Lorentz Force and Larmor radius I have the following exercise: I have a charge (q) accelerated by a potential difference ($ \Delta V$) which enters a region in which a magnetic uniform field is present (the field is not know). I have the distance FO and I have to determinate the distance OS, where 'S' is the point of impact on the "Schermo". The magnetic field B and the particle speed v forms an angle of 90°

That's my idea:
The electrostatic field is conservative so I can write this:
$$ \Delta K = W_T = - \Delta U_e = q\Delta V $$
so: $ 1/2 m v^2 = q\Delta V => v^2 = 2q\Delta V / m $
Now when the charge reaches the "magnetic region" moves in circular motion because of the Lorentz Force with this radius: $ qvB = mv^2 / R => R = mv/qB $
I can't go on with the exercise because I haven't got the magnetic field B.
Can you please help me completing this?
Thank you so much :(
 A: From the comments

I just know it's 90° with v and makes q go down.

I conclude that the angle of impact on the bottom wall is $90°$. If this is not the case then my answer would not address the question.

Answer: In a uniform magnetic field with $\vec{B} \perp \vec{v}$ the particle trajectory is a circle. Hence since we know the final angle the solution is
$$\overline{OF} = \overline{OS}.$$
The magnetic field is not needed since the geometry already specifies the trajectory, which is probably what honeste_vivere meant in his comment.
A: 

From the first Figure 
\begin{align}
OS^{2}  & =KS^{2}-OK^{2}=KS^{2}- \left( KF-OF \right)^{2}
\tag{001}\\
& =R^{2}-\left(R-OF\right)^{2}=OF\cdot\left(2R-OF\right) 
\end{align}
so
\begin{equation}
OS =
 \begin{cases}
\sqrt{OF\cdot\left(2R-OF\right)} & \text{for $0 < OF \le 2R $} \\
\text{no impact} & \text{otherwise}
\end{cases}
\tag{002}
\end{equation}
\begin{equation}
R\equiv \dfrac{mv}{qB}=\sqrt{\dfrac{2m\Delta V}{qB^2}}
\tag{003}
\end{equation}
We must have the value of $\:B\:$.
If we want impact normal to Schermo then $\:OF=R\:$ that is
\begin{equation}
OF\cdot B =\dfrac{mv}{q}=\sqrt{\dfrac{2m\Delta V}{q}}
\tag{004}
\end{equation}
