The physics
A charge q in a field E feels a steady force qE. A + charge feels it along E.
F = q E between plates: E = V / d
A charged drop floats when the electric force balances its weight. Its charge is a whole number of electrons.
n e E = m g
A bob in a sideways field tilts. The thread lines up with the sum of qE and mg.
tan θ = q E / m g T = √((mg)² + (qE)²)
A big sheet of charge makes the same field σ/2ε₀ at every distance.
A steady field acts like extra weight. It moves a spring's rest point, and it changes a pendulum's g.
x₀ = q E / k geff = g + q E / m T = 2π √(L / geff)