Proton Beam Hitting Its Target

If there is no deflection, then the two opposing forces must cancel each other.
In other words,
Electric force $\displaystyle =$ Magnetic force

Hence,
$\displaystyle qE = Bvq$ ,
where $\displaystyle q$ is the charge of a single particle, and $\displaystyle v$ is the velocity of that particle.

$\displaystyle\Rightarrow v = \frac{E}{B}$

Now,
$\displaystyle i = \frac{dq}{dt} = \eta\times e$ ,
where $\displaystyle \eta$ is the no. of particles crossing a point per unit time, and $\displaystyle e$ is the charge of a proton

$\displaystyle\Rightarrow \eta = \frac{i}{e}$

Finally, we find the force exerted on the target.

$\displaystyle F = \eta\frac{dp}{dt} = \eta\times(p_{final} - p_{initial}) = -\eta\times p_{initial} = -\eta\times m_p v$

$\displaystyle |F| = \eta\times m_p v$

Substituting the values of $\displaystyle \eta$ and $\displaystyle v$ , we get,

$\displaystyle |F| = \frac{i}{e} \frac{E}{B} m_p \approx \boxed{2\times 10^{-5}N}$