Why Doesn't Calculating Chord Slope Work for Electron Deflection Angles?

[maccha]

When asked to find the deflection angle for an electron passing through a parallel plate capacitor, for some reason my initial approach was to find the horizontal distance traveled and the vertical distance and then find the angle created by them. I got the wrong answer doing this and looked at the solution which explained that you have to use the final vertical/horizontal velocities and the angle created by them. This makes sense to me, but I still don't understand why my method wouldn't work? Any clarification would be appreciated. Thanks!

 

[tiny-tim]

Hi maccha!

It's because you're finding the slope of the chord from entry to exit (which will be roughly parallel to the tangent at half-way), but the question asks for the slope of the tangent on exit, which is about twice as much.