{v}\times\vec{B}$ to have the opposite direction to $\vec{F}$. Since $\vec{v}\times\vec{B}$ points to $+y$, the negative scalar q will cause $q( \vec{v}\times\vec{B} )$ to point to $-\mathrm{y}$, like $\vec{F}$ does. Now, we use the right hand rule (see fig. 3.19 on p. 53) : Outstretch your right arm and turn your hand so your thumb points up. The fingers are pointing to the right (the fingers represent $\vec{v}$ in the $+x$ direction, and the thumb $\vec{v}\times\vec{B}$ in the $+y$ direction). Swipe your fingers roughly for a right angle. The fingers now point to the front of you. This is the $-z$ direction, when compared to the diagram. This is where $\vec{B}$ points.