Final answer:
The magnitude of the electric field needed to bring a proton to rest from a speed of 600,000 m/s cannot be determined without additional information such as charge, distance, or time.
Step-by-step explanation:
To find the magnitude of the electric field that brings a proton to rest from an initial speed of 600,000 m/s, we will need to know more information, such as the amount of charge on the proton, the distance over which the proton is brought to rest, or the duration of time over which the proton decelerates. Without additional information, it is not possible to directly calculate the magnitude of the electric field. Therefore, based on the data provided, the answer to the question 'What is the magnitude of the electric field?' is Cannot be determined.