Final answer:
We cannot calculate the magnitude of the electric field without information about the force experienced by the particle.
Step-by-step explanation:
The magnitude of the electric field can be calculated using the equation E = F/q, where E is the electric field, F is the force, and q is the charge. In this case, we are given the charge of the particle as 0.640 nc. To calculate the electric field, we need to know the force experienced by the particle when it passes through undeflected. Unfortunately, the question does not provide this information, so we cannot calculate the magnitude of the electric field.