Deflection in an electric field
In an oscilloscope tube, electrons fly to the screen along a parabola.
The electrons are first accelerated through a voltage U₁. Between the deflection plates they feel a constant electric force in the perpendicular direction, so they move like a horizontally launched projectile along a parabola; after leaving the plates they travel in a straight line at constant velocity to the fluorescent screen. The deflection is proportional to U₂ and inversely proportional to U₁, and independent of the particle's charge and mass.
Screen deflection vs. deflecting voltage
The slope of the line is inversely proportional to U₁; the gaps at both ends mean the deflection is too large and the electrons hit the plates.
Think about it
Why would protons give the same deflection in the same oscilloscope tube?
From y = U₂L²/(4dU₁), the deflection depends only on the voltages and the plate dimensions: a larger charge feels a larger force, but it also gains more kinetic energy during acceleration, and the two effects cancel exactly. (Protons must be accelerated in the opposite direction, and they are deflected the opposite way.)
Model assumptionsGravity and plate edge effects neglected · Electrons accelerated from rest · Screen distance 15 cm
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.