How to play Oil Drop
- Choose a test and read the drop's mass, the voltage or the electrons, and the gap on the card.
- Use q·V ÷ d = m·g for a hovering drop, q = n·e, and e = 1.602 × 10⁻¹⁹ C.
- Type your prediction and press Start. The voltage rises for 2 s, then holds.
- Watch whether the drop hovers. Your orange drop, or the orange rungs of the charge ladder, show what your answer predicts.
Charge comes in electrons
In 1909 Robert Millikan and Harvey Fletcher sprayed tiny oil drops between two metal plates and watched them through a microscope. Some drops picked up a few extra electrons. With a voltage V across plates d apart there is an electric field E = V ÷ d, which pushes on a charge with the force q·E. By adjusting the voltage until a drop hovered, they made the electric force equal to the weight, q·V ÷ d = m·g, and so found the charge. Drop after drop, the charges were always whole numbers of the same tiny amount, e = 1.6 × 10⁻¹⁹ C, the charge of one electron. Nothing ever carried half of it. That is why charge is called quantized: it comes in steps of e, the way money comes in whole cents.
A focused learning model
The drop is a sphere of oil (886 kg/m³) in still air. Drag keeps it at its terminal speed at every moment, v = (F_E − W) ÷ (6π·η·r), so it never speeds up. The field between the plates is uniform. The voltage rises steadily for 2 s and then holds for 2 s, so the drop sinks a little before the field is full. A drop that reaches a plate sticks to it. The drops are drawn enlarged, and the microscope view is magnified with a 0.1 mm grid. Answers within 2% are right.