Fundamentals: velocity selector

The charged particle is undeflected in the presence of E and B field in a direction orthogonal(prependicular)to each other. If E is vertically down and B is into the page will work for both positive and negative charges. The electric force and magnetic force act in opposite direction and if the value of E divide by B is velocity of charge than the charge will remain undeflected (continue in straight line). similarly is E is vertically up and B must be out of the page to produce the same outcome.